Thin film chalcogenide photovoltaic materials

Resume : Cu2ZnSnSe4 solar cells, containing only abundant elements, with Ag alloying recently reached efficiency of 10.2% [1]. The open-circuit voltage in CZTSSe devices is believed to be limited by absorber band tailing caused by the exceptionally high density of Cu/Zn antisites. By replacing Cu in CZTSe with Ag, the density of I–II antisite defects (e.g Cu/Zn disorder) is predicted to drop. It was shown by neutron diffraction that CZTSe crystallizes in the kesterite type structure (space group I4 ̅) [2]. So far, only X-ray diffraction used for structural characterization of Ag2ZnSnSe4 was reported in the literature, and it suggests that it also shows the kesterite type structure [3]. The aim of this study was to clarify the crystal structure of the room temperature modification as well as to deduce possible cationic point defects, paying a special attention to the presence of Cu/Zn disorder in these compounds. The results allow us to suggest that in spite of both end members of the (Ag1-xCux)2ZnSnSe4 solid solution being reported to crystallize in kesterite type structure, at least the solid solution with x=0.17 and 0.46 crystallize in the stannite type structure (space group I4 ̅2m). Here the site 2a is completely occupied by the divalent cation (Zn), whereas the site 4d is occupied by Cu and Ag randomly. The latter corresponds to the sites 2c and 2d in the kesterite type structure, in which the Cu/Zn disorder in kesterite type CZTSe occurs. In this way the possibility for the formation of Cu/Zn disorder is completely blocked in (Ag1-xCux)2ZnSnSe4. [1] T. Gershon, et al Adv. Energy Mater. 2016, 1502468 [2] S.Schorr, Solar Energy Materials and Solar Cells, 95 (2011)1482. [3] W. Gong et al, phys. stat. sol C., 1-4(2015)

Resume : We have studied the fabrication of polycrystalline Cu2ZnGeSe4 solar cell absorbers by H2Se selenization of sequentially deposited metal layer stacks. The fabricated layers are polycrystalline with grain size of the order of 1 µm and a band gap of about 1.4 eV. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent XRD spectra. We have found that mainly Cu3Ge and ZnSe phases form very rapidly at temperatures below 350°C. The formation of Cu2ZnGeSe4 happens through the reaction of Cu9Se5, Cu3Ge and ZnSe phases. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds and yields layers with different levels of grain size and adhesion to the Mo back contact. Based on these results we have then designed an optimized layer stack. Solar cells were fabricated with this optimized absorber layer and these cells showed a maximum power conversion efficiency of 5.7 %. The resulting solar cells show good series and shunt resistance behavior, an ideality factor of 1.9 and a saturation current density of 2 10-9 A/cm2, but seem to be limited by bad minority carrier collection under forward bias conditions, leading to a relatively low fill factor. Admittance spectroscopy results seem to indicate that a surface defect is at least partially responsible for the low lifetime of the absorber. This project has received funding from the European Union?s Horizon 2020 research and innovation program under grant agreement No 640868.

Resume : Cu2ZnSn(S,Se)4 (CZTSSe) based solar cells are assigned to the second generation of photovoltaic devices. This thin film technology stands out by the use of cheap and earth abundant elements as well as its tunable direct band gap and high absorption coefficient. However, the record efficiency for this type of solar cell was obtained using CdS as the buffer material [1]. Since Cd is a toxic element and also causes parasitic absorption in the short wavelength region of the spectrum, the substitution of CdS by an environmentally friendly, high band gap material is one of the key challenges for CZTSSe based solar cells. In this work In2S3 and Zn(O,S) were investigated as alternative buffer materials using different chemical and physical deposition techniques, namely chemical bath deposition (CBD), atomic layer deposition (ALD) and sputtering to figure out the optimal deposition process. First investigations on In2S3 and Zn(O,S) buffered devices resulted in solar cells with similar open circuit voltage (Voc) and short circuit current densities (JSC) compared to CdS buffered devices. The difference in the VOC values of In2S3 and Zn(O,S) is most likely caused by a better band alignment of the atomic layer deposited In2S3 to the absorber materials, which is indicated by JV-T measurements. However, Zn(O,S) was deposited by sputtering, which could be the reason for the worse band alignment due to the introduction of sputtering damage on the surface of the absorber. A comparative investigation of Zn(O,S) from chemical deposition techniques is subject of ongoing work. Furthermore, Raman microscopy is used to determine the S/S+O ratio by carefully evaluating the positions of the ZnS- and ZnO-like peaks according to the method reported by Guc et al. [2]. Additionally, Time-of-flight secondary ion mass spectrometry (ToF SIMS) measurements were performed on the different investigated devices to obtain information about the diffusion behavior of the different buffer materials. Finally, the complete devices were characterized by JV-measurements and UV-Vis spectroscopy to obtain the characteristic solar cell parameters and information about losses inside the device.

Resume : Several positive impacts have been reported when Ge is incorporated in tin-kesterite [Cu2ZnSn(S,Se)4] absorber films, including better crystallization of the material, reduced Voc-deficit, and reduced Cu2ZnSn(S,Se)4/CdS interface recombination. The findings suggest that Ge-kesterites [Cu2ZnGe(S,Se)4] may be a promising absorber material for thin film photovoltaic devices, however the highest efficiency reported so far is 5.5%, for a Cu2ZnGeSe4 solar cell. In this study, Cu2ZnGeSe4 was grown by Selenization of metal stacks. Reference solar cells prepared with a CdS buffer layer exhibit efficiencies of 5%, mostly limited by interface recombination [according to electrical (IV, EQE) characterization]. Characterization of the absorber layer by complementary techniques (XPS, Raman, XRD, SEM, photoluminescence) reveals the presence of ZnSe at the absorber surface and a non-optimal Cu2ZnGeSe4/CdS energy level alignment. In order to address these issues, we employed two parallel strategies, i.e. removal of secondary surface phase and replacing the standard CdS with a Zn(O,S) buffer layer. Removing ZnSe by HCl etching improves Voc, but reduces the overall efficiency because of FF and JSC (due to collection length) reduction. After HCl etching oxidized Se phases are found at the surface. For their removal different wet chemical (passivation) routes have been studied. Resulting (etched/passivated) Cu2ZnGeSe4/CdS-based solar cells reach efficiencies > 7%, thanks to an improved Voc (+15 %) and current (+10%). The optimization of the preparation route for the alternative Zn(O,S) buffer finally results in efficiencies >7% for non-etched/passivated Cu2ZnGeSe4. This work describes significant improvement in the efficiency of Cu2ZnGeSe4-based thin film solar cells above the state of the art. Further optimization based on the utilization of both strategies is expected to lead to further improvements in device performance. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 640868

Resume : Two major issues of pure-sulfide Cu2ZnSnS4 (CZTS) solar cells are their unfavorable interface properties and the strong dependence of their performance on process conditions. In this contribution we will present details of the CZTS/CdS interface band diagram as a function of process parameters. The interface band edge positions are measured by x-ray photoemission spectroscopy (XPS) and the near-interface band gaps are extracted by spectroscopic ellipsometry using a newly developed method. The additional ellipsometry measurement is a crucial step for detecting the effect of interdiffusion on the materials’ band gaps in the near-interface region. The interface properties are found to be unfavorable under all investigated process conditions, either due to a cliff-like conduction band offset or to substantial band gap narrowing of CZTS near the interface. The least harmful process conditions for the interface are a low CdS deposition temperature, either with or without post-annealing. Even under such conditions, a decrease of at least 200 mV in the Shockley-Queisser limit for the open circuit voltage is expected due to a narrowed band gap at the interface or in the region of interdiffusion. Cd interdiffusion into CZTS appears therefore to be a detrimental effect for the interface. A buffer layer with a different cation than Cd could solve the problem. Finally, we will show how the proposed measurement method can be applied to other types of chalcogenide solar cells.

Resume : The p-type semiconductor material Cu2ZnSn(SxSe1-x)4 (CZTSSe) with its beneficial properties such as high absorption coefficient, tunable bandgap and abundant raw materials gained attention in recent years as an absorber material in photovoltaic applications. The monograin technology in combination with a cost efficient roll-to-roll application has the potential for a large scale industrial production. Material efficiencies of around 12 % were measured. However the stability in photovoltaics is crucial for a reliable and economic implementation in the photovoltaic market. Despite the importance of the degradation mechanism, very few papers were published so far. In this paper we investigated differently stable CZTSSe monograin layer (MGL) devices and the impact of thermal degradation on the back and frontside of the devices and its parameters. Diffusion processes, especially of copper, was identified as a major driver of the degradation. Temporary barriers at the CZTSSe-graphite interface of the back contact are observed and investigated by current-voltage (I/V), X-ray photoelectron spectroscopy (XPS) and Kelvin probe (KP) measurements. External quantum efficiency (EQE) and photoluminescence (PL) measurements show a change in wavelength dependent charge carrier extraction, which suggests a change at the p-n junction.

Resume : First Solar?s thin-film CdTe solar cells have been certified at 22% cell efficiency. Fundamental work by others has demonstrated that a shift away from copper-doped CdTe to a dopant from the 5th column of the periodic table (?Group-V?) should be possible and it may enable pathways to even higher efficiency. Over the last two years, First Solar has made steady progress on implementing Group-V doping in polycrystalline CdTe. The first milestone was the consistent demonstration of greater than 10e16 cm-3 hole concentration with a flat doping profile and narrow depletion width. Following this success, we produced >900 mV Voc on CdTe devices free of Se. It has been more challenging to produce similar Voc with Group-V doped CdSexTey devices. The failure mechanism is not well understood, however, a primary suspect is increased front junction recombination. Despite the challenges, we were able to fabricated devices >19% efficiency and >850mV Voc on Group-V doped CdSexTey devices. High-temperature long-term stability tests demonstrated stable doping and performance level. First Solar?s leading thin-film manufacturing capabilities are in process of ramping the new utility-power optimized Series 6 module. Series 6 modules will have initial power up to 440 Watts with an optimized design to lower installation and other balance of system cost. It will preserve First Solar existing CdTe module power density advantage: superior temperature coefficient, spectra and shading response.

Resume : CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield, as attested by the remarkable market performance. Typically the junction is made with n-CdS which, however, absorbs some of the light spectrum due to its band-gap at 2.4 eV. So, in order to exceed 20% efficiency it is crucial to increase the spectrum of light convertible by the absorber. One possible alternative buffer that, at the same time, allows a larger transparency and a proper band alignment with CdTe is Magnesium Zinc Oxide (MZO). We have already shown that Magnesium Zinc Oxide (MZO), deposited by RF magnetron sputtering, can be used as an alternative high resistance transparent (HRT) layer for CdS/CdTe thin film solar cells. The application of a MZO layer on Indium Tin Oxide (ITO) has delivered an increase of more than 2 % in absolute efficiency compared to only ITO contacted cells and however they result in higher efficiency also compared to ZnO/ITO-contacted devices . MZO results in higher transparency not only compared to CdS but also to ZnO, and, more important, it gives a better band alignment with CdS: MZO band gap can be tuned changing the substrate temperature during the deposition. This results in an increase in efficiency, in particular in terms of current density as well as fill factor. An analysis of MZO properties and their electrical behaviour with CdS will be presented, moreover CdTe devices without CdS buffer layer will be shown and characterized.

Resume : Copper dopant incorporation near the surface of CdTe absorber is a common approach of fabricating low resistive back contacts for CdTe solar cells. However, it is difficult to carefully control this process, which often results in electrical shunts, and therefore poor device performance [1]. P-type ZnTe as a back-contact layer provides a suitable alternative, as it is chemically stable and has a near-perfect valence band alignment with CdTe [1, 2]. We used metal organic chemical vapour deposition (MOCVD) to grow arsenic ⤓ doped polycrystalline ZnTe layers as a back-contact layer on CdCl2 passivated CdTe absorber for the first time. Different arsenic dopant flows (ranging between 0-10 sccm) were used in the growth process for ZnTe thin film on uncoated glass, yielding varying carrier concentrations. High carrier concentrations of up to ~9×1018 cm-3 with a corresponding low resistivity of ~1.42Ω. cm was achieved as a result of arsenic doping (10 sccm), as determined from Hall-effect measurements. CdTe cells with undoped ZnTe back-contact showed very poor J-V characteristics with conversion efficiencies of ~0.2%. The insertion of a ZnTe:As back-contact layer resulted in a conversion efficiency of >~10% (for 3-10 sccm flow), with particular improvement in JSC, correlating to the improved red response observed in device⤙s EQE. References [1] S. Uliĉná, P. J. M. Isherwood, P. M. Kaminski, J. M. Walls, J. Li, C. A. Wolden, Development of ZnTe as back contact material for thin film cadmium telluride solar cells, Vacuum, 139, 159-163 (2016), references therein [2] N. Amin, A. Yamada, M. Konagi, Effect of ZnTe and CdZnTe Alloys at the Back Contact of 1-µm-Thick CdTe thin Film Solar Cells, Japan Journal of Physics, 41 (2002).

Resume : As one of the most promising absorber materials for high-efficiency and low-cost solar cells, CdTe holds the highest market share among thin film photovoltaics technologies. Recent improvements of CdTe solar cells have involved a CdTe(1-x)Se(x) layer as part of the front side of the absorber. The used alloy has a lower band gap than CdTe, which increases the photocurrent. However, the different band gap of CdTe(1-x)Se(x) can result in band misalignments when it replaces CdTe in the traditional device structure. In this work we investigate the weaknesses in solar cells with CdTe(1-x)Se(x) absorbers. With temperature-dependent current-voltage measurements we attempt to identify barriers in homogeneous CdTe(1-x)Se(x) absorbers caused by band offsets between CdTe(1-x)Se(x) and the MgZnO window layer as well as the back contact. Additionally, we investigate the impact of a CdTe(1-x)Se(x) layer on the size of the space charge region, as with copper doping CdTe(1-x)Se(x) has a lower achievable charge carrier concentration than CdTe. We compare these results to high-performance solar cells with a CdTe(1-x)Se(x) absorber layer at the front of a CdTe layer.

Resume : Based upon the product catalogues of CIS-based thin-film PV modules commercialized by the companies, such as Solar Frontier, Hanergy-Solibro and AVANCIS, FF is currently lower than that of other PV technologies. If CIS-based thin-film PV modules achieved the same level of FF (FF≧0.756) as other PV technologies, the nominal output power of 200Wp and higher would be achieved. Although at this moment, we do not have any definitive solution for the question why FF of commercialized CIS-based thin-film PV modules is lower than that of other PV technologies, detailed analyses of device structure and various resistances in the device will help to enhance the FF over 0.8. FF of 0.8 had already been achieved in the small-area (less than 0.5 cm2 as a cell size) CIGS solar cells in which CIGS absorber was prepared by a coevaporation (i.e. three stage process) and a CdS buffer layer by a chemical bath deposition was applied. In the current champion small-area (1.04 cm2) CIS-based thin-film solar cell prepared by Solar Frontier and measured by AIST, FF was 0.797, in which CIS-based absorber was prepared by a sulfurization after selenization and Cs doping as a post-deposition treatment and a CdS buffer by a chemical bath deposition were applied. This was the highest ever in Solar Frontier as well as a selenization process group for CIS-based absorber formation. This also indicates that CIS has high potential to achieve the FF of over 0.8 by optimizing the interface or improving the junction quality. So far, it was not a common goal to focus on achieving the FF of over 0.8. In the present contribution, it will be discussed how to optimize the interface mainly through consideration for the sensitivity to light soaking effect of commercialized products by Solar Frontier. It would be a guideline to understand the production process more deeply and a focusing point what we should do. Furthermore, it would also contribute to enhance the competitiveness of CIS-based thin-film PV technology.

Resume : We report accelerated heat degradation studies on fully encapsulated CIGS modules as a function of film growth parameters, in particular back contact selenization (pre-Se), as well as the impact of bias (light/voltage) during heat degradation. We show that pre-Se conditions have a profound effect on the heat stability of the device, whereby reduced pre-Se, while increasing initial efficiency, results in strong heat degradation, mostly driven by reduced minority carrier lifetime (as evident from EQE) in the light-soaked state. This is also accompanied by a stronger increase in the shallow acceptor concentration (as measured by CV) in the degraded state suggesting that the Cu-Se divacancy mechanism is likely responsible. In this case, appearance of a high concentration of deep states accompanies increased doping, with the former reducing bulk lifetime and the latter further affecting electron collection due to narrow depletion width. This result suggests that bulk structural properties of the absorber film are strongly impacted by the back contact selenization conditions, making the film more susceptible to heat degradation. In the second part of this paper we show that electrical or light bias during heat exposure reduces degradation, in particular almost fully eliminating the above Jsc loss. This is a surprising result as usually the positive effects of bias are attributed to interfacial changes, while our results demonstrate that bulk properties can be improved as well.

Resume : Building-integrated photovoltaics (BIPV) are considered to be the most suitable PV installation feature particularly for global trends of urbanization (e.g. distributed and on-site electricity generation system). Among the various physical requirements for being BIPV, colorful and aesthetic characteristics have become very important because a coloration of building is considered as a work of art in most modern city. Chalcopyrite thin-film PVs (e.g. CuInxGa1-xSe2, CIGS) will be an attractive candidate because they are durable enough and cost effective with high efficiency. Therefore, it would be very promising for BIPV applications if the authentic aspect is successfully satisfied with the least energy loss. In this study, to fabricate light-transmitting, colorful, and cost-effective power-generating windows, we introduced solution-processed CIGS photovoltaics combined with 1D photonic crystal (PC) dichroic filter/mirror films. By the application of various colored 1D PC dichroic films to the outside or inside of the CIGS photovoltaic modules, colorful and aesthetic value-added CIGS thin-film photovoltaic modules were realized. The details of the fabrication method and characterization of the solution processed colorful CIGS photovoltaics will be discussed in the presentation.

Resume : CuIn1-xGaxSe2 (CIGS in general and CISe with x = 0) photovoltaic (PV) cell has enough potential, such as long-term stability, non-toxicity, high efficiency and so on, to be commercially used for eco-power generation. Nevertheless, it is still challenging to increase solar cell market share of CIGS as much as expected probably due to slump in the unit cost of silicon solar cell. In other words, it might be hard for CIGS to draw attention in cost aspect so that it is needed to consider other applications of CIGS that silicon solar cell can’t realize. As PV technologies have been improved over the past decades, the public starts to require not only efficient but also the beautiful solar panel. However, it is hard to satisfy consumer’s needs with heavy and indistinctive Si-based solar panel. In this study, in an attempt to realize aesthetics of solar cells, we applied the photoelectric effect in a one-pot electrodeposition process using Se aqueous solution containing Cu and In chlorides. As a result, CISe area exposed to light was approximately 1.4 times thicker in thickness than the other part that is not illuminated. In addition, [Cu] to [In] ratio, which is a pivotal factor for high performance, was decreased which is attributed to facilitated In ion reduction reaction. After the annealing process, the morphology of each part is different upon Cu to In ratio, which realized the surface pattern of CISe. Thus far, photo-electrochemically deposited CISe with the pattern attained 8.62 % efficiency.

Resume : Our study focuses on the structure of the photovoltaic modules based on Cu(In,Ga)Se2 thin films. Passing from a solar cell to a photovoltaic module using the current monolithic interconnection decreases the conversion efficiency about 5% abs. In fact, the architecture of the modules induces optical and resistive losses due to the thick ZnO layer. Additionally the P1 P2 P3 scribings reduce the active area about 10%. Thus, we put forward an alternative architecture of modules consisted of a serial interconnection between cells from the front contact to the back contact directly. For this reason, the Molybdenum (Mo) back contact scribing is carried out using a photolithography process and optimized to minimize the dead zone. Then, the thickness of the thin film is also adjusted. This new concept allows to decrease 1) the number of scribes, 2) the ZnO thickness, 3) the air exposure of Mo and 4) the inactive area. Using the same deposition process, we synthesize solar cells and alternative modules consisting of a series of 2 cells. Our preliminary results are very promising and show a high fill factor of 71% for modules and 75% for cells. This suggests a small loss caused by series resistance. 17% efficiency has been attained for cells while for modules it reached 12%. This decrease may be due to the difference of Voc: 700mV for cells vs 1.2V for modules. Future work will concentrate on better understanding how these alternative modules can improve the photovoltaics efficiency.

Resume : Light management and rear surface passivation are essential for high efficiency ultra-thin Copper indium gallium (di)selenide (CIGS) solar cells. While light scattering with efficient rear reflectance and surface passivation can be achieved by the passivated emitter and rear cell- (PERC-) like technique [1] in combination with a bi-layer stack of dielectrics [2], antireflection coatings (ARC) boost light in-coupling. In this work, we first propose a multi-ARC based on MgF2 applied to a 750-nm thick CIGS cell. Deploying HFSS, a 3D finite element method Maxwell’s equation solver [2, 3], the refractive index of three porous MgF2 layers has been graded for minimizing reflection between 300 nm and 800 nm, improving the photo-generated current density (Jph) by 6.34% (from 28.37 to 30.17 mA/cm2). Then, we report on our 3D-to-2D hybrid opto-electrical modelling framework [4] for simulating the performance of an ultra-thin CIGS solar cell endowed with the abovementioned MgF2-based multi-ARC at the front and a previously optimized PERC-like structure at the back [2]. The resulting modelled 3D optical situation has been fed to TCAD Sentaurus [5] for solving the inherently-2D semiconductor equations. The current density-voltage characteristics have been studied as function of rear dielectric to total area cover ratio. [1] B. Vermang et. al., IEEE JPV (2014) [2] N. Rezaei et.al., OPEX (2018) [3] ANSYS white paper, “ANSYS HFSS” [4] M. Zeman et al., SOLMAT (2013) [5] N. Guerra et al., SOL ENERGY (2017)

Resume : Halide-substituted lithium-argyrodites, Li6PS5X (X=Cl, Br, I) are a promising family of lithium-ion solid electrolytes, with potential applications in all-solid-state lithium-ion batteries. Changing X from I to Cl produces a strong increase in lithium-ion conductivity, which has been attributed to increased crystallographic disorder for X and S ions across 4a and 4c sites. A previous molecular dynamics study [1] has predicted that efficient long-ranged Li-ion transport is only achieved for such X/S disordered systems, with this attributed to changes in the rates of lithium-ion jumps between lattice sites. A microscopic explanation for this change in lithium-ion dynamics, however, is lacking. To study this behaviour, we have performed a series of ab initio molecular dynamics simulations of ordered and disordered Li6PS5X. In contrast to previous computational studies, we have analysed the lithium-ion dynamics in terms of transitions between local potential minima (inherent structures) in lithium-ion configuration space, which allows us to resolve non-trivial lithium motion [3]. We find that in fully ordered Li6PS5X, the lithium ions form six-coordinate coordination octahedra around the (4a/4c) S anions. Interestingly, this is the case for X ordered over the 4a sites and over the 4c sites, indicating a strong preference for separated (4a/4c) S ions to be octahedrally coordinated by Li. Lithium motion consists of highly correlated processes within individual octahedra; octahedral rotations, and reorganisations via trigonal prismatic configurations; neither of which give long ranged lithium diffusion. For anion-disordered Li6PS5X, this preferred octahedral coordination is disrupted. We observe a number of LixS coordination environments with x ? 6, and 6-fold coordination environments increasingly deviate from ideal octahedral symmetry. This increased Li- coordination disorder facilitates long-ranged Li-ion diffusion between LixS polyhedra. We note that for pairs of S ions in adjacent 4a / 4c sites, it is not possible for both S ions to simultaneously achieve ideal octahedral coordination. We propose, therefore, that the capacity for long-ranged lithium transport in anion-disordered Li6PS5X arises from geometric frustration of preferred octahedral lithium configurations, which produces a highly disordered network of lithium-ion polyhedra, with fast lithium-ion diffusion. References: [1] Rao et al. Sol. Stat. Ionics (2013), 230, 72. [2] de Klerk et al. Chem. Mater. (2016), 28, 7955. [3] Stillinger and Weber, Science (1984), 225, 983.

Resume : Cu(In,Ga)Se2-based (CIGS) solar cells with an ultra-thin absorber layer (<500 nm) allow manufacturing cost reduction but exhibit low efficiencies due to poor light absorption and back-contact recombination. To overcome this efficiency drop, we develop new back-contact layers based on a reflective and passivating nanostructured mirror for CIGS solar cells. First, numerical electromagnetic simulations of complete solar cells with different CIGS thicknesses and various mirror materials have been performed. We demonstrate the possibility to reach a short-circuit current of Jsc = 36.3 mA/cm² for a 150-nm-thick CIGS absorber with a Ag nanostructured mirror. Experimentally, we have first replaced the conventional Mo back-contact with flat mirrors encapsulated in transparent conducting layers. This reflective back mirror leads to an improvement of the Jsc of 5.2 mA/cm² (from 20.8 to 26.0 mA/cm²) for 480-nm-thick CIGS solar cells, and is fully compatible with nanopatterning for further light-trapping improvement. Regarding rear passivation, we have developed a cost-effective process to pattern nanoholes in an Al2O3 layer on 5x5 cm². Using nanoimprint lithography and wet etching, we demonstrate a Voc increase from 606 to 627 mV compared to bare molybdenum. We will show that a combination of these building blocks could lead to 20% efficiencies for CIGS solar cells with absorbers thinner than 500 nm.

Resume : CuIn(1-x)GaxSe2 (CIGS) solar cells tend to show non negligible losses in the spectral response close to the bandgap which can have different causes. Possible charge carrier collection losses can be distinguished from incomplete optical absorption by comparison of optical simulations of solar cell devices with experimental data, leading to insights into devices limitations such as recombination at the back contact or an imperfect collection function. In this contribution we present refractive indices for all layers present in a CIGS solar cell with high efficiency, and especially absorption coefficients for CIGS over the full composition range for high-efficiency devices. The reliability of the CIGS absorption data is assessed by the good match of light absorption simulations with optical measurements of the absorbers. The charge carrier collection losses are estimated from the difference between devices optical simulations and the experimental EQE. Little to no significant wavelength-dependent collection losses could be evidenced in high-efficiency devices. By contrast, a significant current loss is observed in the case of Ga-free CuInSe2 absorbers, especially for carriers generated deep in the absorber layer. For such low bandgap absorbers the introduction of a Ga-containing graded layer at the back of the absorber efficiently reduces the collection losses, and leads to a large improvement in the device open-circuit voltage.

Resume : XS (X=Cd, Sn) thin films with molar concentration (MX = 0.05M, MS=0.5M) are deposited by spray pyrolysis technique on heated glass substrate at 350°C. The physical properties of the film are characterized by several techniques in order to study their structural, optical and electrical properties. It is observed from X-ray diffraction (XRD) analysis that the CdS film is mainly composed with hexagonal Wurtzite structure with a preferred grain orientation along (101) plane and SnS film is mainly composed with orthorhombic crystal structure with a preferred grain orientation along (111) plane. From optical measurements, the average optical transmission is 80 % (CdS) and 70% (SnS). The band gap value is found 2.4 eV (CdS) and 1.8 eV (SnS). The Hall Effect electrical measurements show that the sample is n-type for CdS and p-type for SnS. The values of the electrical resistivity 3x10-2(?.cm) for CdS and 2.75x10-2 (?.cm) for SnS are obtained.

Resume : Usually, in chalcogenide thin film solar cells such as CdTe, Cu(In,Ga)Se2, and Cu2ZnSn(S,Se)4, the absorber layers are made of polycrystalline materials. Planar defects like stacking faults can be formed easily in the absorber layers in such cases, and the device efficiency can be significantly influenced accordingly. Some recent experimental studies show that another type of planar defects called antisite domain boundaries can be formed in Cu2ZnSnS4. However, our knowledge of their effect on the band structure was far from complete. In this regard, we quantitatively investigated the stability and the electrical properties of stacking faults and antisite domain boundaries in Cu2ZnSnS4 and Cu2ZnSnSe4 using the first-principle calculation method. We found that the stacking faults act as electron barriers, whereas the antisite domain boundaries are electron captures in the materials. Both types of defects do not significantly affect the valence band.

Resume : We have previously observed that the band gap of Cu2ZnSnS4 (CZTS) is increased when annealing Cu-Zn-Sn-S precursors in an atmosphere with a high sulfur partial pressure for a short time. Since the sulfur partial pressure drops during the annealing a lower band gap was observed in CZTS annealed for longer durations. In this work we investigate the effect of the modified band alignment at the CZTS/Zn1?xSnxOy (ZTO) interface as a result of different CZTS annealing conditions and ZTO growth conditions. The conduction band of ZTO can be increased by reduction of the growth temperature. We investigate three different ZTO growth temperatures on CZTS with two different apparent band gaps to obtain insights into the band alignment. The results are compared to CdS reference devices where a cliff-like band alignment is expected. Contrary to the expectation it was observed from JV characteristics that the CZTS/ZTO devices with the widest absorber band gap were more blocked than devices with narrower band gaps. This indicates that both conduction and valence bands were downshifted relative to ZTO in CZTS annealed in high sulfur partial pressure. The result implies that the band gap widening observed in high sulfur pressure annealed CZTS is caused by a larger downshift of the valence band edge relative to the conduction band.

Resume : In this work, we present experimental studies of three non toxic and earth abundant thin film solar cell materials: Cu2ZnSnS4, SnSb4S7 and Zn(S,O). Cu2ZnSnS4 and SnSb4S7 are among the most promising candidates to substitute CIGS as absorbers in solar cells, whereas Zn(S,O) is the most interesting candidate to replace CdS as a buffer layer. Using different characterization techniques such as UV-Vis-NIR spectroscopy, X-ray diffraction, atomic force microscopy, we demonstrate that all these materials exhibit structural, morphological and optical properties that are suitable for solar cell applications. The optical constants of the films were calculated from the analysis of the transmittance and reflectance data in the spectral range 300-1800 nm. The calculated band gap energies were 1.55, 1.45 and 3.08 eV for Cu2ZnSnS4, SnSb4S7 and Zn(S,O) films, respectively. X-ray diffraction patterns show that Cu2ZnSnS4, SnSb4S7 and Zn(S,O) films were polycrystalline in nature with a preferential growth along (112), (-21-3) and (002) planes, respectively.

Resume : Sodium doped Cu2ZnSnS4 (Cu2ZnSnS4: Na) thin films were successfully deposited by vacuum thermal evaporation method. Na concentration varied from 0 to 7 at.%. The effect of Na doping on the optical and electrical properties of the Cu2ZnSnS4 thin films was investigated. The optical, structural and electrical properties were studied using UV-Vis-NIR spectroscopy, X-ray diffraction (XRD) and AC impedance spectroscopy, respectively. The optical properties of Cu2ZnSnS4: Na thin films were calculated the transmittance and reflectance data. With the increase of the Na concentration, the band gap of the films increased gradually from 1.52 to 1.90 eV for sodium doping of 0-7%. The XRD analysis confirms the existence of Cu2ZnSnS4 phase with the preferential plane (112). Electrical properties have been investigated by AC impedance spectroscopy over a wide range of temperature up to 285 °C starting from room temperature in the frequency range 5 Hz?13 MHz. The complex impedance plots display one semicircle with equivalent circuit functions as typical parallel RC.

Resume : A promising technology in photovoltaics, aimed at improving the efficiency of solar cells, is the use of ?tandem? cells, which combine layers of different absorbers to achieve a wider spectral range of the sunlight conversion. Chalcopyrite-type Cu(In,Ga)(S,Se)2 mixed compounds can make a good complement to Si-based bottom cells insofar as their band gaps are sufficiently wide and their structural parameters adapted to that of silicon. The most obvious way of tuning to reach these objectives is to change the chalcopyrite?s chemical composition. However, the defects and dopants, either intentionally added or technically unavoidable, bring about unexpected disturbances. They may, e.g., displace the equilibria of crystalline phases, induce variations of the band gap, create "parasite" energy levels in the latter, and thus strongly influence the performance of materials. In this work, the effects of the chemical composition on the thermodynamic and optoelectronic properties of chalcopyrites are investigated at the first-principles level (within the density functional theory). Hybrid functional methods combined with supercell approach are used to study the structural and electronic properties of CuGaxIn1-x(SySe1-y)2 and CuxX1-xAB2 (with X = Na, K, Rb and Cs, A = Ga and In, and, B = S and Se). For a number of commensurable trial compositions, the equilibrium structural parameters, substitution energies, bang gap variations, as well as thermodynamic, dynamic and dielectric properties are determined and discussed, and conclusions concerning the prospective use of such mixed chalcopyrite cells in tandem with silicon are drawn.

Resume : Today, silicon is the leading material used in photovoltaic solar cells based on its various advantages such as cost efficiency, stability, high reliability, well balanced electrical and optical properties and well established processing techniques. As an alternative to single crystal based solar cells, a search for highly efficient low cost inorganic solar cells has occurred over the last two decades. Research has been focused on using various materials in solar cells, such as; amorphous Si, organic, II-VI and I-III-VI2 materials. Most of today?s thin-film solar cells are based on CuInSe2 (CIS), CuInxGa1-xSe2 (CIGS) and CdTe semiconductor materials, which are consisted of rare, expensive and toxic elements ( In, Te, Ga, and toxic Cd). Although these solar cells have reached reached the commercialization stage with high power conversion efficiencies, the requirement of using rare and expensive elements severely limit the mass production and deployment of them. Therefore, in recent years, a considerable research effort has been focused on development of new photovoltaic absorber materials that can embody earth-abundant, low-cost, and environmentally benign constituent elements for the the realization of higly efficient thin film solar cells at lower cost. Cu2ZnSnS4 (CZTS) compound has recently emerged as a potential photo-absorber material for thin-film solar cells due to their superior physical and chemical properties, which addresses all the issues related to the drawbacks of the aforementioned materials. In this study, high quality CZTS thin films for the realization of highly efficient low cost core-shell like Si-nanowire /CZTS structured solar cell were successfully obtained by using sol-gel technique. For the fabrication of stoichiometric high quality thin films, a production route based on a combination of spin-coating and thin film deposition by sol-gel processes was tested, which is believed to address many issues associated with difficulties reported so far in literature in obtaining highly stoichiometric CZTS thin films. The combination of an absorber thin film layer (CZTS) with a high absorption coefficient and the technology of silicon enables the realization of cost-effective high efficient solar cells. CZTS thin films were deposited on both n-Si-wafer and n-Si nanowires (NWs) fabricated through the electroless etching technique. Post-annealing process at 400-550 oC under inert gas flow was applied to the deposited films to obtain stoichiometric mono-phase CZTS thin films. The structural (XRD), optical (Reflection), morphological (AFM and SEM) and electrical properties (resistivity) of the CZTS thin films deposited on planer-Si was compared with that of deposited onto Si-NW arrays. Results have shown that there is significant modification in these properties following the incorporation of Si nanowires into CZTS thin film matrix for the construction of a third generation solar cell. In other words, it couples the efficiency of the 1st generation solar cells with the benefits of the 3rd generation solar cells. Silver dot contacts were evaporated by thermal evaporation to form the ohmic top-contacts of the solar cells structure by using a copper-shadow mask. As a back contact of the solar cells, the back side of the n-Si wafer was coated with a 150-nm thick silver thin film layer via thermal evaporation technique. The photovoltaic behaviors of the constructed prototype solar cells were examined under standard test conditions (AM 1.5G). From the recorded Current ? voltage characteristics of the solar cells under dark and light illumination conditions, the solar parameters including open-circuit voltage (Voc), short-circuit current (Isc), series-/-shunt resistances and power conversion efficiency were calculated.

Resume : Owing to its suitable electrical and optical properties, Cu2ZnSn(S,Se)4 (CZTSSe) is one of the most potent candidates to be substituted for Cu(In,Ga)Se2 (CIGS) material. Many research groups are trying to keep up with the performance of CIGS solar cells, however, the best power conversion efficiency (PCE) of CZTSSe solar cells (12.6%) is still below that of CIGS (22.6%). In this research, high-performing CZTSSe thin films were successfully fabricated and their optoelectrical properties on surfaces were examined using Kelvin probe force microscopy (KPFM) under diverse light-illumination conditions: red (640 nm), green (532 nm), and blue (405 nm) laser lights. Since the latest researches revealed that upward energy band barrier in grain boundaries (GBs) in high-efficient CZTSSe thin-film surfaces, we have suggested different conduction behaviors. Furthermore, the surface potential variation of the thin-film inside indicates downward potential bending, while that of the surface shows upward potential bending at the GBs. Under illumination, as the surface can be charged by photons, the energy band bending can be affected to be reduce barrier at the GBs. Though the GB barrier decreases, the conduction band of the material can be lower to help the carrier separation between the interfaces. Therefore, we can propone the presence of interfacial states altering the band alignment to assist the carrier transport in the solar cell devices.

Resume : CuSbS2 (CAS) is an emerging absorber material for thin film solar cell applications with the chalcostibite structure, which shows high optical absorption co-efficient and favorable energy band gap. It also has low cost, low toxic and earth abundant constituent. Theoretical investigations shows that CAS has similar spectroscopic limited maximum efficiency (SLME) with CuInSe2 (CISe). Despite these promising characteristics, photovoltaic performance depends on some other factors such as the proper band alignment at the interfaces in particular with the buffer and the back contact, defect concentrations and carrier recombination. In this study, to fabricate non-toxic and low cost CuSbS2 solar cells, we used non-vacuum methods using hybrid inks through the sulfurization process. Sulfur ratio was varied during the sulfurization process to investigate the effect of sulfur flux on the structural and electrical properties of the CAS thin films while Cu/Sb were kept constant. In our study, we found that CAS solar cell performances was strongly correlated to the sulfur flux condition during the sulfurization. Therefore, optimum sulfur flux is needed to get better solar cell performance. The different sulfur ratios samples showed notable differences in surface morphology and interficial properties. The structural and electrical properties of CAS thin film solar cells with the change of sulfur ratios are analyzed by means of EDS, XRD, SEM analysis as well as external quantum efficiency, temperature dependance IV and capacitance-voltage measurements.

Resume : Zn(O,S) film is a promising low-cost and environment-friendly Cd-freebuffer layer for chalcopyrite and kesterite thin film solar cells. However,CZTSSe/CBD-Zn(O,S) solar cells with general thickness buffer layer (20?50 nm) have suffered from the poor interface performance,which may arise from the large positive conduction band offset(CBO) between CZTSSe and Zn(O,S), the low conductivity ofZn(O,S) layer, and the ZnO and Zn(OH)2 secondary phases inthe Zn(O,S) layer. In this study, the band fluctuation caused by ZnO secondary phase inZn(O,S) layer is identified as the main reason deteriorating the deviceperformance. By a concentrated ammonium etching and subsequent softannealing treatment, the detrimental ZnO and Zn(OH)2 secondary phasesare eliminated from the Zn(O,S) layer and the hetero-junction performanceis improved significantly. The Zn(O,S)/CZTSedevices showed quite small series resistance of 0.23Vcm2, andrelatively high fill factor of about 60%.Consequently, the power conversion efficiency ofthe Zn(O,S)/CZTSe solar cells was improved from 1.17% to a favorablevalue of 7.2%. Temperature dependent J?V properties reveal a defect levelassisted charge carrier transport mechanism across the Zn(O,S)/CZTSeinterface. These encouraging results imply that Zn(O,S) buffer layer is apromising substitution for toxic CdS in future manufacturing of highperformance thin film solar cells.

Resume : The development of Cu2ZnSnSe4 (CZTSe) solar cells has been driven by the know-how accumulated for the related compound Cu(In,Ga)Se2 (CIGS). In terms of device structure, and similarly to CIGS, the CdS buffer is the preferred n-type semiconductor counterpart, in spite of the environmental concerns about this material (Cd toxicity), and the non-well known band alignment between CZTSe and CdS. In this comparative work, we explore different strategies to substitute/reduce CdS in the CZTSe technology including: Cd-free buffer layers (Zn(O,S), In2S3), bi-layer structures (ZnS/CdS, In2S3/CdS) and ternary compounds (ZnxCd1-xS). All the buffer layers were grown by chemical bath deposition onto Glass/Mo/CZTSe, with a careful optimization of the deposition parameters. Solar cell devices made with the different buffer layers were then fully characterized using illuminated J-V curves, EQE, CV-T and JV-T methods, and the different complex buffers using ICP-OES, SEM, XPS and T-R measurements. Our results show that Cd-free buffer layers perform systematically worse than CdS reference, most probably due to a large spike band-alignment. Nevertheless, when these layers are combined with small quantities of CdS in a bilayer structure or ternary compound, devices with efficiencies comparable to the CdS reference are obtained. The relevance and characteristics of this more complex buffer structures will be discussed for the improvement of CZTSe based solar cell properties with reduced Cd content.

Resume : The improvement of kesterite based solar cells efficiency over 13% requires to finding technological solutions for relevant problems, among them the optimization of the buffer/absorber interface. This optimization includes the reduction of the surface recombination, the passivation of the surface, and the correct band alignment between the absorber and the buffer layer. In this context, in Si based solar cells technologies, ion implantation has proved to be a powerful tool to modify the surface properties and improve the devices performance. However, in the Cu2ZnSnSe4 (CZTSe) case these methodologies have not been well implemented and studied yet. This work presents the physico-chemical characterization of CZTSe layers implanted with different ions, using two distinct implantation doses and energies to obtain box-like depth profiles, and including: Cl to induce a CZTSe n-type buried layer, Er to improve the transport properties, and Si/S to modify the CZTSe surface band-gap and in consequence the CZTSe/CdS band alignment. Afterwards, the implanted layers were treated under different soft annealing conditions, and characterized using multi-wavelength Raman spectroscopy, SEM, EDX, XRD, and XPS. We will show how the implantation process induces different grades of structural damage, regardless of the implanted ion, and can be partially recovered after a soft annealing. Finally, a correlation of the surface properties with the devices characteristics will be performed, in terms of the effects of the different implanted ions.

Resume : Scarcity of key elements (In, Ga, Te) in most relevant chalcogenide thin film photovoltaic technologies (CdTe and Cu(In,Ga)Se2), has driven the investigation in the photovoltaic field towards other materials, formed exclusively by more abundant elements. One of these new candidates is the chalcostibite compound (CuSbS2) as an alternative to CuIn(S,Se)2, due to the relative abundance (thus, lower cost) and nearly equal ionic radius of Sb in comparison to In. This makes very similar both compounds opening interesting options for the possible partial replacement of In by Sb in the thin film photovoltaic field. In this work, CuSbS2 thin films where prepared by a sequential process based in the reactive thermal annealing under S atmosphere of Cu-Sb evaporated precursors, exploring different annealing conditions. Both, Cu and Sb layers where deposited onto Mo coated SLG substrates by thermal evaporation. A systematic study was done by varying the Cu layer thickness in relation to the Sb one, then selecting the most promising Cu/Sb ratios and carrying out different sulfurization processes. The photovoltaic cells where completed by chemical bath deposition of CdS buffer and sputtered i-ZnO/ITO as top contact. The final devices SLG/Mo/CuSbS2/CdS/i-ZnO/ITO were characterized by X-ray fluorescence, X-ray diffraction, AM1.5 solar light simulator, external quantum efficiency, Raman spectroscopy and scanning electron microscopy, reporting a record device with 1.9% efficiency.

Resume : The transparent electrodes of optoelectronic devices such as solar cells, organic light-emitting diode (OLEDs), require low resistive and high transmittance. We investigated InTiON/Ag-Ti/InTiON multilayer films with a low resistance and a high transmittance by co-sputtering system at room temperature. The electrical and optical properties of InTiON/Ag-Ti/InTiON multilayer films depended on the insertion of a nano-size Ag-Ti layer. The optimized InTiON/Ag-Ti/InTiON multilayer films that Ag-Ti thicknesses of 14 nm and InTiON thickness of 40 nm showed a sheet resistance of 4.13 Ohm/square, an average transmittance of 87.68 % at visible range (400 nm~800 nm) and work function of 4.69 eV. In addition, the InTiON/Ag-Ti/InTiON multilayer films had a very smooth surface morphology without surface defects. Furthermore, we examined the mechanical properties. The InTiON/Ag-Ti/InTiON multilayer films had a constant resistance change(ᅀR/R0) within an outer bending radius of 6 mm. All properties are better than Sn-doped In2O3(ITO) film that have mainly been used as transparent electrodes. Consequently, InTiON/Ag-Ti/InTiON multilayer films are promising alternative to ITO films for transparent electrodes.

Resume : Various chemical deposition techniques have been widely employed for the fabrication of chalcogenide thin films including spin coating, doctor blading and chemical spray pyrolysis. Chemical spray pyrolysis deposition technique offers a lot of advantages for the processing of thin films due to its simplicity, cost effectiveness, extremely large choice of precursors and its application for large area deposition. The processing conditions such as spray parameters (e.g. type of salts, solvent, deposition temperature etc.) and annealing parameters (e.g. annealing temperature, annealing atmosphere, amount of chalcogen etc.) play a critical role and have a significant impact on the properties of the chalcogenide thin films and hence the device performance. In this study, chemical spray pyrolysis was employed to prepare environmental benign and earth abundant ternary Cu2SnS3 thin films. The Cu2SnS3 layers were processed under ambient air by using only water as a solvent. This deposition step was followed by an annealing step in chalcogen atmosphere. The influence of different processing conditions on the structural and morphological properties of the prepared thin films were investigated using SEM, XRD and Raman spectroscopy. Cu2SnS3 based thin film solar cells were fabricated and the resulting efficiencies are discussed in the context of the varied parameters. Highest reached efficiencies were in the neighborhood of 2%.

Resume : Quaternary chalcogenide Cu2ZnSnS4 (CZTS) compound, a potential material for application as absorber layer in thin film solar cells, is successfully synthesized by direct melting of the constituent elements taken in stoichiometry compositions. Alkali element Na was incorporated into CZTS thin films synthesized by thermal evaporation method, in order to further improve the structural and optical properties. X-Ray diffraction, Raman spectroscopy and optical spectrophotometry were used to characterise the phase purity and optical properties. It showed that the diffusion of Na ions is uniform in the films after annealing. The XRD analysis showed that the films possess polycrystalline structure and the preferred orientation being in (112) direction. Na ions incorporation in CFTS thin films could improve the cristallinity, the graine size and the absorption coefficient. For CZTS: Na 5%, optical results revealed an direct optical band gap of 1.6 eV with p-type conductivity. Keywords: CZTS; Semiconductor; Thin films; Thermal evaporation; Na-doped; Optical properties.

Resume : In recent years, Cu-Sn-S system has been investigated in several research areas and they have several phases such as Cu2SnS3, Cu2Sn3S7, Cu4SnS4, etc. Among them, Cu2SnS3 (CTS) has been spotlighted as a potential absorber material for thin film solar cell applications because of its high absorption coefficient (> 104 cm-1) and lack of Fermi level pinning. It also has low cost, low toxic and earth abundant constituent. Until now, 4.63% conversion efficiency was reported by 2 step process with vacuum processed precursor. In this study, we used cost-effective non-vacuum process using hybrid inks through the sulfurization to obtain CTS thin films. The structural and electrical properties of CTS thin films were analyzed by EDS, XRD, SEM, TEM and the fabricated CTS solar cell showed 2.943% conversion efficiency. Finally, other photovoltaic performances (such as EQE, IVT and CV) will be presented.

Resume : II?VI semiconducting materials have emerged with high potential in applications for fabricating optical devices which include short wavelength emitting laser diodes and light emitting diodes operating in the blue region due to their wide direct band gap properties. It is a subject of great interest in regard to chalcogenide-based semiconducting materials which are used in making devices like photodetectors and photovoltaic devices. ZnS layers were electrodeposited from an aqueous electrolyte containing 0.3 M ZnCl2 and 0.03 M (NH4)2S2O3 in 800 mL of de-ionized water. Electropurification of the ZnCl2 was carried out for 48 h prior to the addition of (NH4)2S2O3 in order to remove any possible impurity ions present in the solution. Finally, the pH of the electrolyte containing both precursors was adjusted to 3.00 ± 0.02. The temperature of the electrolyte was 30°C. Uniform and transparent ZnS layers were cathodically deposited on cleaned glass/?TO substrates using a simple two-electrode deposition system at a cathodic potential of 1550 mV established using a cyclic voltammogram. The deposited layers using an average deposition current density of ~65 ?A?cm?2 and deposition time of 60 min have thickness of ~150 nm. These were then annealed in air at 350 °C for 10 min. Prior to the deposition of Cu2ZnSnS4, the glass/?TO/ZnS substrates were cleaned with methanol and deionised water. The deposition of Cu2ZnSnS4 layers was also done using a two-electrode system at a cathodic deposition potential of 1450 mV also established using a cyclic voltammogram. The Cu2ZnSnS4 deposited on glass/?TO had a thickness ~300 nm while that deposited on glass/?TO/ZnS had a thickness ~150 nm. This therefore brings the total thickness of the ZnS/Cu2ZnSnS4 bi-layer to ~250 nm comparable to the ~300 nm of Cu2ZnSnS4 grown on glass/?TO. The CdTe deposition electrolyte contained 1 M CdSO4 (99.0%) and 1 mM TeO2 (99.999%) in 800 mL of de-ionized water. To do this, a cyclic voltammogram was recorded using the two-electrode system, to determine the reduction potential of Cd2 . The TeO2 was first dissolved in H2SO4 and then added into the bath after the electro-purification of CdSO4, and the pH of the electrolyte adjusted to 2.00 ± 0.02. Optical absorption and transmittance measurements on the various deposited thin film layers were carried. The result of using ZnS as the buffer/window layer is directly reflected in the improved high short-circuit current density (Jsc) as well as improved open-circuit voltage (Voc), fill factor (FF) and ultimately, the conversion efficiency (?) of the 3-layer device, compared to the device as shown in Figure 3a,b. The measured Voc values of 640 mV and 630 mV are not as large as expected and are indicative of the presence of leakage paths which is also evident in the low fill factor values obtained. However, to ensure that the observed high Jsc values are genuine, the diodes producing them were isolated by carefully removing the CdTe material around them and repeating the I-V measurements. It is therefore possible in these solar cells for photons with energy lower than the energy bandgap of CdTe to create useful electron-hole pairs that contribute to photo-generated current. These capacitance values suggest that the glass/ITO/ZnS/Cu2ZnSnS4/CdTe/In solar cell has a wider depletion region compared to the glass/?TO/ Cu2ZnSnS4/CdTe/?n 2-layer solar cell. Both figures display a slow response of 1/C2 with applied reverse bias voltage.

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Resume : CdZnTe thin film is a promising material for photovoltaic applications since it has a tunable direct band gap properties and it is used as an absorber layer due to its high intrinsic mobility, high absorption coefficient, and high atomic number. CdZnTe thin films are deposited by techniques like physical evaporation, chemical vapor deposition, sputtering, close-spaced sublimation (CSS), molecular beam epitaxy and electrodeposition. However, the physical evaporation technique has many advantages such as low cost, good reproducibility and environmentally friendly. In this study, the effect of substrate temperature on CdZnTe thin films deposited by thermal evaporation have been investigated. Optical properties is studied using UV-VIS spectrophotometer. Crystallographic properties and chemical compositions are analyzed by X-ray Diffraction (XRD) and Energy Dispersive X-ray analysis (EDX). In addition, Morphological properties are examined using Scanning Electron Microscopy (SEM). Finally, the rectification properties using I-V measurements and capacitance-voltage (C-V) measurements have been also performed. As a result, an optimization process has been carried out for substrate temperature of CdZnTe thin films.

Resume : One of the major issues for CdTe solar cells is the activation treatment step, generally done by deposition of CdCl2 and subsequent annealing of the stacks in air. It is very well known that without this process-step no high efficiency devices can be obtained. However CdCl2 is carcinogenic and highly soluble in water, making it quite dangerous during the fabrication process, for this reason MgCl2 has been recently introduced. In this work we have studied devices (exceeding 14% efficiency) prepared by low-substrate temperature CdTe deposition and activated with MgCl2 treatment deposited by methanol solution. It is very interesting the fact that MgCl2 treated cells perform high current density without post deposition annealing of the back contact; in our process this step is generally needed for efficiencies above 10% because it is crucial to diffuse copper into the CdTe bulk to improve the carrier concentration. For this reason finished cells have been analyzed in terms of deep level transient spectroscopy and photoluminescence and the results compared with our previous work where cells were analyzed by capacitance voltage and drive level capacitance profiling. Moreover quantum efficiency has been also used for comparison. The effect of magnesium in the CdTe activation treatment will be presented and discussed.

Resume : One of the several keys to increase efficiency of CdTe solar cells is the improvement of light transmission of the buffer layer. CdS, with a band gap of 2.4 eV, results to be opaque at low wavelength regions, for this reason, in order to gain in blue and UV light, extremely thin CdS has to be applied, otherwise the buffer has to be substituted with wide band gap materials. On the other hand, by thinning the CdS layer we give place to pinholes and shunts also due to the wide intermixing of CdTe and CdS layers and subsequently consumption of CdS. For this reason we have applied and compared different CdS post-deposition treatments in order to stabilize the layer and avoid post treatment shunts. . Treatments at temperatures above 500 °C in argon and chlorine atmosphere on 80 nm thick CdS have performed higher stability compared to not treated CdS, resulting in improved performance with a 10% increased current density and a 5% increased open circuit voltage and fill factor, with an overall efficiency above 14%.. Moreover light transmission of the CdS treated layers from 300 to 450 nm is increased of about 10%. In this paper a study of treated thin CdS with novel recrystallization treatments will be presented and analysed by means of X-ray diffraction, atomic force microscopy, micro-Raman. Finished CdTe devices made on thin CdS will be shown and their performance characterized in terms of current-voltage, capacitance-voltage and quantum efficiency

Resume : Earth abundant and low toxicity CZTSSe based photovoltaics offers potential of covering future energy demands in the TW level. Currently, a strong voltage deficit seems to hinder its desired marketability. Multiple charged defects and lateral composition inhomogeneities resulting in band tailing are identified as triggers. Among the most detrimental defects, the Sn related ones stand out due to their deep electronic nature within the CZTSSe bandgap and easy formation linked to highly volatile SnxS(e)y species. Recently, addition of Ge resulted in a more efficient balance of the Sn presence during the annealing, changing the CZTSSe reaction mechanism from tri to bi-molecular and improving devices performance. Thus, deeper understanding of the absorber synthesis seems clearly still necessary. Oxide precursor based routes have not been as much explored as well as established Cu-Zn-Sn w/o chalcogen based ones. An intrinsic more stable nature of oxide containing species when compared to their chalcogen counterparts, in particular SnOy species, could shed more light on current absorber limitations. In this work, CZTO and CZTS precursors are prepared using single quaternary targets and PLD, which offers the possibility of fine composition tuning. Multiple reactive annealing variations are investigated by analysing absorber microstructure, composition and optoelectronic features of absorbers and solar cells. Results obtained with SEM, Raman, XRD, EDX, EQE, J-V and PL will be reported

Resume : Silicon-based tandem solar cells are one of the promising approaches to increase the efficiency of photovoltaic devices beyond the single-material detailed-balance limit while benefiting from the well-established technology of the conventional silicon solar cells. However, an inexpensive, earth abundant, stable and non-toxic material with a proper band-gap has not yet been identified for the upper cell. We suggest CZTS, a chalcogenide with a bandgap of 1.5 eV, as a promising material for the top absorber. The presence of a high temperature (500-600 oC) annealing process in a sulfur-rich atmosphere during synthesis of CZTS makes monolithic integration challenging. Thus, a barrier layer is necessary between the two absorbers. This barrier layer must act effectively as a diffusion barrier and allow exchange of charge carriers between the two cells. In this research, we investigated various layers of TiN and TiO2, deposited with (PE)ALD, as potential materials for the barrier layer. For this purpose, simple stacks of Cu/(~25nm)TiN/Si and Cu/(~80nm)TiO2/Si were prepared, and systematic isothermal annealing experiments at 550 oC in vacuum were performed to simulate the CZTS synthesis conditions. Subsequently, the samples were characterized by XRD, XPS, and sheet resistance measurements to evaluate the effectiveness of TiN and TiO2 as diffusion barriers. Furthermore, optical simulations were done to determine the optical performance of these layers in the near infrared spectrum.

Resume : One strategy to enhance the photocurrent of a CdTe solar cell is increasing the transparency of the CdS window layer. Using an alternative window material with a higher bandgap is a common method. In this study we reduce the thickness of the CdS layer in order to increase the photocurrent and we passivate the front-contact by means of an oxygen plasma in the CSS-process to maintain the open circuit voltage. In general thin CdS-layers agglomerate during the chlorine activation which causes a defect-rich interface between CdTe and SnO2. Those ?Pinholes? reduce the open circuit voltage dramatically due to charge carrier recombination. We show that the use of the oxygen Plasma Enhanced CSS-technology during the CdS-deposition leads to less recombination at this interface. EBIC-measurements show a bright signal at the front contact for cells with oxygen plasma treated CdS layer. Indeed, the measured open circuit voltage increases with raising oxygen flow into the plasma. It is assumed that oxygen can passivate the interface between CdTe and the front contact and counteracts the loss in voltage. Simultaneously, the photocurrent stays high, which results in an improvement of the cell efficiency. Consequently, the EQE signal shows an almost rectangular behavior. This new PECSS-technology for depositing CdS affords the opportunity to produce CdTe solar cells with a photocurrent of more than 27 mA/cm² and an open circuit voltage above 800 mV.

Resume : Cu2ZnSnS(e)4 (CZTSSe) is an emerging material as absorber for solar cells, which have recently attracted substantial interest. Currently, the record efficiency of CZTSSe solar cells is 13.6%. However, this value is still far below the 22.9% record efficiency of Cu(In,Ga)S(e)2 and further development is needed. Apart from secondary phases, grain boundaries (GB) and other extended defects, such as stacking faults and dislocations, can also significantly affect the electrical and optical properties of the absorbers. Detrimental deep traps within the band gap (non-radiative recombination centers) may be associated with them. Hence, a better understanding of the chemistry of structural defects is desired. In a previous study, we observed by atom probe tomography (APT) for a Cu-poor CZTSe low temperature precursor that the GBs are Cu-enriched and no impurity segregation. For the annealed precursor the GBs were decorated by Na and K impurities and showed (almost) no change in the concentration of the matrix elements. Here, we investigate Cu-poor CZTSe thin films by APT. Thanks to a doubled atom detection efficiency we detect a larger variety of chemical fluctuations. Na impurities can also decorate the mainly Cu-enriched GBs in the precursor. For the absorber, we also clearly observe fluctuations of the matrix elements for the mainly by Na decorated GBs. The fluctuations vary from one GB to another. We will discuss possible effects of the observed phenomena on the cell performance.

Resume : The Cu-poor and Zn-rich kesterite Cu2ZnSnS4 (CZTS) thin films were sintered on Molybdenum-coated glass substrates from CZTS-based ink. The main identified challenge to prepare photovoltaic device-relevant CZTS film from CZTS-based ink was to produce dense CZTS film with large and uniform CZTS grain size. The present work was deliberately aimed to study the microstructural evolution of CZTS film as a function of sintering parameter and type of sintering additive. Based on structural phase analysis, it was found that sintering of CZTS films from 450 °C to 620 °C in low vacuum quartz tube furnace leads to severe CZTS decomposition into its binary phase constituents. On a contrary, sintering of CZTS film under 1 atm. nitrogen gas and sulfur vapor successfully prevents CZTS from detrimental decomposition. The tetragonal kesterite phase of CZTS film was stable up to sintering temperature of 620 °C, just below the softening point of glass substrate. Nevertheless, the effective densification and recrystallisation of CZTS film from CZTS-based ink takes place only when alkalis of Na and /or K-sources were added into CZTS ink, as revealed by electron microscopy. Na-acetate gradual addition up to 10 mol. % enhances as-sintered CZTS film grain size as well as suppresses CuS secondary phase growth. On the other hand, the presence of K-carbonate as small as 2 mol. % dramatically densifies and recrystallize CZTS film with much less of CuS secondary phase. This suggests K superiority over Na in mediating microstructural evolution of CZTS film. The alkalis in the sintering of CZTS film may act as sintering fluxes that densifies CZTS films, leading to the tetrahedrally-shaped CZTS grains.

Resume : CdCl2 activation treatment is an indispensable technique for producing efficient CdTe solar cells. However, the process is complicated due to presence of oxygen, resulting in segregation of CdCl2·2CdO residual phases on the GBs and high Cl concentration (over 1019 cm?3) in the CdTe lattice. High concentration of residuals strongly limits the density and mobility of holes in CdTe by self-compensation and cause hygroscopicity of the cells. In a previous study we proved that controlled thermal annealing in closed evacuated ampoules and in a process tube at 500 - 600 oC is feasible tool to remove residual phases from CdTe:Cl films grown by close spaced sublimation (CSS), and as a result significant improvement in film optoelectronic properties has been observed. Here we report a systematic investigation of the impact of those processing steps on properties of CSS CdTe/CdS cells by analysing I-V and EQE measurement results. An improvement in conversion efficiency from 11% characteristic of the cell with classical CdCl2 activation in air, to 14 % for annealed cell was recorded. This enhancement is mainly caused by improvements in VOC from 780 mV to 820 mV, and FF from 60 to 72 %. The EQE showed enhanced response in the long-wavelength region, implying better collection efficiency in CdTe. This is in good agreement with increased p-type carrier concentration previously observed in annealed CdTe:Cl films. C-V measurements are in progress to determine the doping concentration.

Resume : Photovoltaic conversion efficiency of single-junction solar cells is intrinsically limited by Shock-ley-Queisser limit, with significant energy losses due to thermalization. A viable way to overcome this limit is to develop multijunction solar cells. Perovskite solar cell represents an ideal candidate for wide band gap top cell in combination with low band gap Cu(In,Ga)Se2 (CIGS) bottom cell. In the last years, we observed a significant improvement in perovskite/CIGS tandem performances with efficiencies approaching 24%. Those achievements refer to devices grown on glass substrates. The possibility to develop highly efficient perovskite and CIGS solar cells on flexible substrates lays the foundations to lightweight flexible tandem devices by high-throughput roll-to-roll manufacturing. Here, we report a multi-stage perovskite deposition approach by combining vacuum- and solution-based techniques. We develop NIR-transparent perovskite solar cells with efficiency approaching 14%, grown on a flexible front sheet which is used for flexible CIGS module encapsulation. Eventually, we demonstrate flexible perovskite/CIGS tandem with 19.5% efficiency measured in four-terminal configuration. Our work represents a step forward towards new concepts of flexible thin film tandem devices, and provides insights into engineering of PbI2 growth, which underlines the versatility of hybrid vacuum-solution perovskite deposition.

Resume : To our best knowledge as yet results of ageing experiments of Kesterite Solar Cells have not been published. However, at least in crystalsol detailed investigations of monograin-based solar cells have been performed proving very different stabilities depending on the composition of the Kesterite. Although completely stable solar cells even without any sealing have been demonstrated internally, in this paper we report on ageing effects observed for internally called ?highly unstable? Cu2ZnSnS4 solar cells aged for 8.5 and 95.5 hours at 105 °C hours in air. During these times the initial efficiency of 6.4 % decreased by 84 % and another 93 %, respectively. The photocurrent generation in cross sections of the CZTS / CdS / ZnO active junctions was investigated by electron beam induced current measurements in an electron microscope. Besides a pronounced decrease of the EBIC signal also a shift of the signal much further into the Kesterite material was observed, which is explained by diffusion of a recombination center from the surface into the CZTS (probably out-diffusion of Cu or Sn atoms into the contacting CdS buffer layer). After the main signal observed close to the interface has vanished, a minimum in the EBIC signal in a depth of around 400 nm becomes visible, which can be explained by the depth at which the Fermi level crosses the position of a main recombination center, maximizing the loss of charge carriers in this depth.

Resume : Thin films of Cu3SbS3 were deposited on non-heated glass substrates by vacuum thermal evaporation technique. The as-deposited Cu3SbS3 thin films were placed into programmable furnace (type Nabertherm-Germany) and annealed at different annealing temperatures (Ta). The as-deposited films were annealed in air atmosphere for 2 h in the temperature range 100 - 350°C. Their structure and chemical compositions are studied by X-ray diffraction (XRD), dispersive X-ray spectroscopy analysis (EDAX) and scanning electron microscopy (SEM). The variations of the microstructural parameters, such as crystallite size (D), dislocation density (?) and strain (?), with annealing temperature were investigated. Optical measurements show that all the Cu3SbS3 thin films have relatively high absorption coefficient between 105 and 106 cm-1 in the spectral range 1.4 ? 2.4 eV with p-type conductivity. Electrical properties have been investigated by ac impedance spectroscopy over a wide range of temperature up to 593 K starting from room temperature in the frequency range 5Hz?13 MHz. The complex impedance plots display one semicircle with equivalent circuit functions as typical parallel RC. By increasing the temperature, the Impedance spectroscopy analysis shows that the resistance decreases from 105 ? to 103 ?. In addition, the analysis of conductivity indicates that both AC and DC conductivities of materials increase with increasing temperature. The activation energy values calculated from DC conductivity and angular frequency relaxation are almost identical, indicating that the conduction mechanism was thermally activated and was assured by hopping between localized states.

Resume : Currently material solar cell efficiencies above 12 % are obtained for Kesterite monograin solar cells in crystalsol. Although the monograin powder particles are surprisingly homogenous with a standard deviation of less than 14%, all grains show pronounced variations in photocurrent (PC) and in luminescence intensity for both, electrical as well as light generation. Therefore highly spatially resolved photocurrent measurements of single grains can directly be correlated to photoluminescence (PL) and electroluminescence (EL) measurements. Whereas PL and PC measurements reveal regions of high photovoltaic activity, corresponding EL measurements show the opposite effect. Regions of high PL and PC activity show a low EL signal and vice versa. These results can be understood by assuming different barrier heights at the CZTS/CdS interface as will be shown in the paper. This investigation method allows the characterization of powder materials even before solar cells are prepared, avoiding variations due to the differences in solar cell making.

Resume : To surpass 30% conversion efficiency, tandem solar cells with crystalline silicon (c-Si) sub-cell seem to be one of the most promising architectures regarding the theoretical efficiency. Furthermore a monolithic two-terminal approach with wide bandgap thin films does not require a significant modification of the solar modules fabrication. CuIn1-xGaxSe2 is a promising candidate thanks to its good efficiency around 22% for x = 0.3 (single junction). Nevertheless its bandgap is lower than the optimum value for a top-cell, which is around 1.7 eV. Such a value corresponds to x = 1. However the CuGaSe2 record efficiency is too low (? = 11.9%) for a tandem integration with c-Si. Hence, an optimization is needed to improve CuGaSe2 co-evaporation process and adapt it for the substrates used on tandem applications. We have chosen a CuPRO (Copper Poor / Copper Rich / Copper Off) process to grow CuGaSe2 absorbers on SLG/Molybdenum. During the process, the second stage allows the formation of large grains and the third reduces the defects concentration. In our work, we focus on the morphological and electrical impacts of the copper content during the first stage. However, since the CuGaSe2 growth depends on the nature of the substrate, further work is required in order to adapt the CuPRO process for silicon and oxides (SiO2, ZnO ?) substrates, used for tandem solar cells.

Resume : The semiconductor compound CdZn(S,Se) (CZSSe) is considered as one of the ideal photovoltaic absorber layer materials for low-cost thin film solar cells, since CZSSe has a large absorption coefficient and all the constituent elements are naturally abundant. In this work, we investigate the influence of the bath temperature (40-80°C) on the chemical bath deposited CdZn(S,Se) (CZSSe) nanoparticles and films. The films have been characterized by X-ray diffraction and scanning electron microscopy for the structural and morphological study, respectively. The mixture was cooled to 00C with stirring until the mixture turned into a clear solution and then purged with argon. Na2S2O3 (1.85 mmol), SeO2 (1.85 mmol), and Na2S2O3 (0.925 mmol) with Na2Se (0.925 mmol) were applied as chalcogenide sources to fabricate CdZnS (CZS), CdZnSe (CZSe), and CZSSe nanoparticles. The solutions of Na2S2O3, SeO2, and Na2S2O3 with SeO2 were also purged with argon and cooled to 00C, respectively, followed by a quick transfer into the metal sources to react with CdCl2 and ZnCl2, for 2 min. The CZSSe films coated on Al substrates fabricated were applied to the preparation of CZSSe solar cells. The CZSSe solar cells with a structure of Al/p-CdS/CZSSe/In were fabricated.

Resume : In this study we report the grown of zinc selenide (ZnSe) thin films by radio frequency magnetron sputtering onto indium tin oxide (ITO) coated glass substrates, used as window layer for cadmium telluride (CdTe) based photovoltaic structures, for space applications. The structural, optical, morphological and electrical properties of fabricated ZnSe layers were analyzed by X-ray diffraction, optical spectroscopy and spectroscopic ellipsometry, scanning electron microscopy (SEM), and temperature dependence of the electrical resistivity measurements, respectively. The prepared films are polycrystalline with a marked (111) texture and optical band gap values of 2.83 ? 2.84 eV. CdTe absorber layer was obtained by thermal evaporation (TVE) and its thickness values were larger than 2 ?m. The photovoltaic structures were completed by copper (Cu) and gold (Au) commixture thin films deposited by TVE and acting as back electrode. Specific parameters together with the electrical and photo-electrical behavior of ITO/ZnSe/CdTe/CuAu devices was analyzed before and after irradiation with protons (3 MeV, fluency 10-13 cm-2), and the effect of irradiation was discussed. Keywords: ZnSe films, CdTe, ZnSe/CdTe heterojunction, rf-magnetron sputtering

Resume : CdTe/CGS/CdS/ZnO solar cell is proposed in this research work. Numerical analysis of single and multi-junction solar cell based on cadmium telluride (CdTe) and copper gallium sulfide (CGS). Performance of a cell is analyzed by using SCAPS (Solar Cell Capacitance Simulator) software. By changing the physical parameters using SCAPS software, electrical characteristics of CdTe/CGS/CdS/ZnO based multijunction solar cell is analyzed. Under AM1.5 illumination simulation of J-V (current versus voltage) and quantum efficiency results are to be done. The cell structure is, the top cell is a CGS solar cell and the bottom cell is CdTe cell used in tandem configuration. Efficiency of CGS/CdS/ZnO solar cell is 10.578 % (with FF=83.697%, Voc= 0.820V and Jsc= 15.403 mA/cm2) and efficiency of CdTe/CGS/CdS/ZnO is 20.701%. (With FF=83.650%, Voc= 0.873V and Jsc= 28.318 mA/cm2).

Resume : It is known that most record-efficiency kesterite solar cells have been typically fabricated using a high-resistivity cadmium sulfide (CdS) buffer layer deposited by chemical bath deposition (CBD) in order to avoid the formation of undesirable shunt paths due to sputtering damage during the deposition of the subsequent zinc oxide (ZnO) window layer. However, the use of cadmium is undesirable from the viewpoint of environmental safety. Moreover, the quantum efficiency of the kesterite solar cell drops at short wavelengths due to the optical absorption loss from the CdS layer. This implies that further improvement in the short-circuit current can be achieved by replacing CdS with other appropriate wider bandgap buffer materials. In recent years, Zinc sulfide (ZnS), an important II-VI compound semiconductor with a wide direct band gap (Eg = 3.6~3.8 eV), has have attracted intensive attention for its potential application as Cd-free buffer layer material in kesterite solar cells. For photovoltaic applications, ZnS thin film is nearly transparent in all wavelengths of solar spectrum and has high absorption for the wavelength below 520 nm as compared to CdS. Many techniques including sputtering, molecular beam epitaxy, pulsed laser deposition, chemical vapor deposition, successive ionic layer adsorption and reaction, spray pyrolysis, chemical bath deposition (CBD), and photochemical deposition (PCD) have been proposed to fabricate the ZnS thin films. Among these methods, PCD is most attractive because it can be employed as the large-area growth without vapor deposition related to physical techniques and free of some inherent problems associated with high temperature fabrication. Moreover, this technique can be used to deposit ZnS in selected areas. In this study, Zn(O,S) thin films were deposited on ITO glass substrates by pulsed UV photochemical deposition (PCD). In the deposition process, ozone is fed into the deposition solution to incorporate oxygen into ZnS to form Zn(O,S) film. Oxygen content is easily controlled by modulating ozone flow rate. The structure, morphology, transmittance and optical bandgap of Zn(O,S) thin film were investigated by X-ray diffraction, scanning electron microscope and UV-vis spectrophotometer. Our results showed that the as-prepared samples exhibited high transparency in the visible regions. The optical bandgaps of Zn(O,S) films, ranging from 3.2 to 3.7 eV, can be readily tuned by modulating oxygen content in the samples. This study confirmed that Zn(O,S) is suitable as Cd-free buffer layer material in the kesterite solar cells.

Resume : CdTe with direct energy gap of 1.45 eV is well adapted to the spectrum of solar radiation. Several studies have been reported on the electrical conduction of CdTe films for photovoltaic application. Investigations deal with the effect of film’s composition by varying growth parameters leading to influence the electrical conduction of CdTe films. The as deposited n- or p-type CdTe thin films shows lower conductivity, is due to non-stoichiometric effect as well as the heavy compensation between native defects. Hence, the higher n-type or p-type conductivity of CdTe films can be obtained under control by deviating from their stoichiometric composition as well as alternating the deposited potential [2]. In this study, the p-i-n structure of homojunction CdTe solar cell was fabricated by electrodeposition. The kinetic model based on the Butler-Volmer equation has been developed for a variety of deposited conditions, and numerical simulation is used for the deposited process of CdTe thin film by varying the electrodeposited parameters. The potential of perfect stoichiometric (PPS) can be obtained. Therefore, the main strategy is easily achieved to improve the intrinsic quality of i-layer resulting in higher efficiency of CdTe solar cell. In addition, based on the kinetic model, theoretical calculation and numerical simulation, the intrinsic CdTe thin film with near-stoichiometric and the self doping n-type Cd-rich and p-type of Te-rich CdTe with non-stoichiometric can be prepared successfully. By the precise potential-control of the electrodepositing process, the properties of electrodeposited CdTe thin films are consistent with the theoretical prediction, to be confirmed experimentally. Technologically, the defect engineering process, based on the theoretical approach of non-stoichiometric effect, is not limited only to optimize the effective type-conversion but also to improve the quality of each electrodeposited CdTe layers as well as higher the performance of solar cell. Actually, the defect engineering was designed to improve the effective self-doping of n-type and p-type, and also the as-deposited near-stoichiometric (PPS) layer. By preceding the type conversion process sequentially, the efficiency of p-i-n CdTe photovoltaic cell is enhanced. In short, the electrodeposited CdTe thin film with its process of special defect engineering leads to very well commercial advantages such as lower cost and larger area size on CdTe thin-film solar cells [1-2]. Finally, the near stoichiometric and non-stoichiometric compositions were obtained and electrodeposited by following with the results of simulation. Film’s conductivity by can be improved by defect engineering process, which can be achieved better by effective type conversion, and optimized by correlating with the non-stoichiometric effects [1-2]. [1] R.D. Engelken and T.P. Van Doren, J. Electro -chem. Soc., I32 (1985) 2910. [2] H.Y. Ueng et al., J. Electron. Mater. 37, p.1821, 2008.

Resume : Solar cells based on the Cu(In, Ga)Se2 (CIGS) absorber are among the most promising candidates to replace costly silicon solar cells, thanks to high efficiencies and low costs of CIGS solar cells. The recent progress in their efficiency is mainly due to incorporation of different alkali metal atoms into the absorber layer [1]. However, the microscopic mechanisms how the alkali metal post deposition treatment (PDT) improves the efficiency of CIGS solar cells are, despite extensive research, not yet clear [2]. It has been found experimentally that solar cells after heavy alkali (K, Rb, and Cs) PDT have (i) smoother morphologies of the absorber layer, (ii) higher open circuit voltages, and (iii) higher efficiencies than those after light alkali (Li, Na) PDT. In the present study we explain why the effects of different alkali metal atoms vary remarkably. Our first principles investigations show that light alkali metal atoms prefer to accumulate on the Cu sublattice as impurities and incorporation of heavy alkali metal atoms contributes to the phase separation [3]. Starting from thermodynamics of impurities and extending to their kinetic parameters, our aim is to gain a comprehensive understanding of the absorber microstructure after the PDT treatment. Formation of secondary phases during the PDT process has been predicted by considering which are the possible reactions and calculating their enthalpies. Some of the phases can appear only at high temperatures and certain impurity concentration ranges. Therefore, we discuss also which are the conditions for preferable formation of mixed and separated phases. The properties of these phases have been analyzed and conclusions regarding the ensuing materials and device characteristics are drawn. 1. Jackson, P., Wuerz, R., Hariskos, D., Lotter, E., Witte, W. and Powalla, M., Phys. Status Solidi RRL 10, 583–586 (2016).  2. P. Reinhard, B. Bissig, F. Pianezzi, et al. Chemistry of Materials 27, 5755 (2015) 3. M. Malitckaya, H.-P. Komsa, V. Havu, and M.J. Puska. J. Phys. Chem. C 121, 15516–15528 (2017)

Resume : Recent record cell efficiencies of Cu(In,Ga)Se2 (CIGS) solar cells have been achieved by post-deposition treatment (PDT) with the heavy alkali elements Rb and Cs. So far it is unclear if this is caused mainly by a bulk or a grain boundary (GB) effect of these heavy alkali elements. The diffusion of the heavy alkali element rubidium (Rb) in CIGS layers was investigated over a temperature range from 148 °C to 311 °C by outdiffusion from a rubidium fluoride (RbF) layer deposited on a finished CIGS layer. The diffusion profiles were measured by secondary ion mass spectrometry. By using CIGS layers with different grain sizes, diffusion along GBs could be distinguished from diffusion into the grain interior. Based on this data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation D(V, Rb) = 3.2 ∙ 10-8 exp(-0.43 eV/kBT) cm2 s-1 and the fast diffusion along the GBs by D(GB, Rb) = 7.3 ∙ 10-9 exp(-0.30 eV/kBT) cm2 s-1. Additionally, the ion exchange mechanism with Na was investigated. In Na doped CIGS layers the diffusion of Rb into the bulk of the grains is hindered and Rb mainly diffuses along GBs. In contrast, in case of alkali free CIGS layers Rb diffuses into the grains as well as into the GBs. Finally, the effect of Rb on the solar cell parameters of CIGS thin-film solar cells was investigated. RbF PDT was found to enhance the open-circuit voltage, the fill factor and the carrier density in a similar way as observed for K and Na.

Resume : Cu(In,Ga)Se2 solar cells are the most efficient ones among all thin film photovoltaics. The recent push in record efficiencies was mainly realized by applying a RbF post deposition treatment (PDT) to the absorber. However, it is not yet fully clear why the introduced Rb improves the solar cell performance. In order to investigate the beneficial effect of Rb, a Cu(In,Ga)Se2 absorber was grown on a Mo coated alkali free substrate and subjected to a RbF PDT. This pure RbF PDT leads to a significantly higher conversion efficiency. A thin cross sectional lamella was cut out of the layer stack and investigated via a combination of different electron microscopy techniques and synchrotron based X-ray fluorescence analysis. It is evident that Rb segregates at random grain boundaries and dislocation cores, where it likely passivates defects. In contrast, Rb does not segregate at benign Σ3 twin boundaries. Additionally, Rb agglomerates at the interface between the absorber and the MoSe2 layer. Our results thus provide clear indications of the origin of the beneficial effect of Rb in Cu(In,Ga)Se2 solar cells [Schöppe et al., Nano Energy 42 (2017) 307]. Subsequently, we investigated a high efficiency solar cell grown on a glass substrate and subjected to a RbF PDT, thus providing a conversion efficiency of over 20 %. Applying the same combination of analysis techniques, we clearly demonstrate that our conclusions for a pure RbF PDT are also valid for state of the art devices.

Resume : It has been widely demonstrated, that the performance of Cu(In,Ga)Se2 (CIGSe) thin film solar cells can be increased by post deposition treatments using alkali-fluoride compounds. Several models have been given in the literature to explain the beneficial effects in RbF-treated samples. However, there is no general agreement with respect to which recombination mechanism is connected to the improvement in efficiency. In this study, temperature dependent IV measurements (IVT) together with supporting measurements (GDOES, CV-profiles, EQE) have been performed to analyze the recombination mechanisms in samples with varied RbF incorporation, bandgap profiles and composition (CGI ratio). The results demonstrate that the values of the bandgap energy (Eg), the activation energy of the bucking current (EA) extracted from the extrapolation of the open circuit voltage (Voc) at T = 0 K and from the slope of the Arrhenius plot are all in reasonable agreement. The diode factors were generally around 1.5 for T > 200 K, thus showing no evidence of a fundamental change in recombination mechanism with RbF-PDT. While a gain in Voc is generally observed with the RbF-PDT, maintaining a high fill factor is more challenging. The latter effect is connected to an increasing deviation between dark and illuminated (Isc-Voc) characteristics in the IVT measurement with RbF.

Resume : Evaporation of relatively heavy alkali-metal fluorides such as KF, RbF, or CsF onto Cu(In,Ga)Se2 (CIGS) thin films, a process often referred to as alkali-metal post-deposition treatment (KF-, RbF-, or Cs-PDT), has been recognized to be a promising technique to boost CIGS solar cell performance. Use of these techniques is also known to lead to morphology modifications manifested by wormhole formation at CIGS film surfaces. In this study, we examined the effects of RbF-PDT on ternary CuInSe2 (CIS) films. It was found that there were specific facets where granular particles (RbF or related compounds) were preferably deposited and as a consequence a large number of wormholes were formed. It was also found that granular particles concentrated in the grain boundaries present at CIS film surfaces as well as the specific facets. Electron backscatter diffraction (EBSD) measurements revealed that the preferred facets where granular particles tend to concentrate are on other than (111)-planes, namely the preferred facets were (220)/(204)-related planes rather than the (112)-planes of chalcopyrite CIS. CIS solar cells fabricated using RbF-PDT CIS films showed a substantial enhancement in device performance, similar to the case for quaternary CIGS devices. These trends were, however, not present for In-free CuGaSe2 films and devices, implying that the effect of relatively heavy alkali-metals strongly depends on the group III elemental composition in CIGS.

Resume : To increase the cost efficiency and reliability of CIGS solar cells we aim to make thinner CIGS layers of ~ 500 nm. This will not only reduce the amount of material, but also the number of defects in the bulk that may cause performance instabilities. One of the instabilities in CIGS is caused by alkali elements, which are required to achieve high performance, but are also mobile and their concentration should be controlled. To investigate the effects of alkali of thin CIGS layers, we deposited the CIGS by 1-stage coevaporation on Mo/Si(O,N)/SLG substrates, with Si(O,N) as alkali barrier. The alkali are deposited by spincoating or evaporation, either before or after CIGS deposition. Photoluminescence (PL) and time resolved PL (TR-PL) were performed on CIGS/CdS samples and electrical characterization was done on finished solar cells in standard AZO/i-ZnO/CdS/CIGS/Mo configuration. There was no significant change in solar cell performance between the different Na treatments. K treatment however showed significant changes in PL, TR-PL and solar cell performance. Generally, a larger blueshift and broader emission peak was measured for the K treated sample, implying higher degree of compensation. Higher lifetime was measured as well. Efficiencies of K treated solar cells increased more than 2% due to higher Rshunt and Voc. This work received funding from the European Union’s H2020 research and innovation programme under grant agreement No. 715027.

Resume : Cu(In,Ga)Se2 photovoltaic technology has become very attractive because of the very high efficiencies achieved (close to 23%) compared to other thin film materials. The use of flexible substrates paves the way to roll-to-roll deposition techniques, and to new PV applications. Among flexible substrates, polyimide holds the record efficiency (20.4%). Recent research showed that a combination of Na with a heavier alkali produces the highest efficiencies mainly through an improvement of the Voc, but the exact mechanisms are still under investigation. On polyimide, only few studies exist on NaF and KF post deposition treatments (PDT), despite the key importance of understanding the mechanisms and the effectiveness of these processes at low temperature. In this work, the electrical performances solar cells prepared on polyimide with different PDTs will be analyzed and discussed in light of structural (GD-OES, XPS, XRD …) and optical (Raman, PL, EQE) characterization data, supported by physical modeling. In particular, we show that strong improvement of the Voc can be obtained by adding indium during the alkali PDT. The absorber modification obtained using indium in combination with KF leads to an improvement of Voc of about 20 mV with respect to NaF/KF PDT and of almost 50 mV with respect to NaF-PDT only. Our results show a record efficiency of 18.1 % (without anti-reflective coating), a value which is close to the highest reported value of 20.4% (with ARC). Other alkalis are currently under investigation and the latest results will be presented.

Resume : The beneficial effect of KF-PDT for CIGS solar cells has been frequently reported. However, the buffer layers used in most of these studies have been grown conventionally by a wet CBD method. In this contribution, we have applied ALD grown Zn(O,S) buffer layers. The intention is to study the impact of KF-PDT on devices with buffer layers grown by a dry, vacuum-based deposition process. Devices were fabricated with the structure: SLG/Mo/CIGS-KF/[CBD CdS or ALD Zn(O,S)]/i-ZnO/ZnO:Al/metal grid. It was found that a wet-chemical surface treatment of the CIGS-KF surface is required before growing the ALD Zn(O,S) layer. Different surface treatments could be attributed to changes in the I-V characteristics. Furthermore, the effect of KF-PDT was evaluated for both buffer layer processes by varying the KF flux during the PDT process. Regardless of buffer layer, the KF-PDT led to a significant increase in Voc. However, the presence of a K-In-Se rich surface phase resulted in lower FF values for the ALD Zn(O,S) devices. It was found that a comparable device efficiency to CdS (η ≈ 18%) for the Zn(O,S) buffer layer could only be attained by etching away the K-In-Se rich phase prior to ALD growth, followed by a low-temperature anneal of the finalized device. While the mechanism behind the low-temperature anneal is not fully understood, it indicates that the K-In-Se rich surface phase in itself is not responsible for the KF-induced Voc gain observed for the Zn(O,S) devices.

Resume : Both, RbF and KF post deposition treatments (PDTs) have been proven to enhance the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells by increasing VOC and FF. Here we compare the impact of both PDTs on the material and device properties of CIGS thin films and corresponding solar cells. Thereby we spotted different dependencies of the PDTs’ effectiveness on the copper content ([Cu]/([Ga]+[In]) = CGI). Compared to untreated references, KF-treated solar cells show higher efficiency gain at lower CGI, while RbF-treated devices exhibit best performances close to stoichiometry. An additional benefit of the PDT can be achieved by adapting the CdS buffer layer: typically its deposition can be made shorter, leading to higher JSC. Therefore we investigated the growth of chemical bath deposited CdS on alkali-treated CIGS layers. To do so we carried out a combined study of Raman scattering, analyzing the PDTs effect on the CdS growth stages and its growth rate, and scanning electron microscopy (SEM). Latter reveals e.g. that both PDTs lead to a closer coverage of the CdS layer compared to the reference. Finally we used standard device analytics at four selected times during the CdS-growth to correlate the observed modifications with device properties. It turns out, that both PDTs lead to strongly improved VOC and FF especially after short CBDs validating the results of the Raman/SEM-study. All results will be discussed with respect to the recently reported models regarding alkali-induced effects.

Resume : The chalcopyrite compound Cu(In, Ga)Se2 (CIGS) has the potential to be used as a semiconductor material in thin-film photovoltaic devices with high conversion efficiencies. In 2013, Swiss Federal Laboratories for Material Science and Technology (EMPA) demonstrated the effectiveness of subjecting the CIGS surface to KF post-deposition treatment (KF-PDT) and many studies on high-efficiency CIGS solar cells with KF-PDT have been reported. However, CIGS solar cells with KF-PDT cannot always retain these high conversion efficiencies for a long time; that is, the degradation of conversion efficiency frequently occurs. Although the degradation mechanism of CIGS solar cells has not yet been clarified, the acceptor concentrations in CIGS layers easily decrease when the cells are kept in the dark, even at room temperature. Light soaking and current injection by positive bias have been reported as an effective method to increase the acceptor concentrations of CIGS layers. Light or current induced acceptors are thought to be metastable, and high acceptor concentrations are generally considered beneficial for high conversion efficiency. In this study, we examine whether heat-light soaking and current injection at high temperature are useful to enhance the conversion efficiency of CIGS solar cells with KF-PDT, and the best cell efficiency of 22% is achieved after 25 hours of heat-light soaking.

Resume : The addition of the alkali metals to CuInGaSe2 (CIGS) thin films through post deposition treatments (PDTs) or incorporation during film growth has been of interest for quite some time as a means of increasing photovoltaic device performance. Alkali metals alter recombination processes within these devices and an in-depth knowledge of these changes is critical to optimizing PDTs such that recombination losses are minimized. Device measurements indicate significant changes in both interface and bulk recombination after PDT application. Cathodoluminescence (CL) microscopy is a powerful technique to probe changes in charge carrier recombination behavior at sub-micron scale features such as grain boundaries. In this contribution, CL analysis of CIGS thin films exposed to various PDTs will be presented with a detailed discussion of how alkali PDTs alter recombination behavior at grain boundaries and in grain interiors. In addition to standard CL analysis, temperature and injection level are also included as experimental variables during acquisition of spectrum imaging data. This provides another layer of detail related to the spatial variations in recombination behavior to further clarify how PDTs alter recombination process in CIGS photovoltaic devices.

Resume : We employ ac and dc electrical measurements and photoluminescence to investigate aluminum oxide passivation layers deposited by plasma-assisted atomic layer deposition (ALD) on polycrystalline Cu(In,Ga)Se2 (CIGS) for application in thin-film solar cells. Already a few monolayers of aluminum oxide result in non-ohmic highly resistive current-voltage characteristics, which indicates a controlled ALD process and formation of a closed and compact dielectric layer on the rough CIGS surface. Despite the excellent growth, as-deposited aluminum oxide layers do not provide any meaningful surface passivation. Bias-dependent admittance spectra show clear evidence for a broad distribution of interface defects with defect concentrations around (1–5)e12 eV-1cm-2. The capacitance step associated with these interface defects however vanishes below our detection limit after annealing for a few minutes, even at moderate temperatures of 350 °C. As a result, the photoluminescence yield improves drastically, and the effective carrier lifetime increases from 2 ns to 20–30 ns. For such moderate annealing conditions, we do not detect any evidence for the formation of fixed charges in the aluminum oxide, and the improved interface passivation is thus purely related to a chemical passivation. Nevertheless, the photoluminescence yield of the annealed sample, even without field-effect passivation by fixed charges, is comparable or superior to reference samples with a state-of-the-art CdS buffer layer.

Resume : Spectral conversion has been widely applied in the field of luminescence and is potentially beneficial in photovoltaics. Among different approaches, down conversion shows a theoretical potential to enhance the limit of energy conversion efficiency of a single-junction solar cell from about 32% to 40%. For CIGS solar cells, the main current loss in the short wavelength region (< 550 nm) comes from absorption of the CdS buffer layer with a relative small bandgap. As the spectral response of CIGS has an optimal efficiency in the longer wavelength (550-1000 nm) region, spectral down converter that converts photons with wavelength shorter than 550 nm to longer wavelength at a high luminescent efficiency has the potential to reduce the above current loss for CIGS solar cells. In this work, transparent ceramics of Y3Al5O12: Ce3 have been fabricated and applied as the down convertor for CIGS solar cells. Their absorption and photoluminescence properties have been investigated. The device conversion efficiencies of CIGS solar cells with and without the down converter have been measured and compared. The results show that optimized Y3Al5O12: Ce3 ceramics may bring about 9% enhancement to the current density, which enhanced the efficiency of CIGS solar cells from 17.3% to 18.9%. Compared with a pure antireflection coating with an enhancement of ~6%, the great enhancement in down-converter device demonstrates that the Y3Al5O12: Ce3 ceramics has a potential to further improve the performance of CIGS solar cells.

Resume : Cu rich Cu(In,Ga)Se2 has less defects & better transport properties than Cu-poor. Still, Cu-rich Cu(In,Ga)Se2 cells suffer from low open circuit voltage. To understand the difference, we performed admittance measurements (ADM) on Cu-rich CuInSe2 cells, which revealed the presence of an admittance step with activation energy of around 200meV. This step could be a bulk or an interface defect or a transport barrier. To investigate more about this step, ADM were performed on Schottky contacts & results showed the presence of this 200meV step, indicating it is an effect of the bulk of the absorber and not at the interface. IV(T) was performed & activation energy of the series resistance as function of temperature was extracted showing values much less than 200meV observed by ADM indicating that this step is not a barrier, but is a defect in the bulk of the absorber. This defect disappeared with introducing post-deposition treatments (PDT): In-Se & KF treatments. To further understand the nature of this defect, a PDT was performed using only Se on Cu-rich absorbers. ADM were performed and the 200meV defect disappeared indicating that the 200meV step is a bulk defect related to Se. Interestingly, Se PDT succeeded in increasing efficiency of Cu-rich CIS from 8 to 9.4% with increase in VOC by nearly 30mV & increase in FF by 10% absolute. Se PDT was able to increase efficiency due to removing the 200meV Se-related bulk defect, which appears to be one of the problems of Cu-rich CuInSe2.

Resume : This contribution presents first results on bifacial Cu(In,Ga)Se2 (CIGS) solar cells that use hydrogen-doped In2O3 layers (IOH) as a transparent back contact. In order to enable sufficient carrier collection and at the same time allow for reasonable light absorption, an absorber layer thickness of only 600-700 nm was chosen. The thin absorber layer results in an increased impact of the back contact recombination velocity (i.e. interface defect density) and emphasizes the need for a back surface field. The back contact properties affect the photovoltaic performance even more at illumination from the back side. A reduced collection efficiency for electrons generated close to the CIGS/IOH interface is usually the major limitation for back-side illuminated cells. Here, we investigate the passivation effect of Al2O3 films deposited on the IOH layers in order to mitigate the corresponding losses in short circuit current. Furthermore, the effect of the Na supply technique (from glass substrate or NaF pre-deposition) and of a variation in IOH thickness are studied. The cell with the best combined efficiency (?) shows values of ? = 11.0% at front side illumination and ? = 6.0% at back side illumination. Standard electrical characterization (IV, EQE) is supported by optical measurements and microstructural/chemical investigations on the cross-sections (SEM & TEM). The observed trends will be evaluated and the potential of further modifications discussed.

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Resume : We have studied annealing process at temperatures less than 500°C in the two-step CIGS process, which is essentially required for the use of polymer films. The annealing process higher than 550°C has a limitation in substrate selection and needs to be lowered under 500°C for flexible CIGS solar cells based on polymer films. Before RTP annealing process, the Se film was deposited on the sputter-deposited In/CuGa layer by a thermal evaporator. In a CIGS absorber layer annealed at 480°C, Ga was found to be severely present more near Mo/CIGS interface, which lowered the open circuit voltage characteristic in the device. In order to solve this problem, Sb was added to the precursor, and it was confirmed that the Ga distribution in the thin film was changed according to the amount of Sb added. As the thickness of Sb increased to 10nm, Ga was observed to diffuse more easily to the surface and Voc was also increased in the device. The solar cell containing Sb exhibited better device performance with an efficiency of 9.13%.

Resume : The effects of Na on the electrical properties of Cu(In,Ga)Se2 (CIGS) based solar cells are well known to improve the values of power conversion efficiency by increasing the open-circuit voltage (Voc) and fill factor (FF). Many reports have investigated the effects of Na in CIGS pointing out that its incorporation increases the free carrier density, due to a decrease in compensation. However, the exact influence of Na is still debated in the CIGS community. In this work, we studied two samples: a reference one (15% efficiency), prepared according to standard industrial procedures and a sample completely alkali-free grown on an Al2O3 substrate (7% efficiency). Completed solar cells were characterized by J-V and, as expected, significant losses in Voc and FF for the Na-free sample, were observed. Additionally, the lack of Na doping leaded also to a very low free majority carriers density and very large depletion regions, as shown by C-V measurements. Photoluminescence (PL) measurements on the reference sample showed a single, broad, and asymmetric band, whereas on the Na-free one, a second component at lower energies is identified. The impact of the decrease in the free majority carriers density on the PL and external quantum efficiency results, is discussed in the scope of the role of Na changing the compensation ratio.

Resume : The performance of chalcopyrite-based thin-film solar cells has recently been improved by performing alkali post-deposition treatments (PDT), where recent records have been reached using Rb [1] and Cs [2,3]. We used hard and soft x-ray photoelectron spectroscopy to study the impact of NaF/RbF PDTs on the chemical and electronic properties of low temperature processed CIGSe absorbers as a function of RbF PDT deposition rate. We find a stronger copper depletion at the absorber surface with increasing Rb content in agreement with Ref. [4]. Furthermore, we were able to identify a second Se species, scaling with Rb content. To monitor the formation of the CdS buffer layer/CIGSe absorber interface, we studied a buffer layer thickness series deposited with chemical bath deposition. The observed difference in the attenuation of the absorber lines suggests a faster and/or more compact growth of the CdS layer for the RbF treated absorbers. We will present and discuss the impact of the NaF/RbF PDT on the chemical and electronic structure of the CdS/CIGSe interface as a function of RbF PDT deposition rate. Ultimately, we aim to link our findings to the increase in efficiencies of respective CIGSe solar cells. [1] Jackson et al., Phys. Status Solidi RRL 10, No 8, 583?586 (2016) [2] Solar frontier (http://www.solar-frontier.com/eng/news/2017/1220_press.html, 2018-01-12) [3] PVSEC 27, session 2ThO4.3, talk by Jyh-Lih Wu (Solar Frontier) [4] Avancini et al., Chem. Mater. 29 (22), 9695?9704 (2017)

Resume : In order to lower the cost of chalcogenide based solar cells, a novel and non-vacuum Electrostatic Spray Assisted Vapour Deposition (ESAVD) process has been developed to deposit CIGS and CZTS absorber layers for thin film solar cells [1-4]. During ESAVD process, the aerosol containing a mixture of chemical precursor is charged and directed towards a heated substrate where it would undergo decomposition and chemical reaction to deposit a stable solid film onto the substrate [3]. ESAVD is a scalable process for industrial level and it can be operated in open atmosphere. It is also very easy to be adapted for large area deposition with the deposition uniformity as high as 90% under optimised conditions. In our work, CIGS and CZTS absorber thin films with high uniformity (? ±5%) have been achieved after optimizing the deposition conditions of ESAVD process. The composition, purity and grain size of absorber have significant influence on the device performance of the fabricated solar cells. The composition ratio and absorber thickness were further optimized based on the combined results of XRF, SEM, Raman, EDX, and photovoltaic properties of devices. The optimized CIGS solar cell with Cu/In+Ga= 0.78 and In/In+Ga=0.29 showed the best efficiency of 9.55% with Voc = 0.518V, jsc= 28.79 mA cm?2, and FF = 64.02%. Further improvement of films quality is crucial to obtain enhanced carrier collection efficiency in solar cells. Thus, copper(Cu) deficient CIGS films were intentionally designed to avoid the dangerous KCN post-treatment for the removal of extra high conductive minor phase of CuxSe. Subsequently, the as-deposited absorber was dipped into aqueous NaCl solution before selenization to increase the grain size and p-type conductivity of the absorber. Photovoltaic results from the j-V curve have demonstrated that dipping in 0.2M NaCl for 20minutes resulted in an increased Voc and jsc due to the enhanced grain size and p-type conductivity, which showed the jsc, Voc, FF and ? of the solar cell of 29.30mA/cm2, 0.564V,65.59% and 10.83%, respectively. ESAVD has also been used to fabricate earth abundant absorber CZTSSe. Different alkali metals such as Na, Li and Rb were incorporated in ESAVD deposited CZTSSe compounds to further improve the photovoltaic performances of the related devices. Photovoltaic properties results showed that Li, Na and Rb incorporation can increase power conversion efficiency of CZTS devices up to 5.5%. The introduction of a thiourea treatment, has further improved the quality of the absorber/buffer interface, pushed the device efficiency up to 6.3%.

Resume : Cu(In,Ga)(S,Se)2 (CIGS) solar cells have reached to power conversion efficiency (PCE) of 22.6% on rigid substrates. Taking a step forward with a commercialization stage, CIGS solar cells are expanding its application field, employing bendable substrates. CIGS solar cells on polymer substrates have already achieved the PCE of 20.4%. To catch up with the performance of conventional CIGS solar cells, it should be important to figure out the energy loss mechanism and propose a suitable condition for post-deposition treatment (PDT). Especially, the role of Na ions in CIGS thin-films is well-known, and its incorporation in Na-free flexible substrates would be critical. Hereby, NaF layers were applied as PDT and the annealing temperature of CIGS thin-films was varied. In consequence, we were able to create CIGS thin-films with four-different PCE of 0, 6, 12, and 15%, and they were optically and electrically probed by micro-Raman scattering and Kelvin probe force microscopy (KPFM), respectively. As a result, high-efficiency CIGS thin-film displays enhanced crystallinity and higher energy band barrier near the grain boundaries (GBs). This can ameliorate carrier transportation, improving device performance. Furthermore, 640, 532, and 405-nm wavelength laser modules were used for investigating the effects of surface photo-voltage, analyzing the recombination process in the materials indirectly. Also, the phase distribution on the surfaces were determined by the difference among other phases; the work function of CIGS (?CIGS) is approximately 5.0 eV.

Resume : Dense, coarse grained CuInSe2 thin-films are manufactured with vacuum-free processing based on nanoparticulate precursors. Bimetallic copper-indium-, elemental selenium, and binary selenide (Cu2-xSe and In2Se3) nanoparticles were synthesized by wet-chemical methods and dispersed in organic solvents as nano-inks. Different combinations and layer-stacks from these nano inks were printed on molybdenum coated float-glass-substrates via doctor-blading. The temperature treatment to transform these layer-stacks into dense CuInSe2-films was also investigated, using a variation of temperatures, a face-to-face technique and mechanically applied pressure to overcome issues like high porosity, oxidation, or selenium- and indium-loss. All thin-films were characterized with, SEM, EDX and XRD. CuInSe2 formation takes place between 320 °C and 390 °C and MoSe2 is observed at 550 °C. Dense layers with a thickness of about 3 µm were formed in this work which show potential to be used as solar cell absorber.

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Resume : The synthesis of high efficiency Cu(In,Ga)Se2 (CIGSe) devices requires Cu-poor stoichiometries, which could induce different mechanisms for the Cu deficit accommodation, thus impacting in the final devices opto-electrical performance. The thin film Cu poor chalcopyrites structural characterization is not evident due to the similitude between the CIGS itself and their related Ordered Vacancies Compounds (OVC). On the other hand in the case of low concentration of Cu vacancies, the detection of defective CIGS is not obvious. In this context, Raman spectroscopy (RS) using resonant conditions has demonstrated being a suitable tool for the characterization and detection of both OVC and defective CIGS since it is highly sensitive to crystal quality, punctual defects, and OVC phases formation. In this work, a complete RS analysis (including resonant and non resonant conditions), supported by XRD, XPS and PL, is performed on samples with compositions from Cu-rich to Cu-poor CuInSe2 (CIS) (Cu/In=1.4-0.4) thin films. This analysis allows detecting the presence of a gradual transition between the stoichiometric CIS, the defective CIS and the formation of OVC phases, based on the asymmetry of the 173 cm-1 peak and intensity increase of the 232 cm-1 peak and the XRD changes. A similar RS characterization is then applied on high efficiencies CIGS samples (18.5-20%) and a correlation of the intensity of the CIGS related defects peak with the cell performance is founded and discussed.

Resume : The thermal stability of a bottom device is paramount for reliable application in tandem devices. Ideally, it will permit processing of a top device at required optimum process temperatures. Here we investigate the degradation behavior for CdS-buffered Cu(In,Ga)Se2 (CIGS) thin film solar cells with and without Na-incorporation under thermal stress in ambient air and vacuum with the aim to gain a more detailed understanding of the degradation mechanisms. Previous studies of CdS/CIGS devices primarily attributed the degradation after application of thermal stress to Cd diffusion into the CIGS absorber layer. On the other hand potential-induced degradation of CIGS devices has been associated with the migration of alkali elements (mostly Na) within the device. It is further known that air and humidity exposure will affect Na diffusion within CIGS devices. For the devices studied, we observe severe degradation after annealing at 250 °C in air and at 300 °C in vacuum. The electrical, morphological and compositional properties of the samples before and after defined application of thermal stress are studied. Unlike literature reports, we find no pronounced Cd-diffusion into the CIGS absorber layer. For Na-containing samples the observed degradation can be explained by the formation of Na-induced acceptor states in the TCO front contact. Thus, supported by 1-D numerical device simulation using SCAPS, various possible degradation models are discussed and correlated with our findings.

Resume : Having an absorber layer thickness below 1 µm for a regular CIGS solar cell reduces the pathways for electron collection which leads to reduced recombination losses in the bulk absorber layer. Additionally, it reduces material costs and production time. Yet, having such a thin absorber decreases the cell efficiency significantly, mainly due to incomplete light absorption and high Mo/CIGS rear-surface recombination. The aim of this paper is to implement some innovative rear surface modifications for a 500-nm thick CIGS absorber layer to reduce these affects: a passivation layer to reduce the back-surface recombination and point contact openings using nano-particles (NPs) (CdS, Ag, etc.) to create electrical contacts, with some NPs also improving the optical confinement. For example, one approach is to use a layer of thermally evaporated and uniformly deposited Ag, which is then annealed to create Ag NPs with typical diameters of 100 to 150 nm. Ag NPs create the electrical contacts and improve light scattering. An Al2O3 layer is used to passivate the Mo/CIGS rear surface which also prevents the NPs from completely dissolving into the absorber during the absorber deposition. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analysed in this paper. This work received funding from the European Union?s H2020 research and innovation program under grant agreement No. 715027.

Resume : One of the recent evolutions of CIGS technology is thin absorber layer deposited on flexible substrates. Contrary to ?classical? CIGS solar cells, those substrates don?t provide an alkali source, which has proven to be a factor for efficiency increase. This lack of alkali source is replaced by alkali post deposition treatment (PDT) right after the absorber deposition on the front rear side of the cell. As the deposition process evolves, it becomes necessary to study the alkali elements diffusion over the CIGS absorber layer but also to understand the surface chemistry right after the post-deposition treatment. This work addresses those two critical issues by implementation of XPS measurements as an efficient tool to perform this control by the determination of the composition and chemical environments in constancy and complementary of GD-OES measurements bringing information on alkali depth distribution. The comparison between NaF and KF PDT will be emphasized. Both as deposited samples, to conserve the initial CIGS chemical surface, and freshly rinsed, to reproduce the process stage before the buffer layer deposition, are investigated. Specific spectroscopic signatures and alkali incorporation are observed depending on the nature of the alkali. With air ageing unusual surface chemistries are evidenced, with the absence of oxidized Se contribution and different alkali mobility toward the surface are shown. These process particularities will be discussed here.

Resume : Inhomogeneities in Cu(In,Ga)Se2 thin films have been reported to lead to band-gap or electrostatic potential fluctuations, which may reduce the photovoltaic performance of the corresponding solar cells via enhanced recombination. The issue of where these inhomogeneities occur in the Cu(In,Ga)Se2 absorber has so far not been discussed in detail in literature. The present work gives an overview of spatial variations in composition, net doping, and lifetime on various scales, also with respect to their occurrences at interfaces and extended structural defects. Impacts of these spatial variations on the device performance of the corresponding solar cells are discussed. It can be shown that compositional inhomogeneities possibly affecting the device performance are only present at (partial) dislocations and at the Cu(In,Ga)Se2/buffer interface, and that inhomogeneous distributions of excess charges at line/planar defects as well as of net doping concentrations affect considerably the potential landscape within Cu(In,Ga)Se2 thin films.

Resume : Electrostatic potential fluctuations in Cu(In,Ga)Se2 solar absorbers may enhance the recombination of excited charge carriers and therefore may be one origin of the limited open-circuit voltage in the corresponding solar cells. The present contribution reports on lateral fluctuations of the widths of the space-charge region, w_SCR (proportional to the net doping density N_A^-0.5), and of the diffusion lengths L_D (proportional to the lifetime of electrons ?^0.5) in Cu(In,Ga)Se2 solar cells, which were accessed by means of electron-beam induced current (EBIC) measurements in a scanning electron microscope appliedusing on cross-sectional specimens. Solar cells with low ([Ga]/([Ga] [In])=0.30) and high ([Ga]/([Ga] [In)=0.66) Ga content and with various chemical-bath-deposited buffer layers (CdS, Zn(O,S), and InxSy) were analyzed . Complementary energy-dispersive X-ray spectroscopy (EDX) were carried out, which show that apart from the Ga/In gradients, no substantial, spatial changes in composition are present in the investigated Cu(In,Ga)Se2 thin films. Therefore, we rule out the presence of relevant band-gap fluctuations. By extracting EBIC profiles acrossperpendicular to the p-n junction, it was revealed that w_SCR and L_D exhibit substantial fluctuations depending on the utilized buffer layer, while for devices with the same buffer layer, the Ga concentration in the Cu(In,Ga)Se2 thin film has no influence on the average fluctuation amplitudes. The average values for w_SCR and L_D (about 400-500 nm and 1-2 µm) were confirmed by capacitance-voltage and quantum efficiency measurements.

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Resume : There have been increasing interests of Cu(In,Ga)Se2 (CIGS) thin film solar cells due to their high energy conversion efficiency among thin film solar cells. CIGS solar cells consist of several layers and resulting interfaces, whose optical and electrical properties can greatly affect the performance of the devices. The structural and compositional properties of CIGS absorber depend on characteristics of molybdenum (Mo) back contact. Although properties of Mo thin films have been widely studied for CIGS solar cells, a step toward deeper understanding of effects of Mo on CIGS solar cell is necessary for ensuring further progresses. . In this work, we have investigated influences of Mo back contact on CIGS solar cells. Mo layers with two different orientations, 110- and randomly orientations, were deposited on the soda-lime glass using DC magnetron sputtering. CIGS absorbers were prepared by a three-stage co-evaporation. To confirm effects of the orientations of Mo layers on the CIGS absorbers and resulting devices, we performed structural and compositional analyses on both of the Mo layers and CIGS absorbers. With in situ thermo-Raman spectroscopy and secondary ion mass spectroscopy, the randomly oriented Mo back contact was found to release a greater amount of alkali ions (i.e., Na and K) into the CIGS absorber than the 110-oriented one. Accordingly, the different Na incorporations induced by the orientation of Mo back contact turned out to significantly influence performances of the resulting devices, indicating the CIGS solar cells with randomly-oriented Mo exhibited superior device performances to the other. Device characterizations, including temperature-dependent IV, CV, admittance spectroscopy, and drive-level capacitance profiling, revealed that the alkali ions? release by the orientation of Mo back contact affected not only carrier transport at the CIGS/Mo interface but also recombination mechanism in the space charge region. The more details will be discussed in the conference.

Resume : An advantage of metallic substrates over polyimide foils is the excellent temperature tolerance due to their high melting point. Hence they are well suited for the deposition conditions of CIGS solar cells. However, while Mo foil can be used as both the substrate and back contact in this system, the stainless steel despite the necessity to include a diffusion barrier layer remains economically more interesting. In this work we will compare the impact of flexible molybdenum and stainless steel foils on the properties of the CIGS solar cells. In previous work, efficiencies up to 14 % with Mo foil were reached while the preliminary tests on stainless steel without an impurity diffusion barrier layer yielded cell efficiencies up to 12.2%. In order to have a better understanding of these low efficiencies and to improve them, 4 different stainless steel substrates were characterized and compared to Mo foil. The CIGS absorber layers were deposited at 480°C and 550°C. Both substrates and CIGS absorber layers were characterized by XRD, GD-OES, SEM, AFM, and optical measurements. It will be shown that regardless of the benefits of alkali treatment, such as Na and K, or Ga gradient, which can improve cell performances, a crucial factor influencing the increase of the efficiencies of our cells remains the impact of the back contact. The impact of the different characteristics of the substrates on the cell performance will be discussed.

Resume : We studied crystallographic and optical properties of CuInSe2, CuIn3Se5, and CuIn5Se8 in Cu2Se-In2Se3 system [1]. The crystal structure changed from chalcopyrite to stannite-type with increasing In2Se3 content. CuIn3Se5 and CuIn5Se8 had a wider band gap than that of CuInSe2 and had a deeper valence band maximum (VBM) from the vacuum level. In Ag2Se-In2Se3 system [2], AgIn5Se8 exists in the Ag-poor side of AgInSe2. In this study, we prepared (Cu1-xAgx)InSe2 and (Cu1-xAgx)In5Se8 solid solutions and characterized their crystallographic and optical properties. Chalcopyrite-type (Cu1-xAgx)InSe2 and stannite-type (Cu1-xAgx)In5Se8 were successfully synthesized by mixing the elemental powders and sequential heating in N2 gas. Tetragonal lattice constants a and c of (Cu1-xAgx)InSe2 and those of (Cu1-xAgx)In5Se8 increased with increasing Ag content. The band-gap energy of the (Cu1-xAgx)InSe2 system increased from 1.00 eV for x=0.0 to 1.22 eV for x=1.0. For the (Cu1-xAgx)In5Se8 system, the band-gap energy increased from 1.21 eV for x=0.0 to 1.40 eV for x=1.0. Energy level of the VBM from vacuum level was measured by photoemission yield spectroscopy (PYS) and that of the conduction band minimum (CBM) was also determined by adding the band-gap energy to the VBM level. The VBM of the (Cu1-xAgx)InSe2 system decreased from -5.29 eV for x=0.0 to -5.80 eV for x=1.0. The VBM of the (Cu1-xAgx)In5Se8 system also decreased with increasing Ag content, while the CBM was almost constant. [1] T. Maeda, W. Gong, and T. Wada, Jpn. J. Appl. Phys. 55, 04ES15 (2016). [2] G. Petzow and G. Effenberg, Ternary Alloys, VCH Verlagsgesellschaft, Weinheim, Geamany 2, 308 (1988).

Resume : CuIn1-xGaxSe2(CIGS) has been considered as one of the best candidates in opto-electronic devices for photovoltaics with high-conversion efficiency. For decades, many efforts have been conducted to increase the efficiency of CIGS solar cells by improving the major factors which are short-circuit current density(Jsc), fill factor(F.F) and open-circuit voltage(Voc). Another research is also underway to develop CIGS solar cells on flexible substrates like a stainless steel, polyimide, etc. In the case of SUS, impurities (Ni, Cr and Fe) diffused from the substrate cause deterioration of the characteristics of the device. Therefore, it is very important to deposit a diffusion barrier onto substrate to prevent the diffusion of impurities, and also to analyze the defect of the devices due to impurities. Deep-level transient spectroscopy (DLTS) is a very useful method for defect analysis in the devices. In this research, a defect analysis study of the device was performed using DLTS equipment with or without diffusion barrier.

Resume : We reported alkali-metal effects of Li, Na, and K in CuInSe2 (CISe) and CuGaSe2 (CGSe) studied by first-principles calculation [1]. The substitution energy of Li for Cu (LiCu) was much lower than those of NaCu and KCu. Then, we successfully synthesized (Cu,Li)InS2 [2] and (Cu,Li)In(S,Se)2 [3] solid solutions. Chalcopyrite-type samples were obtained for the (Cu1-xLix)InS2 with 0.0 ? x ? 0.1 and the (Cu1-xLix)In(S0.5Se0.5)2 with 0.0 ? x ? 0.2. In this study, we synthesized (Cu1-xLix)GaS2 samples and characterized their crystallographic and optical properties. Single-phase chalcopyrite-type (Cu1-xLix)GaS2 solid solution could be obtained for 0.0 ? x ? 0.35. Crystal structures were analyzed by Rietveld refinement using XRD data. The tetragonal lattice constants a and c increased with increasing Li content, x. The band gap energy linearly increased from 2.44 eV for x = 0.0 (CuGaS2) to 2.67 eV for x = 0.35. The ionization energies were measured by photoemission yield spectroscopy (PYS). Energy positions of the valence band maximum (VBM) and conduction band minimum (CBM) were determined from the ionization energies and band bap energies. The VBM level of (Cu1-xLix)GaS2 solid solution slightly decreased with increasing Li content, x and the CBM level slightly rose. [1] T. Maeda, A. Kawabata, and T. Wada, Jpn. J. Appl. Phys. 54, 08KC20 (2015). [2] T. Maeda, C. Zhao, and T. Wada, Thin Solid Films 633, 172 (2017). [3] T. Kusumoto, T. Maeda, and T. Wada, submitted to Jpn. J. Appl. Phys.

Resume : Micro concentrator solar cells theoretically have higher power conversion efficiency and use less precious semiconductor than normal large area non-concentrated solar cells. Cu(In,Ga)Se2 is suitable for such cells due to its high efficiency potential, and because it can be fabricated in micron sized areas using a two-step synthesis. We employ electrodeposition, which has a material utilization efficiency of over 90%, to deposit arrays of 200 µm diameter dots of Cu, In, and Ga metal stacks. After selenization, Raman spectra show a peak at 174 cm-1 and room temperature photoluminescence spectra show a radiative transition at 1.02 eV, both consistent with CuInSe2, suggesting that gallium is segregated at the back contact. First devices show a power conversion efficiency of 1.8% with an open-circuit voltage (Voc) of 182 mV and a short-circuit current (Jsc) of 24.2 mA cm-2 under 1 sun. External quantum efficiency measurements reveal a band gap of 1.0 eV consistent with a front surface of CuInSe2. To study the performance under concentrated light, the cells were illuminated with 650 nm light over 3 decades of intensity. From 1 to 33 suns, the efficiency rises from 1.8 to 4.7% due to the increase in Voc, and a maximum diode factor of 2.6 is calculated. These results are comparable to other selective area deposited CuInSe2 state of the art micro solar cell devices.

Resume : The power conversion efficiency of Cu(In,Ga)Se2 (CIGS) based solar cell is improved for slightly Cu-poor thin films. Besides the deviation from stoichiometry the distinct atomic species induce a large diversity of native point defects and compositional fluctuations. Thus, Cu-poor CIGS is typically highly doped and highly compensated, and its optoelectronic properties are governed by the presence of electrostatic and bandgap fluctuating potentials. A set of CIGS samples with a constant value of [Ga]/([Ga]+[In])~0.3 and without any Cu-transitions, were prepared. The [Cu]/([Ga]+[In]) (CGI) ratios studied were 0.53, 0.71, and 0.84. The electrical performance of the samples was studied by J-V measurements, and the experimental evidences of fluctuating potentials influence were discussed from external quantum efficiency (EQE) and photoluminescence (PL) measurements. The samples with the best electrical performance are the ones which present a lower effect of fluctuating potentials relatively to the other two samples. With decreasing electrical performance the effect of fluctuating potentials increases. A clear relation between the electrical performances of the resulting solar cells with the average depth of fluctuating potentials is demonstrated and discussed.

Resume : Latest record efficiencies of Cu(In,Ga)Se2 (CIGS) solar cells were achieved by means of a rubidium fluoride (RbF) post-deposition treatment (PDT). To understand the effect of the RbF PDT on the surface chemistry of CIGS and its interaction with sodium that is generally present in the CIGS absorber, we performed in-situ X-ray photoelectron spectroscopy (XPS) on RbF-treated CIGSe thin films as-deposited by a three-stage co-evaporation process and after RbF-PDT. The sample transfer from the deposition to the XPS analysis chamber was in part performed via an ultra-high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali-treated absorbers. Beside an expected reduction of Cu and Ga at the surface of RbF-treated CIGSe films, we find that Rb penetrates at the grain boundaries of CIGSe and, contrary to the fluorine, it is not completely removed by ammonia etching. The remaining Rb contribution at 110.56 eV may hence belong to an Rb-In-Se phase, since the shift in the Indium Auger transition is similar to the one previously ascribed to K-In-Se. Na is driven towards the surface of CIGSe absorber as a direct result of the RbF PDT. Although both RbF and air exposure represent a driving force for Na, a remaining signal is observed after etching with ammonia. This indicates that air exposure, i.e. oxygen and/or oxides from the CIGSe surface, ensure a complete transfer of sodium from bulk CIGSe to its surface.

Resume : A wide variety of thin film growth methods have been used to fabricate Cu(In,Ga)Se2 (CIGS). The most versatile one is co-evaporation, which allows for a near-perfect control over elemental distribution in time and in depth of the film. An optical study by photoluminescence (PL) of the influence of different Ga profiles on the electronic structure of CIGS is presented. We prepared a set of CIGS samples with different [Ga]/([Ga]+[In]) (GGI) profiles: i) a three-stage sample (A) with a typical Ga notch profile; ii) a one-stage sample (B) with a decreasing linear GGI profile, from 55% at the back to 25% at the front; iii) a flat-gradient sample (C) with flat evaporation profiles with a constant value of GGI~0.3. Despite a blueshift of the luminescence with the increase of the excitation power, striking differences were observed for the three samples. For sample A, two emission bands are observed in the range 0.92-1.16 eV, being asymmetric the one located at a low energy position which progressively dominates the spectrum as the excitation power increases. For sample B, at low excitation power values, a single asymmetric band is observed, whereas with the increase of the excitation power, two new bands are evident for higher values of energy. For sample C, two asymmetric bands are always observed in the excitation power range investigated. The influence of the different Ga profiles on the fluctuating potentials is discussed and related with the J-V and external quantum efficiency results of the resulting solar cells.

Resume : Thin film solar cells made of Cu(In,Ga)Se2 (CIGSe) suffer from high costs, since the rare elements indium and gallium are used for a multitude of technical applications. The concept of micro-concentrator CIGSe solar cells allows substantial material saving. Therefore a reliable bottom up process for the preparation of the absorber islands is required. In addition, this also results in an increase of solar cell efficiency. We show that the indium islands can be grown at predefined locations by novel methods of surface treatment before indium deposition. By this, regular island patterns with a well-defined morphology have been obtained. The controlled deposition of copper, followed by selenization in a PVD system, and selective etching of copper selenides yields polycrystalline CIGSe micro islands with promising material properties. For the samples, we proof ways to place a non-conducting layer in between CIGSe islands to insulate the front and back contact. This layer is obtained by spin-coating and curing a novolak based resist. At first, the photovoltaic active absorber islands is completely covered by the resin. The top of the islands can be stripped by plasma etching for front contacting. A characterization of the obtained devices provided evidence for their functionality.

Resume : It is known that post-deposition treatment (PDT) with heavy alkalis of small bandgap Cu(In1-x,Gax)Se2 (x ~ 0.3) solar cells improves the electrical parameters. In this work we investigate the influence of heavy alkalis (KF, RbF and CsF) on wide bandgap CIGSe samples. It is found that the PDT leads to a reduced VOC-deficit and thus allows the open-circuit-voltage (VOC) to partially overcome its typical limitations for high gallium concentrations. The study of red-light VOC(t) transients, where t is the time, allows to differentiate between dominant bulk and interface recombination. No-PDT samples show a change from bulk (dVOC/dt>0) to interface recombination (dVOC/dt< 0) as the bandgap increases. KF treatment partiallly quenches interface recombination. Comparing CIGSe devices (same Eg) with different alkali treatment, a different slope (dVOC/dt) was measured. Wide bandgap CIGSe samples treated with heavy alkalis (RbF and CsF-PDT) illustrate a smaller slope in comparison to no-PDT samples. Independent from the alkali element, the open circuit voltage increases for cells with a bandgap value of 1.4 eV and reduces sharply for higher bandgap values. In addition, the CIGSe devices treated with heavy alkalis of RbF and CsF-PDT show a higher open circuit voltage compared to the case of KF-PDT or no-PDT.

Resume : Among the various efforts to improve the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells, the control of energy levels has been a key issue because the depth directional profile of energy levels largely affects the carrier transport. Nowadays, thus, the In-Ga-Se/Cu-Se/In-Ga-Se multi-stage CIGS co-evaporation process has been widely used to make moderate Ga-gradient in CIGS films because the Ga-to-In ratio determines the energy band gap and the conduction band minimum. The world record efficiencies are achieved mainly using this technique. However, conventional multi-stage technique should be modified when using heat-vulnerable plastic substrates such as polyimide (PI) since the elemental inter-diffusivity substantially changes with temperature. In this work, we engineered the process parameters and suggest an optimal elemental profiles in CIGS absorber for highly efficient solar cells at limited thermal budget. The CIGS thin films were fabricated by using independently controlled Cu, In, Ga, and Se effusion cells. The evaporation flux and duration time of each element are controlled at different substrate temperatures under 450 °C. The elemental compositional profiles in depth direction were successfully controlled. The properties of the fabricated films were characterized by a secondary ion mass spectroscopy, an X-ray diffraction, a scanning electron microscopy, and an energy dispersive spectroscopy. To measure the photovoltaic performances, we measured a current density-voltage curve on the solar cell devices with a structure of Al/Ni/ITO/Zn(O,S)/CIGS/Mo/PI. The slope, minimum point position, and height of the compositional Ga-to-In ratio largely influenced on the minority carrier transport and eventually the solar cell efficiency.

Resume : The present study was conducted in order to shed light on the question which are the microscopic material properties in a high-efficiency Cu(In,Ga)Se2 (CIGS) solar cell. A CIGS device with stacking sequence ZnO:Al/(Zn,Mg)O/CdS/CIGS/Mo/glass, of which the CIGS absorber underwent a RbF postdeposition treatment, was investigated. Its conversion efficiency was almost 21% without antireflection coating. Electron backscatter diffraction (EBSD), energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements were performed on cross-sectional specimens in order to get insight to microstructural, compositional, electrical, and optoelectronic properties. The average grain size was determined to 0.5 µm by EBSD. From the EBIC results, the width of the space charge region, wSCR, and the minority-carrier diffusion length in the quasi-neutral region, LD, were determined. Average recombination velocities at grain boundaries calculated from the CL data were about 5x10^3 cm/s in average. EDX elemental distribution maps did not show any substantial change in composition in the lateral direction; only the Ga/In gradients perpendicular to the substrate were detected, which can be correlated well with the peak shifts visible in the hyperspectral CL map. In contrast, w_SCR and L_D were found to be inhomogeneously distributed along the p-n junction, indicating lateral fluctuations in the net doping and the carrier lifetime. The present contribution discusses these fluctuations as possible origin for the limited open-circuit voltage of CIGS solar cells.

Resume : Most of the Cu(In,Ga)Se2 (CIGS) absorbers for high-efficiency solar cells have been formed by physical vapor deposition as typified by the ?3-stage process?. In the first stage of the ?3-stage process?, a (In,Ga)2Se3 precursor layer which was the framework of the CIGS crystal was prepared by the deposition of In, Ga, and Se atoms [1]. The preferred orientation of CIGS films could be controlled by controlling the orientation of the (In,Ga)2Se3 precursor layer [2]. In this study, we investigated the crystal structures and optical properties of (In,Ga)2Se3 system. We prepared the (In1-xGax)2Se3 powders with 0.0 ? x ? 1.0 by mixing the elemental In, Ga, and Se powders and heating at 550? in N2 gas. In2Se3 (x=0.0) has a layered defect wurtzite-type crystal structure [3] and Ga2Se3 (x=1.0) has a defect zinc-blende-type structure. The XRD patterns showed that the (In1-xGax)2Se3 have a layered defect wurtzite-type structure for 0.0 ? x ? 0.6 and ideal wurtzite-type structure for x=0.7. The samples with 0.8 ? x ? 0.9 are a mixture of ideal defect wurtzite-type (In,Ga)2Se3 and defect zinc-blende-type Ga2Se3. The direct band-gap energy of the (In1-xGax)2Se3 increased from 1.95 eV for x=0.0 (In2Se3) to 2.20 eV for x=0.5 and then decreased to 1.85 eV for x=0.6. The samples with 0.7 ? x ? 1.0 have the indirect band gap and their band gap energies increase with Ga content. On the basis of the present results, the deposition of high quality CIGS films with high Ga content is discussed. [1] T. Wada and T. Maeda, Jpn. J. Appl. Phys. 50, 05FA02 (2011). [2] S. Chaisitsak, A. Yamada, and M. Konagai, Jpn. J. Appl. Phys. 41, 507 (2002). [3] A. Pfitzner and H. D. Lutz, J. Solid State Chem. 124, 305 (1996).

Resume : This contribution demonstrates how replacing the conventional intrinsic ZnO 2nd buffer in Cu(In,Ga)Se2 (CIGS) solar cells with a tailored Zn1-XMgXO (ZMO) film enables the use of an optically superior ZnO:B (BZO) transparent conductive oxide (TCO) as front electrode, instead of the standard ZnO:Al, while maintaining Voc and FF. The ZMO is deposited in a pulsed DC sputtering process from a compound target. By tuning the Mg content of the sputter target and the ZMO film thickness, the band structure can be designed in such a way that the use of BZO in the device stack is possible, without losing Voc. For our Mo/CIGS/CdS cell stack the ZMO process window is quite broad, with an optimal [Mg]/([Zn]+[Mg]) ratio of 0.05-0.07 and film thickness around 100nm. The BZO layer is produced in an industrial size prototype low-pressure chemical vapor deposition tool, and has excellent optical properties also at lower sheet resistances of about 10 ?/?. IV, EQE, XRD and optical measurements illustrate how the absorption losses in the TCO are reduced, due to the higher carrier mobility, sharper absorption edge in the blue region and advantageous film texture of the BZO. The increased current output corresponds to an absolute efficiency gain of around 0.8% units, compared to the standard window layer (WL). As proof of concept we have produced non-gridded cells (0.11cm2), implementing this improved WL on absorber material from our pilot production line, reaching efficiencies up to 21% (AR-coated).

Resume : The difference of open-circuit voltage (Voc) compared to the Shockley-Queisser limit is an indicator of the recombination losses in a solar cell device. Its computation requires the precise knowledge of the absorber bandgap, especially for absorber alloys such as Cu(In,Ga)Se2, Cd(Te,Se), Cu2ZnSnSe4 or perovskite. However the characterized value of the bandgap depends on the choice of the determination method, and comparison of Voc deficits reported by various sources remains ambiguous. In this contribution we review methods for bandgap determination, such as processing of the quantum efficiency curves (notably inflection point and Tauc plots) and optical measurements of bare absorbers. The bandgap determination methods are compared for different solar cell technologies, and we report systematic differences in the bandgap value related to the choice of determination method. The presented results should facilitate the analysis and interpretation of bandgap and Voc deficits values reported by various sources.

Resume : To achieve high efficiency utilizing Cu(In,Ga)Se2 (CIGS) absorber layers alkaline incorporation to the CIGS layers is mandatory. Generally for high efficiency devices, alkaline elements are introduced by diffusion from the soda-lime glass (SLG) substrate and by post deposition treatment (PDT) with alkaline fluoride deposition. In this work, CIGS layers are grown on substrates of polyimide and SLG covered with SiOx alkaline diffusion barrier by the multi stage thermal co-deposition. Co-evaporation of alkaline fluorides was attempted during the growth processes of CIGS layers in addition to the PDT. SEM observation revealed that the microstructure changed with NaF co-evaporation during the 3rd stage, in which In, Ga, and Se are co-evaporated. With increasing the temperature of the NaF effusion cell, faceted morphology at film surface altered into rounded morphology. It was shown that co-evaporation of NaF during 3rd stage improved the efficiency of CIGS solar cells. Compared to samples with only PDT (NaF followed by RbF), alkaline fluoride co-evaporation increases in efficiency from a baseline of about 19% to 20%. A correlation between the efficiency and the composition ratio of [Cu]/([In]+[Ga]) was suggested.

Resume : Building-integrated photovoltaics (BIPV) are receiving increased attention as a feasible way to generate renewable electrical power. For the BIPV window applications, highly stable, efficient and transparent for visible light solar cells are essential. In terms of high stability and power conversion efficiencies (PCEs), CuInSe2 (CISe) thin film solar cells are one of the potential candidates for BIPV applications. But there are some challenges to transparency. General approach to semi-transparent solar cells is using ultrathin absorber layers or to remove some parts of absorber layers on transparent conductive oxide (TCO) as a back electrode. The former is easily limited to provide light transmission in the infra-red region and difficult to achieve high efficiency due to high back surface recombination, while the latter may result in material losses. To tackle this problem, in this study, we demonstrated selectively deposited CISe thin films on patterned Mo back contact using electrochemical deposition enables the material savings without the need of further removal of absorber layer. Furthermore, to reduce the back electrode interface recombination and high shunt conductance caused by the edge of patterned Mo, we introduced a passivation layer in the edge area of patterned Mo. As a consequence, we present a first demonstration of a novel fabrication process for semi-transparent CISe solar cells with upto 30% transparency and over 8% of PCE.

Resume : Wide band gap pure sulfide Cu(In,Ga)S2 (CIGS) solar cells are attracting increased attention since the world record efficiency of 17.6% achieved by Solar Frontier for a band gap of 1.55 eV, using a two-step process. Such material is motivated by implementation in next-generation high-efficiency tandem solar cells as top cells, thanks to their wide band gap. The present paper reports on 1.6-1.8 eV wide gap CIGS absorbers successfully made by a two-step process, implementing the electrodeposition technique instead of sputtering. The sulfurization of the stacked Cu-In-Ga metallic precursors (deposited at room temperature, under atmospheric pressure, up to 15x15 cm2) was performed thermally using H2S gas. We investigate the influence of the composition of the precursors and thermal annealing parameters on the properties of the CIGS absorber as well as on the device performance. Increasing the Ga content allowed us tuning the absorber band gap up to 1.8 eV, which is the widest band gap reported using two-steps process. On the other hand, the morphology and crystalline structure were found to strongly depend on the copper content. An optimization of the processing parameters allowed us to successfully fabricate compact and crystalline wide gap CIGS absorbers without parasitic phases. The resulted devices have shown very promising characteristics, Voc as high as 760 mV and Fill Factor of 60.7 %, for a present efficiency of 5% at 1.6 eV were recorded. In depth characterization of materials at different processing stages and devices (Raman, GXRD, GDOES, PL, SEM, Spectral responses and IV?) will be presented, revealing the great potential of this technology for industrial application.

Resume : At the emergence of new concepts based on multijunction solar cells, wide band gap pure sulfide Cu(In,Ga)S2 (CIGS) solar cells have attracted a lot of attention, in particular for implementation in tandem solar cells with Si as top cells. Herein, wide gap CIGS solar cells were fabricated using the same industrial process as Solar Frontier. First, the CIG metallic precursors were deposited in different stack configurations by means of sputtering. The second step consists in sulfurization under H2S-feeled atmosphere. We show that the layer stacking, chemical composition and annealing parameters play a key role on the properties of the CIGS absorber and device performance. Copper-poor absorbers exhibit In2S3 parasitic phases while Cupper-rich ones result in Cu-based secondary phases on both surface and bulk. The chemical composition as well as the deposition sequences were then optimized so as to obtain a compact and crystalline wide gap absorber without parasitic secondary phases. On the other hand, controlling the Ga content and its gradient into the absorber have allowed monitoring the band gap from 1.5 to 1.7 eV. Finally, our best wide gap CIGS cell have shown a 7.3 % efficiency, a Voc of 690 mV, a Jsc of 17.2 and FF of 61%. Thanks to advanced characterization techniques available at IPVF we could identify the limiting factors and evaluate directions for improvements.

Resume : Abstract: Spectral conversion has been widely applied in the field of luminescence and is potentially beneficial in photovoltaics. Among different approaches, down conversion shows a theoretical potential to enhance the limit of energy conversion efficiency of a single-junction solar cell from about 32% to 40%. For CIGS solar cells, the main current loss in the short wavelength region (<550 nm) comes from absorption of the CdS buffer layer with a relative small bandgap. As the spectral response of CIGS has an optimal efficiency in the longer wavelength (550-1000 nm) region, spectral down converter that converts photons with wavelength shorter than 550 nm to longer wavelength at a high luminescent efficiency has the potential to reduce the above current loss for CIGS solar cells. In this work, Ce3+ and Yb3+ doped perovskite nanocrystals are successfully synthesized and applied as the down convertor for CIGS solar cells. Their absorption and photoluminescence properties have been investigated. The device conversion efficiencies of CIGS solar cells with and without the down converter have been measured and compared. The results show that Benefiting from the higher inner luminescent quantum yield, the down convertor could enhance the efficiency of CIGS solar cells effectively.

Resume : CuIn0.7Ga0.3Se2 (CIGS) thin films were deposited by using one-step radio-frequency magnetron sputtering with different substrate temperatures onto glass substrates and an annealing process for improving the crystallinity and the morphology of the prepared films was performed. The annealing step was performed in argon gas atmosphere at 300°C for 1h. The influence of annealing process and the substrate temperatures on the crystalline quality as well as structural, optical and electrical properties of thin layers obtained has been studied. X-ray diffraction showed that the annealed films were highly orientated in the (112) and/or (204)/(220) direction. In2Se3 secondary phase was observed on the samples grown at lower substrate temperatures. The surface morphology of CIGS layers studied by Atomic Force Microscopy (AFM) and Scanning Electronic Microscopy (SEM) has been also discussed. The most surprising and exciting outcome of this study was that the as grown films were of near stoichiometric composition after annealing. Resistivity measurements were carried out using the four point probe method. The optical absorption showed that energy gap values are between 1.13 and 1.18 eV and rather sharp absorption fronts. Thin films resistivities are between 10.7 and 60.9 Ω.cm depending on the experimental growth conditions and annealing. Keywords : Cu(In,Ga)Se2, thin films, rf-magnetron sputtering, Annealing process, Secondary phases.

Resume : Invest cost for industrial mass production CIGS coating tools contributes substantially to the cost of CIGS PV technology. An increased process speed is an effective leverage to increase the output of a single tool and thus reduce the invest cost for a given nameplate fab capacity. In this contribution, the increase of deposition speed, its effect on CIGS layer and device properties on an industrial scale coevaporator tool was investigated. We found that peak cell efficiencies around the center of deposition are less affected, but lateral non-unformities and non-ideal deposition conditions towards the deposition edges increase with increased deposition speed. Whereas the depth dependent element distribution is actually little affected, the film roughness and electronic quality deteriorate to some degree. Optimization efforts in the process for increased deposition speed focused on Ga depth distribution, deposition temperatures, and alkali content. Finally, an equal module efficiency at 20% increased deposition speed could be achieved. By this acceleration, the total equipment invest for a CIGS turnkey line can be reduced by about 7%

Resume : The search for material combinations with suitable energy band alignments is an important challenge in the development of new PV technologies. In this work it is shown how changes in bulk defect chemistry can dramatically influence the functional properties of an interface, even within the same material system. One striking example for this is the Cu2O/ZnO interface, for which many different values for the band offset have been reported [1]. The origin of this phenomenon is explored in a systematic study conducting multiple in-situ photoelectron spectroscopy interface experiments. A large process dependent variation in energy band alignment is observed, with valence band offsets ranging from ?EVB = 1.45 ? 2.7 eV [2]. This variation can be ascribed to a pinning of the Fermi level in ZnO and Cu2O, which can be traced back to oxygen vacancies in ZnO and to metallic precipitates in Cu2O, respectively. The intrinsic valence band offset for the interface, which is not modified by Fermi level pinning, is derived as ?EVB ? 1.5 eV, much lower than previously reported values in literature. The results from this study explain the variation of ?EVB at the Cu2O/ZnO interfaces reported here and in literature, but more importantly provide a general strategy to control the energy band alignment at semiconductor heterointerfaces. [1] M. Ichimura et al. Japanese Journal of Applied Physics, 50 (5), 5100, 2011 [2] S. Siol et al. ACS Appl. Mater. Interfaces, 8 (33), 21824?21831, 2016

Resume : Solar cells based on (Ag,Cu)(In,Ga)Se2 (ACIGS) with varying Ag/(Ag+In) ratio from few percent up to 50 % have been studied in this contribution. The substrates were either a high strain point, K-containing glass or a normal soda lime glass. The efficiencies of the solar cells have been in the range of 18 % or above. Interdiffusion of silver and copper in ACIGS was investigated. Ag was added to the co-evaporation of the ACIGS during one of the three stages in the co-evaporation. In contrast to interdiffusion of Ga and In, which has previously been found to be slow in presence of alkali metals and in particular in presence of K, interdiffusion of Ag and Cu is found to be rapid. Even if Ag is added only in the first stage, corresponding to about 25 % of the total ACIGS thickness, the final profile as analyzed by glow discharge optical emission spectroscopy was found to vary less than 10 % in depth. Similar to solar cells based on co-evaporated CIGS layers, cells with ACIGS layers benefit from a post deposition treatment (pdt) with alkali fluorides, such as KF. We have investigated the surface properties of ACIGS with KF pdt after an ammonia etch (a similar treatment as in the beginning of the chemical bath deposition) by X-ray Photoelectron Spectroscopy and Transmission Electron Microscopy. The results point to that the surface reactions for the ACIGS surfaces as compared to the CIGS surfaces are slightly different. In addition to the formation of GaF3, which is washed away together with remaining KF by the ammonia treatment, also Ag-Se phases are formed. This may explain the increased sensitivity for ACIGS to excess KF leading to reduced voltage and fill factor.

Resume : Up to now, sulfide chalcopyrites lead to less efficient solar cells compared to the selenide counterparts. We have investigated the Cu2S-In2S3-Ga2S3 ternary system through a chemical crystallography approach (X-ray diffraction and chemical analyses) on bulk materials and established the phase diagram of these compounds. The general formulation of the targeted compounds (high temperature ceramic route) is Cu1 z(In1 xGax)1 z/3S2 (0

Resume : Reactive thermal annealing of electrodeposited precursors in elemental Se and S is an industrially attractive technology to produce CIGS thin film solar modules, due to the comparably low equipment costs and high metal utilization. However, effort is still required to obtain the double graded band-gap, with a minimum at 1.15 eV, that enables high-efficiency CIGS. We have demonstrated a bandgap optimization to 1.15 eV, giving an open-circuit voltage above 650 mV, by the adaptation of the gallium and sulphur profile in sequentially deposited Cu(In,Ga)(Se,S)2 absorbers in a 30x30 cm2 research line. Electro-optical modelling shows that straightforward selenisation of electrodeposited precursors results in well-behaved CuInSe2/CuGaSe2 bilayer absorbers. By pre-alloying before and extended soaking after selenisation the bandgap was increased from 1.00 to 1.10 eV. The pre-alloying stabilizes the electrodeposited precursors and by soaking the Ga-profile can be tuned. The minimum bandgap was further increased to 1.15 eV by reacting Cu(In,Ga)Se2 with elemental S to form Cu(In,Ga)(Se,S)2 with a few percent S near the junction. This bandgap and the maximum Voc of 665 mV obtained are higher than values earlier reported for semi-industrial routes based on elemental Se and S reactive thermal annealing. A 15.4% maximum active area efficiency (0.5 cm2) and an 13.3% average efficiency over 10x10 cm2 were achieved.

Resume : A proper control of Ga concentration profile is mandatory to achieve high efficiency Cu(In,Ga)Se2 (CIGS) solar cells. Deep gradients, detrimental for carriers diffusion, are obtained when CIGS is deposited with a standard three-stage process at low temperature on flexible substrates: an optimization of the process is needed. In this study, we show the impact of the depth of the notch by introducing Ga flux during the second stage from 0 nm/min to 1.1 nm/min. We show a better diffusion of the carrier by EQE measurements performed with bias and obtain a higher VOC compensated by a lower JSC du to a higher band gap energy. We then analyze CIGS growth during the second stage by coupling different characterization techniques: GD-OES, Raman and XRD on various absorbers extracted from the deposition chamber at different moments of the second stage. We were then able to compare the growth mechanisms before and after the optimization. We show a better crystallization and morphology for an absorbers grown with an optimized three-stage process. We also show the presence of less binary compounds at the end of the second stage by Raman spectroscopy. Thus, we manage to explain why less deep Ga grading profiles are obtained when Ga is evaporated during the second stage. To conclude, we discuss the impacts on the growth of CIGS during the last stage. Further investigations of the front grading allowed us to achieve a 17.8% solar efficiency without a KF post-deposition treatment and ARC.

Resume : With a tunable band gap and high efficiency, Cu(In,Ga)Se2 (CIGS) solar cells have shown great potential for thin film tandem solar cells. For combining them with perovskite top cells, the bottom cell should preferably be based on a 1.0 eV absorber layer, achieving high efficiency and covering a broad spectral range. In this work we present the development of about 18% efficient CIS based bottom cells with an optoelectronic bandgap of 1.0 eV. The cells consist of a low bandgap front for a large part of the absorber as relevant for tandem applications, while a Ga back grading is applied towards the Mo contact to reduce the recombination loss. By Ga back grading and modifying the doping and the front interface, the open circuit voltage of those devices could be increased to above 585 mV while still keeping a high spectral response close to the 1.0 eV band gap. We demonstrate the suitability of these cells for tandem devices and show that using this approach even current matching for monolithic interconnection with perovskite top cells of 1.6eV bandgap is feasible. The tandem devices in 4 terminal configuration display a significant improvement over the single cell efficiencies.

Resume : From photoluminescence spectroscopy we can determine on a bare absorber several quality indicators of the finished solar cell: the quasi Fermi level splitting, which corresponds to the maximum open circuit voltage, the luminescent efficiency which corresponds to the voltage loss with respect to the band gap of the absorber, and the diode factor of the solar cell. We will discuss the relations between the photoluminescence data and the solar cells properties and how photoluminescence can be used to monitor absorber quality before finishing the solar cell.

Resume : It has been well known for a long time that Cu-Se deposition during the 2nd stage of the famous 3-stage Cu(In,Ga)Se2 (CIGS) co-evaporation process influences the microstructure of the final absorber layer. To gain a comprehensive understanding of the mechanisms and driving forces responsible for the microstructural changes during this stage and its effect on the absorber quality, we have performed a detailed series of in-situ studies, as well as complementing ex-situ studies in the recent past. Our studies provided detailed insights into several correlating changes of the structural and physical properties of the absorber layer during Cu-Se deposition or subsequent annealing, such as defect annihilation, stress relaxation, texture changes, and grain growth. In this contribution, we give a comprehensive overview of these various observations and demonstrate how they can all be explained by a single model of abnormal grain growth, driven by differences of the internal energy of neighboring grains. One of the important consequences is that this abnormal grain growth leads to the reduction of structural defects within the grains. Moreover, from our model we derive limitations and perspectives for future process optimizations of Cu(In,Ga)Se2 deposition by co-evaporation.

Resume : Among the factors limiting the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells, the presence of unpassivated surfaces near the absorber/buffer interface might play an important role, due to possibly high recombination velocities in a region with large charge carrier densities. In this contribution, we report the presence of voids with diameters of up to 200 nm in the CIGS absorber layer, just below the front surface, as observed by transmission electron microscopy (TEM). CIGS thin films are grown by multi-stage co-evaporation. The deposition of a CdS buffer layer by chemical bath (CBD) leads to the passivation of some of the voids’ internal surface, whereas other voids are completely buried within the CIGS film and cannot be reached by the CBD solution. The density of voids is observed by coupled focused-ion-beam etching and scanning electron microscopy. In multi-stage co-evaporation, the CIGS film is grown non-stoichiometrically in a Cu poor - Cu rich - Cu poor sequence. The elemental distribution is analyzed during the different stages of the CIGS growth by scanning TEM coupled with energy-dispersive x-ray spectroscopy (STEM-EDX). We propose that the voids are formed during the final Cu rich to Cu poor transition, due to the out-migration of Cu and Se from non-homogeneously distributed areas with segregated Cu-Se phases. Possible pathways to eliminate or reduce the density and size of such voids are suggested. Finally, we analyze the impact of passivated and unpassivated void surfaces on photovoltaic properties by 2D and 3D simulations using the Sentaurus TCad Suite software.

Resume : The limited photovoltaic performance of the CuIn(1-x)GaxSe2 (CIGSe) thin-film solar cells, when x exceeds the threshold value of 0.4, is under investigation for more than 20 years. So far, the best labscale energy conversion efficiency is achieved for Ga content (x) around 0.3, while theoretical predictions propose a better solar cell efficiency for x around 0.75. In our work, by coupling computational and experimental studies, we introduce a new approach to explain the limited performance for large x, which involves the presence of copper-selenide (Cu-Se) detrimental phases at the bulk of the CIGSe layer. These Cu-Se compounds are created during the CIGSe deposition process and they can precipitate at the GBs or/and within the grains, acting as recombination centers. Our XRD, RAMAN and EDS analyses demonstrate that the copper-selenide behavior differs, at low and high x. Thus, it appears that for large x, Cu-rich compounds precipitate at the bulk of the CIGSe layer, while at low x, Cu-Se phases segregate mainly at the surface and so they can be easily removed by wet-chemical etching with potassium cyanide (KCN). Moreover, our study raises questions about the Cu-Se diffusion within an In-rich or Ga-rich environment. Additional TEM analyses were also carried out, in order to further increase our knowledge about the Cu-Se behavior in the CIGSe layers.

Resume : The highest Cu(In,Ga)Se2 (CIGS) thin-film solar cell efficiencies are gained by depositing CIGS absorber layers with a three-stage co-evaporation process. During the second stage, Cu-Se is deposited on CIGS and heated, transforming the CIGS layer from Cu-poor to Cu-rich in composition. It has been reported that the Cu-(In,Ga) interdiffusion between CIGS and Cu-Se layers in the second stage is essential for the recrystallization of CIGS. However, little has been known about how the atomic structures evolve during this recrystallization. In this study, Cu-Se is deposited on CuInSe2 (CIS) and the recrystallization is directly monitored during in-situ heating in a scanning electron transmission microscope (STEM), which simultaneously reveals the composition changes via electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX). The results show that Cu favours diffusion along grain boundaries. Planar defects play an important role in grain recrystallization: the grains with high-density planar defects tend to be consumed by the grains without planar defects, and this only occurs in case of excess Cu on CIS. Moreover, Cu-In interdiffusion between CIS and Cu-Se layers transforms some CIS grains to Cu2-xSe grains. The newly formed Cu2-xSe grains show a reconstructed superlattice aligned with the tetragonal lattice of the surrounding CIS grains, suggesting a low-energy transformation mechanism between CIS and Cu2-xSe.

Resume : The compound semiconductors Cu(In,Ga)(S,Se)2 (CIGSSe) is used as absorber materials in thin-film solar cells. Currently, a record efficiency of 22.9% is achieved for CIGSSe solar cells. When considering the polycrystalline nature of the CIGSSe thin-films, this efficiency is surprisingly high. For further enhancement of the efficiency it is important to understand how the microstructure affects the electrical properties of the absorber and, hence, the device performance. Therefore, one needs to study the relationship between structural and chemical properties of grain boundaries (GB). Compositional changes at GBs in Cu(In,Ga)Se2 thin films have been already reported. However, there are only few studies, which correlate structural and chemical properties of GBs. Here, we investigate the correlation between crystallographic and chemical information of GBs by combining transmission Kikuchi diffraction and atom probe tomography. For the CIGSe system, we detect an elemental redistribution at random GBs but no chemical fluctuations at twin boundaries. We observe Cu depletion and In & Se enrichment at random GBs and, at some random GBs, a slight Ga depletion. This In on Cu scenario is accompanied by co-segregation of Na and K. The amount of impurity segregation varies from one GB to another but also along an individual GB. For the CIS system, we will present our latest results as well and will discuss possible effects of the observed phenomena on the cell performance.

Resume : Electronic structure including lateral profiles of surface potential of Cu(In, Ga)(S, Se)2 (CIGSSe) surface and CdS/CIGSSe interface used in cells with conversion efficiency above 20% is studied by in-situ photoemission, inverse photoemission spectroscopy (PES and IPES) and UHV-Kelvin probe microscopy (KFM). Impact of post deposition treatment using KF (KF-PDT) on these electronic structures is also studied. The measurements have revealed that the surface of the CIGSSe without KF-PDT is characterized with a high S/(S+Se) ratio of about 0.6, an expanded band gap energy of 1.4–1.5 eV and a high conduction band minimum (CBM) of 0.8–0.9 eV. The KFM analysis shows a small built-in potential at grain boundaries and apparent potential fluctuation originated in crystalline faceting. The interface between in-situ evaporated CdS and the CIGSSe has almost “flat” connection of conduction band, where a large difference in CBM of the CdS and the CIGSSe of about -0.4 eV is compensated with a large interface induced band bending (iibb) of 0.44 eV. The KF-PDT results in a lowering of Fermi level of the CIGSSe and suppression of fluctuations in surface potential and photovoltage. The former causes an increase of built-in potential through the interface. iibb of this interface is, however, equivalent to that of the non-treated CIGSSe, which results in a slightly negative conduction band offset. These results have revealed that the current KF-PDT is beneficial mainly to open circuit voltage.

Resume : Cu(In,Ga)Se2-based (CIGSe) thin-film solar cells have continued to increase prominently in efficiency over the last years, reaching over 22% on a laboratory scale. Such high efficiencies are usually achieved with a Ga/(Ga+In) (GGI) ratio of ~0.30. In order to achieve higher open-circuit voltages, larger absorber band gaps are needed, which can be realized, e.g., by increasing the GGI. For such devices with a large GGI, a comprehensive understanding of the electronic and chemical structure of the CIGSe-based absorbers and their interface with the buffer layer is essential. Absorbers with different bulk GGI ratios of ~0.30, ~0.66, and ~0.95 and their interface with a solution-grown CdS buffer layer were studied using a number of different electron and soft x-ray spectroscopy techniques, including lab-based x-ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and x-ray-excited Auger electron spectroscopy. Combined with synchrotron-based soft x-ray emission spectroscopy, a full picture of the electronic and chemical structure at and near the surface can be drawn, including the band alignment and intermixing behavior as a function of GGI. In our contribution, initial results of this comprehensive study will be shown and discussed in view of their impact on the performance of corresponding CIGSe solar cells.

Resume : Cu(In,Ga)Se2 (CIGS) thin films have been a promising material for solar cells with record conversion efficiencies above 22%. Usually, CdS is used as buffer layer for these devices, but due to the bandgap of 2.4 eV CdS buffers decrease the quantum efficiency of the device in the short wavelength region. Thus, Cd-free buffer materials with wider bandgap are demanded. In this work, three CIGS devices with the same Ga/(Ga+In) ratio (GGI) of about 0.3 deposited by co-evaporation but different solution-grown buffer layers, namely CdS, Zn(O,S), and InxSy, were analyzed. These three devices exhibit conversion efficiencies of 17.0% for CdS buffer, 14.7% for Zn(O,S), and 14.6% for InxSy, respectively. Microstructural investigations by TEM showed that at the bottom of the absorbers pores are present. Moreover, twin lamellas, stacking faults, and other defects exist in the CIGS. The interfacial growth of CdS/CIGS was coherent, while that of Zn(O,S)/CIGS and InxSy/CIGS was incoherent. EDXS results demonstrate that neither the absorbers nor the buffer layers are chemically homogenous on a micrometer scale. In addition to different bandgaps of the buffer layers, it can be supposed that local differences in the GGI and the Cu/(Ga+In) ratio (CGI) of the absorber as well as the buffer/CIGS epitaxial relationship may influence the conversion efficiency of CIGS solar cells, too.

Resume : Replacing cadmium sulfide (CdS) with an alternative buffer material in Cu(In,Ga)(S,Se)2 (CIGSSe) based thin film solar cells has been discussed vastly in the literature over the past two decades. One of the prime candidates introduced for this replacement is indium sulfide (In2S3), which has suitable properties for optoelectronic applications. However, the presence of impurities even at low concentration can strongly influence the optical and electrical properties of In2S3. So-far, several studies documented an intermixed In2S3/CIGS interface, which mainly contains Na, Cu, O and Cl impurities. The existence of Cu and Na in indium sulfide is attributed to the diffusion from the CIGSSe absorber layer into the In2S3 buffer layer. Regarding the anionic impurities, the existence of chlorine and oxygen are due to incomplete sulfurization of the Cl-containing precursor and out-diffusion of oxygens from the ZnO front contact respectively. In spite of the vast experimental confirmation on the existence of impurities, the knowledge of their properties, particularly the role of Cl, in indium sulfide is not well-grounded. In this contribution, we mainly focus on anionic impurities in In2S3 and report on their formation enthalpy, influence on conductivity and optical transition levels. Besides, we present a comparison between the role of cationic (Na and Cu) and anionic (O, Cl) impurities in In2S3. This includes how these impurities influence the absorber/buffer interface, how they compete with each other in seizing available lattice sites, and they contribution in the experimentally observed Photo Luminescences (PL) and persistent electron photoconductivity. All calculations were performed based on Hybrid-Density Functional Theory (Hybrid-DFT) with projector augmented wave pseudopotentials, implemented in Vienna Ab-initio Simulation Package (VASP).

Resume : CdS is most commonly applied as buffer layer in photovoltaic devices employing Cu(In,Ga)Se2 (CIGS) as active absorbing material especially in highly efficient devices. The parasitic optical absorption of CdS (Eg = 2.4 eV) results in a reduced photocurrent density. A simple reduction of the CdS layer thickness, however, leads to a reduced device performance due to a loss in open-circuit voltage (Voc) attributed mainly to insufficient coverage on the rough surfaces and possible sputter damage of the CIGS absorber surface during the deposition of the window layer. In this contribution, alternative window layers are investigated replacing the sputtered ZnO. Highly resistive and transparent oxides, such as SnO2, TiO2, Al2O3, ZnO are deposited by atomic layer deposition (ALD) onto the substrate/Mo/CIGS/CdS stack. ALD is chosen for this purpose since for thin and resistive oxide layers a conformal coverage of the rough surface and a precise thickness control are paramount. Solar cells were characterized by (temperature-dependent) current-voltage and external quantum efficiency (EQE) measurements. It is found that high Voc (~0.7 V) can be retained with thinner CdS in combination with ALD-TiO2 and SnO2, and therefore a gain in current density (~1 mA cm-2), owing to a reduced parasitic absorption of CdS, is achieved. The drawback of this approach is a reduced FF of about 73% compared to 76% of the reference structure. The interpretation of the observations is supported by 1D device simulations using SCAPS.

Resume : The high diffusivity of Cu+ ions in the hexagonal-close-packed structure of Cu2-xS often leads to interesting and unexpected observations. Herein, we present the etching of oxide and sulfide thin film underlayers during the atomic layer deposition of Cu2-xS thin films. The infiltration of the underlayers by Cu+ ions is an essential step that precedes the etching process. However, it is suspected that the eventual etching of the underlayer, and the etch rate strongly depend on the lattice (bond dissociation energy) of the underlayer material. Thin films of ZnS, ZnO, SnS, and SnO were etched to different degrees during the deposition of Cu2-xS while SnO2 exhibited a high resistance to etching. Interestingly, a selective removal of Zn2+ was observed when a ternary Zn1-xSnxO film was used as underlayer. Based on XPS results and findings from other supplementary experiments, a possible reaction mechanism was proposed for the etching process. Finally, the observation was extended to the synthesis of Cu2-xS nanowires that can be used as effective absorbers for photovoltaic cells.

Resume : We present chemical solution deposited cadmium chalcogenide thin films with a well-defined orientation relationship with the monocrystalline substrate,[1,2] which allow to predict and control orientation and morphology of the films. CdS and CdSe films were chemically deposited on (100), (111)A and (111)B oriented GaAs substrates and characterized by analytical transmission electron microscopy, high-resolution scanning electron microscopy and x-ray diffraction. Both CdS and CdSe showed phase change from initial zinc blende at the interface to the stable wurtzite phase when deposited on GaAs(100). Furthermore, the importance of surface polarity was demonstrated for deposition on GaAs(111)A and GaAs(111)B. While in the case of CdS monocrystalline were formed only on GaAs(111)B, CdSe monocrystalline films were obtained also on GaAs(111)A. The monocrystalline films had wurtzite structure, with the c-axis oriented perpendicular to the substrate surface. In addition, the films showed high densities of stacking faults which are attributed to the well-known polytypism in CdS and CdSe. The advanced control over orientation and morphology of thin film enhance the materials potentials in optoelectronics devices. 1. O. Friedman, A. Upcher, T. Templeman, V. Ezersky and Y. Golan, Journal of Materials Chemistry C, 2017, 5, 1660-1667. 2. O. Friedman, D. Korn, V. Ezersky and Y. Golan, CrystEngComm, 2017, 19, 5381-5389.

Resume : Increasing demand for energy soon will force us to seek environmentally clean alternative energy resources. Recent efforts to design ordered assemblies of semiconductor and metal nanoparticles as well as carbon nanostructures provide innovative strategies for designing next generation energy conversion devices. Recently, synthesis of atomically-flat colloidal nanoplatelets (NPLs) have been also demonstrated by using colloidal approaches. Owing to the extended lateral dimensions of colloidal NPLs larger than the exciton Bohr radius, these NPLs exhibit pure quantum confinement along their vertical thicknesses. NPLs which absorb light in the visible range, can serve as sensitizers as they are able to transfer electrons to large band gap semiconductors such as SnO2. Size quantization often becomes an important factor to drive the energetics to more favorable levels. Since colloidal NPLs has shown unique electronic structure, their optical properties significantly different from the spherical shaped colloidal NCs. With the formation of quantum-well like electronic structure, sharp excitonic transitions are observed from the absorption spectra corresponding to the heavy-hole and light-hole transitions. Here, by using surface modifiers, CdSe NPLs have been assembled onto mesoscopic SnO2 films. Femtosecond transient absorption and emission quenching experiments confirm the injection from the excited state of CdSe NPLs into SnO2 nanoparticles. The injected charge carriers in a CdSe-modified SnO2 film can be collected at a conducting electrode to generate a photocurrent. When the SnO2-CdSe composite employed as a photoanode in a photo electrochemical cell, it exhibits a photon-to-charge carrier generation efficiency of 21%.

Resume : Efficiency potential of future generation solar cells such as wide bandgap CIGSe(CuInGaSe2), CIGS(CuInGaS2), CZTS (Cu2ZnSnS4) and CZTSSe (Cu2ZnSn(S,Se )4) solar cells is discussed based on external radiative efficiency (ERE), open-circuit voltage loss and fill factor loss and non-radiative recombination loss. In summary, 1) CIGSe solar cells have efficiency potential of over 26% such as 26.0 and 26.5% with normalized resistance of 0.05 and by improving in ERE into 5% and 10%, respectively from around 0.5%. 2) CIGS solar cells, efficiencies more than 22% from present 16.9% efficiency will be realized by improvements in ERE into more than 1% from about 0.001%. 3) CZTS(Se) and CIGS solar cells have efficiency potential of more than 20% and 22%, respectively by improvement in ERE from around about 0.001% to 1%. Effects of non-radiative recombination losses upon properties of future generation solar cells are discussed. 4) Regarding wide-gap CIGSe solar cells, lattice mismatching between CIGSe active layer and buffer layer, and increase in non-radiative recombination center density with increase in bandgap energy of CIGSe materials and solar cells are suggested as non-radiative recombination losses in wide-gap CIGSe solar cells. 5) Regarding CZTS(Se) solar cells, existence of unknown recombination loss and resistance loss due to low carrier mobility compared to CIGSe materials are shown in addition to surface and bulk recombination losses.

Resume : Tin sulfide (SnS)-based thin film solar cells were fabricated using an novel organic precursor, Sn(dmamp)2 (dmamp, di-methyl-amino-methyl-propanolate). SnS thin films were deposited by MOCVD (metal organic chemical vapor deposition) in the temperature range of 150~300 °C. Effect of post annealing temperature and atmosphere on the cell efficiency were investigated. Also, n-type buffer, Zn(O,S) were optimized by estimation of band structure. X-ray diffraction (XRD) showed that formation of the pure SnS phase was observed on the sample grown between 200~250 °C and partial pressure of H2S at 10 Torr. Transmission electron microscope (TEM) images showed that metallic thin precipitates in the films were observed to disappear at moderate annealing condition. Maximum cell efficiency was measured up to ~2.6%.

Resume : Blocking electrolyte contacts offer a convenient non-destructive way of characterizing thin film semiconductor layers such as the CdS buffer layers that are used in CIGS, CZTS and CdTe solar cells. If the layers are deposited on a highly doped substrate such as fluorine-doped tin oxide coated glass (FTO, the doping density and film thickness can be determined by Mott Schottky plots, which show a change in slope when the applied voltage is high enough that the space charge layer extend to the substrate. Whilst modelling the capacitance behaviour of CdS deposited from a chemical bath onto FTO, we observed that the layers evidently are partly porous. Porosity in buffer layers can give rise to shunting in thin film solar cells, to the photovoltaic performance, as it can cause shunting. We have therefore used cyclic voltammetry with a redox electrolyte to investigate the porosity of thin CdS films on FTO. The cyclic voltammetry response shows that the pores in the film behave like an array of ultramicroelectrodes. A new theory is presented that allows estimation of the average pore sizes and pore number density by combining the results of capacitance and cyclic voltammetry measurements. Typical values for a range of CdS films deposited under different conditions are presented and discussed.

Resume : V2VI3 family compounds such as Sb2(S,Se)3, Bi2(S,Se)3, (Sb,Bi)2Te3 have drawn considerable attention as promising earth-abundant, non-toxic materials for thermoelectric and recently for photovoltaic applications. These compounds exhibit lower than 3 crystal structure dimensionality giving rising to strongly anisotropic optical and electrical properties. For instance, a record certified 7.6% power conversion efficiency has been achieved for Sb2Se3 solar cells by carefully optimizing film orientation, which enhanced carrier transport across the absorber. Therefore, the control of thin film growth and consequently orientation is critical for V2VI3 ?based device performance. In this work we analyse the growth process of Sb2Se3 on various substrates from vapour phase. We investigate the influence of deposition parameters as well as the nature of substrate on Sb2Se3 film orientation, grain size, morphology and composition. We found out that as-deposited Sb2Se3 film orientation primarily depends on the substrate temperature and the substrate?s surface energy with respect to Sb2Se3. Because of anisotropic crystal structure of Sb2Se3, the facets with (hk0) indices generally have lower surface energy and therefore are the most dominant under near equilibrium growth conditions (high substrate temperature, Se-rich atmosphere). In this case, despite the chosen substrate, 1D and 2D growth is observed (nanorods, nanoneedles, nanosheets) and films are oriented horizontally (c-axis parallel to the surface). However, if substrate surface energy is high (Sb2Se3 is unlikely to create strong bonds with substrate) and deposition conditions are further away from equilibrium, the growth will become 3D and grains of Sb2Se3 will be oriented non-horizontally (hkl, l?0). The transition from non-horizontally to horizontally aligned films occurs at higher temperature for substrates with high surface energy. According to the findings we propose several strategies to synthesize high crystalline quality films with favourable orientation on various substrates. The results obtained in this study and observed tendencies we believe can be extended not only to V2VI3 family compounds, but also to other e.g. SnS, CuSbS2 anisotropic photovoltaic materials, where carrier transport is of key importance. Obtaining a better understanding of the growth mechanism and how to control film orientation can facilitate the fabrication of high performance devices.

Resume : A well-defined WO3/Bi2S3 heterojunction comprised of single-crystalline Bi2S3 nanowire (Bi2S3NW) layers on top of the WO3 nanoparticles (WO3NP) was synthesized via an in-situ hydrothermal reaction. The single-crystalline Bi2S3 nanowires were uniformly grown and directly anchored on the surface of the WO3 nanoparticle layer. Compared to those of the other Bi2S3 electrodes, the resulting WO3NP/Bi2S3NW heterojunction showed enhanced photoelectrochemical (PEC) activity. The origin of this enhanced PEC activity is mainly attributed to the enhancement of charge separation on the Bi2S3 layer, due to the effective photogenerated electron transfer from the Bi2S3 conduction band to that of WO3. The uniform heterojunction interface with less interfacial defects generated through the in-situ growth method also leads to lower resistance at the interface between the different domains. Furthermore, the single-crystalline longitudinal structure of the Bi2S3 nanowires can provide a direct electrical pathway through a single domain of nanowires.

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Resume : Replacing the CdS buffer in a Cu(In,Ga)(Se,S)_2 (CIGSSe) thin film solar cell with alternative materials such as InxSy requires reconsidering the interface properties between buffer and absorber as well as between buffer and intrinsic ZnO. Besides the material properties of the buffer, the interfaces could have a major impact on the junction formation of the solar cell. As the charge and defect properties of an interface are not directly measureable, we used numerical simulation combined with extensive material and device characterization to model the i-ZnO/In_xS_y:Na/CIGSSe interfaces. In this study we present a one-dimensional simulation model which is based on measured electrical characteristics of In_xS_y:Na buffer and i-ZnO layers, as well as capacitance, current-voltage and quantum efficiency measurements. The simulation results show good agreement with the experimental data over a broad temperature range. Derived from this, we postulate a novel band alignment at the junction due to an interface-near region which exhibits an inhomogeneous doping and defect density profile in the CIGSSe as well as donor type interface traps at the window/buffer and buffer/absorber interfaces. We conclude, that the combination of simulation and temperature dependent device characterization enables access to crucial interface parameters and is therefore a productive method to investigate the junction properties of new material combinations and strategies for further solar cell optimization including (alkali-) post deposition treatments.

Resume : By now, the progress in manufacturing Cu(Ga,In)(S,Se)2 absorbers used in thin film solar cells led to conversion efficiencies of 22.6% [1]. Applying absorber materials with an intermediate band (IB) energy level in between the valence and conduction bands it is proposed that efficiency can be raised up to 63.3% [2]. In literature [3] different transition elements which may cause an IB within the band gap of chalcopyrite type CuGaS2 have been suggested. In our approach we studied solid solutions following the pseudo-binary section of CuGaS2 and MnS as well CuMnS2. Different series of powder samples were synthesized by solid state reaction of pure elements. The samples were analyzed in terms of chemical composition and phase content using WDX spectroscopy as well as in terms of crystal structure by X-ray and neutron diffraction. In addition we performed photoluminescence (PL) measurements to determine the optoelectronic properties. The presentation will summarize structural trends in dependence on the Mn content in CuGaS2 as well as off-stoichiometry. The observed PL spectra will be interpreted with calculations of the band diagram [4] and correlated with the cation distribution obtained from average neutron scattering length method. Moreover we prove that Mn incorporation does not lead to an IB-Material. [1] Jackson et al., Phys. Status Solidi RRL, 10 (2016) [2] Luque et al., Physica B, 382 (2006) [3] Martí et al., J. Appl. Phys., 103 (2008) [4] Zhao et al., Phys. Rev. B, 69 (2004)

Resume : CuIn0.7Ga0.3Se2 (CIGS) thin films were deposited on both n-type silicon (Si) wafer and Si-nanowire (NW) arrays by one-step thermal evaporation of a stoichiometric powder for the construction of Planer-Si/CIGS and Si-NW/CIGS structured heterojunction solar cells. A core-shell like solar cell was constructed by decoration of electroless-etched silicon (Si) nanowires (NWs) with a 700-thick CIGS thin film. The detailed information about the synthesis of Si NWs have been presented elsewhere [1]. The CIGS thin film with the same thickness was also deposited on an n-type Si wafer (1 cm x 1cm) for the construction of planer counterpart of the core-shell like n-Si-NW/p-CIGS structured solar cell. Following the deposition of CIGS thin films, the whole device structures were annealed at 400 oC under the N2 flow for 30 min. The structural properties of the films deposited on both planer Si and Si-NW arrays were examined using a X-Ray diffractometer in the 2? range of 20o to 70o with a scan rate of 1o/min. Surface morphologies of the deposited films were studied by atomic force microscopy (AFM) operated in tapping mode and scanning electron microscopy (SEM) measurements. Reflectance measurements were performed in the wavelength range of 400-1000 nm to investigate the effect of introducing one-dimensional nanostructures into device structure of solar cells. Results have shown not only that there is a dramatic reduction in the reflection with the incorporation of Si nanowires but also a significant improvement in crystallinity of the deposited CIGS films when compared with those recorded for its planer counterparts. In addition, it was revealed that the post-annealing under the inert gas flow has a significant effect on morphology, electrical resistivity (measured by four point probe) and structural properties of CIGS thin films deposited on Si substrates both in planer and NW configurations. Silver dot contacts (1 mm in diameter) were evaporated by thermal evaporation to form the ohmic top-contacts of the solar cells structure by using a copper-shadow mask. As a back contact of the solar cells, the back side of the n-Si wafer was coated with a 150-nm thick silver thin film layer via thermal evaporation technique. The photovoltaic behaviors of the constructed prototype solar cells were examined under standard test conditions by using a PC-controlled solar simulator. Current ? voltage characteristics of the solar cells under dark and light illumination conditions were recorded and analyzed for the calculation of the solar parameters of the fabricated solar cells including open-circuit voltage (Voc), short-circuit current (Isc), series-/-shunt resistances and power conversion efficiency. [1] H Karaagac, M Parlak, E Yengel, MS Islam (2013) Mater Chem Phys 140: 382.

Resume : Antimony Selenide (Sb2Se3) is a promising material for thin film PV due to its suitable optoelectronic properties, low cost, low toxicity, and its unusual 1D nanoribbon structure allowing benign grain boundaries. The record efficiency currently stands at 6.5% [1] for a thermally evaporated film. Here, we focus on devices produced by close space sublimation yielding efficiencies of 5.5% [2]. Current champion devices benefit from addition of quantum dots at the device back surface to enhance performance but there is still little fundamental understanding of the surface composition or its effect on cell performance. X-ray Photoemission Spectroscopy (XPS) is powerful for characterising surface chemistry and electronic properties in a material. Here, we present the results of in-depth XPS analysis of Sb2Se3 surfaces following various treatment steps. Commonly for Sb2Se3 films have varying amounts of Sb2O3 and Se at the back contact, yet it is unclear how this affects the performance of the device. We investigate how the surface properties can be controlled via different post-growth annealing processes and chemical etches such as CS2 and (NH4)2S and how this influences cell performance. This offers the opportunity to identify which surface characteristics best benefit device efficiency, as well as develop an understanding of the behaviour of Sb2Se3 following different post-growth treatments. [1] C. Chen et al., ACS Energy Lett. 2, 2125 (2017) [2] L. Phillips et al., submitted, 2018

Resume : CIGS is one of the most promising thin film solar cell technologies, and reached a record power conversion efficiency of 22.9% [1]. The most commonly used buffer layer is chemical bath deposition (CBD) of CdS, but these solar cells suffer parasitic absorption losses in the short wavelength region. The conventional Cd-Free buffer layer with wider bandgap is Zn(O,S), but this buffer regularly leads to increased recombination at the interface with CIGS, seen as a lower Voc. In this work, we study both buffer layers together with novel alternatives, and new surface treatments prior to passivation and buffer layer deposition. As surface treatment several etching and cleaning steps like NH3 solution, S(NH4)2 and HBr/Br2 solution are tried. CdS and Zn(O,S) buffer layers are grown by CBD, where CdS is the reference for comparison. TiO2 layer is used as a passivation layer to reduce interface recombination. The properties of buffer layer/CIGS interface with and without TiO2 passivation layer are examined by photoluminescence (PL) and time resolve photoluminescence (TR-PL), which are effective to characterize defects and recombination mechanisms. Electrical characterization is also done on finished solar cells. The effect of the surface treatments and passivation layer on CIGS solar cells will be shown in this paper. This work received funding from the European Union?s H2020 research and innovation programme under grant agreement No. 715027. [1] Solar Frontier, Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 22.9%, Press Release 20/12/2012, Available: http://www.solar-frontier.com/eng/news/2017/1220_press.html (Accessed on 17/01/2018)

Resume : The analysis of the minority charge carrier lifetime in Cu(In,Ga)Se2 (CIGSe) thin-film solar cells via the contactless and non-destructive method of time-resolved photoluminescence (TRPL) is commonly believed to be most straightforward in low injection conditions. Moreover, to prevent rapid degradation of the blank absorber material, TRPL of CIGSe absorbers is usually measured with a CdS buffer layer. However, our time-resolved device simulations show that the presence of a p-n heterojunction strongly influences the shape of TRPL decays measured under low injection conditions. Simulations also predict that this effect depends on the doping concentration of the absorber layer. By varying the doping concentration via a metastable light-soaking treatment of the samples, we were able to experimentally verify these predictions. The main sources of the observed effects were determined to be charge carrier separation and thermionic emission at the heterojunction. These findings need to be taken into account in the interpretation of TRPL lifetime measurements on the CIGSe/CdS system.

Resume : CIGS (Cu(In,Ga)Se2) solar cells present high efficiencies (above 22%) but, to still be competitive in a constant evolving context with new materials and architectures, efficient analyses tools are required. We propose here an innovative coupling of characterization methods, Glow Discharge Optical Emission Spectroscopy (GD-OES) and X-ray Photoelectron Spectroscopy (XPS), to fully determine, at first sight, the chemical composition of CIGS absorber from the surface to the depth and to its further implementation on the final device. By combining these techniques, a high etching rate (GD-OES) and a precise monitoring of elements distribution in depth, enabling an accurate positioning at specific location (buried interfaces) (XPS) directly at the GD-OES bottom crater can be obtained. In a previous study the capability of this coupling has already been established. However, prior to XPS analysis, a preliminary regeneration step after stopping the GD-OES profiling is necessary to eliminate the superficial perturbed layer at the surface of the crater bottom. In this work we study the different strategies that can be employed to proceed to this necessary intermediate step to achieve the regeneration of the initial chemical information: an ex-situ wet chemical engineering (HCl, KCN and Bromine dipping) and a physical curing approach based on the use of Arn cluster sputtering, directly performed inside the XPS analysis chamber.

Resume : A great deal of effort has been made to develop alternative earth-abundant, cost-effective and non-toxic semiconducting materials for photovoltaics, batteries, light-emitting diodes, photo catalysis, and other applications. In particular, tin (II) sulfide (SnS) has attracted more interest due to its structural multiformity, appealing catalytic activity and favorable opto-electrical properties. On the other hand, the complete optical analyzation of SnS thin films is essential for more preponderant designing of devices since the optical properties give the information to understand their electronic properties and band structures. Herein, we deposited SnS thin films on glass substrates as a function of concentration of an environmental friendly complexing agent ?tartaric acid (C4H6O6)? by an extremely facile route, chemical bath deposition using analytical grade stannous chloride (SnCl2.2H2O) and thioacetimide (C2H5NS) as source materials for Sn and S ions, respectively. Further, an exhaustive investigation on their optical properties was made using the transmittance and reflectance measurements. Finally electrical parameters such as resistivity, carrier mobility and carrier density were estimated using optical data. A detailed analysis results will be presented and discussed for a clean understanding of electronic characteristics of SnS thin films.

Resume : Zn(S,O,OH) has been investigated as an alternative buffer material against CdS because of its direct wide band gap (~3.8 eV) at room temperature and non-toxic character [1]. To produce a buffer layer, chemical bath deposition (CBD) process is typically used but in Zn(S,O,OH) deposition, various factors (such as Zn source, S source, complexing agent, pH of solution, bath temperature, stirring speed and deposition time) are required to control. In this study, we focused on the difference of chemical contents depending on time. In the CBD of Zn(S,O,OH), there are two stages: clear solution stage and turbid stage. However, although chemical composition was not changed in clear stage, it shows different content in turbid stage. Therefore, we controlled the thickness of buffer layer by quartz crystal microbalance (QCM) system with certain temperature. The system follows the equation as below: ?m=(Ar ???)/(2f^2 ) ?F --> ?t=?m/(?c?Ar )) ?m-mass change;Ar-surface area of an electrode ; ?-elastic constant of crystal ;?-density of the crystal ; f- standard frequency . The characters of prepared buffer layer were analyzed by TEM and EDS and more observation will be discussed.

Resume : High-efficiency Cu(In,Ga)Se2 (CIGSe)-based thin-film solar cells often use a CdS buffer layer grown by chemical bath deposition (CBD). However, alternatives are being developed that are more environmentally friendly and/or more transparent in order to reduce absorption losses due to the buffer layer. Among these alternatives, InxSy is a very promising candidate, reaching CIGSe cell efficiencies very close to, or sometimes even above CIGSe devices with CBD-CdS buffer layers. To deliberately optimize the buffer layer design, an important prerequisite is detailed knowledge of the chemical and electronic structure of CIGSe absorber surfaces and their interfaces with CBD-InxSy buffer layers. To study these properties, we use laboratory-based electron spectroscopy techniques, as well as synchrotron-based soft x-ray emission spectroscopy. With these complementary spectroscopic methods, important insights into the chemical structure, e.g., characteristic species present at the surface of the investigated samples and intermixing processes at the interface, are gained. Furthermore, the electronic structure, i.e., the band edge positions as well as the band alignment at the CBD-InxSy/CIGSe interface, is studied. Initial results of these investigations will be presented and related to the electrical properties of corresponding CIGSe solar cell devices.

Resume : In this work, we show a method to grow Sb2Se3 thin films which can be used as absorber layer in a solar cell structures. These films were grown on the top of different substrates such as soda-lime glass, Mo coated soda-lime glass and Si. The Sb-Se precursors films were deposited by RF magnetron sputtering and then annealed with an H2Se gas flow. Different annealing temperatures were tested and analyzed. This study also analyses the effects of the use of different substrates on properties of the film. Compositional and morphological analyzes are performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Two techniques are used to phase identification and structural characterization, namely, X-ray Diffraction and Raman Scattering Spectroscopy. The first technique shows that the main crystalline phase is Sb2Se3 with Pmna orthorhombic structure. Raman scattering analysis show the characteristic peaks of Sb2Se3 located at 150 cm-1 and 188 cm-1. Traces of rhombohedral and amorphous Se secondary phases are also observed. Special attention is taken to Raman scattering characterization conditions in order to avoid misidentification of Sb2Se3 phase. Many authors show results with Sb2O3 Raman modes identified as Sb2Se3. In this work we clearly differentiate the modes from each binary compound and it is shown how prevent the oxidation of Sb. Spectrophotometry measurements allowed to extract a direct bandgap with a value close to 1.06 eV.

Resume : Thin film solar cells based on direct bandgap semiconductor materials like Cu(In,Ga)Se2 (CIGS), have light absorption properties far larger than indirect bandgap materials such as Si. So, absorber layers with thickness of 1-2 ?m are enough to absorb most of incoming light, while c-Si requires two more orders of magnitude thicker absorber layers. Nonetheless, CIGS solar cells with ultrathin absorber layers (0.5 ?m) suffer from incomplete photon absorption and increased charge carrier recombination at the rear electrode. Passivation layers are needed to reduce these detrimental effects. To study the detrimental effects of thinning the CIGS layer, we compare solar cells with and without passivation layers. 25nm Al2O3 layers and point structures have been created on the rear contact using e-beam lithography. The solar cells with ultrathin absorbers layers provide devices with a power conversion efficiency value of 8.2% while with nanostructured passivation layers the efficiency reached 9.1%. SEM and XRD measurements revealed that the CIGS layer does not undergo any significant morphological changes, in agreement with the similar values of carrier density (~8x10E16) and depletion region (~160 nm) extracted using C-V. However, J-V, EQE and photoluminescence results show very different optoelectronic behaviour which is indicative of a successful passivation of the rear contact. A detailed comparison between both types of samples is done and presented.

Resume : CIGS thin films with high absorption coefficient and direct band gap are one of the promising absorbing materials for solar cells. CIGS solar cells on flexible substrate offer the advantage to be lightweight and low cost and can be applied to various fields such as transportation and building. In large-area flexible CIGS solar cells, it?s significant to obtain a higher fill factor. However, there are some limits for high efficiency due to low fill factor. Some reason might be caused by leakage current in pinhole or plasma damage of chemical bath deposited CdS. Therefore, it?s very important to find solutions for increase of fill factor. Both buffer layer with high durability and process condition of i-ZnO thin layer with avoidable plasma damage are needed for large-area flexible solar cells. In this study, Mo back contact layer on stainless steel substrate (100×100 mm) are deposited by a DC sputtering and CIGS absorber layer are deposited by a co-evaporation method. As buffer layers, chemical bath deposited CdS and atomic layer deposited ZnS are prepared with 50 nm thick. These are followed by RF sputtered i-ZnO layer with varying input RF power for avoiding plasma damage. Surface and interfacial properties between buffer layer and i-ZnO layer are investigated using TEM, GD-OES, Microwave PCD and Kelvin probe method. Finally, the photovoltaic properties are analyzed with a solar cell simulator and external quantum efficiency measurement systems.

Resume : Metal-Insulator-Semiconductor (MIS) structures with Al2O3, Si3Nx and SiOx as passivation layers in Cu(In,Ga)Se2 (CIGS) solar cells are studied. The structures consisted of Mo/CIGS/insulator/Al. Several measurements were done to quantify the insulator deposition influence on the CIGS surface, namely: Raman scattering, X-ray diffraction and Photoluminescence. To study the electrical properties of the CIGS-insulator interface, the performed measurements were: i) C-V measurements to estimate the number and polarity of fixed charges (Qf) and ii) G/?-f measurements to estimate the density of interface defects (Dit). In summary, the deposition of the insulators at high temperatures (300 ºC) and the use of sputtering causes surface modification on CIGS which compromises device operation if used as front contact passivation layer. Moreover, by varying the SiOx deposition parameters, it is possible to have opposite charges inside the insulator, +2.8×1011 and -6.1×1010 cm-2, which would allow its use for different architectures. All the insulators have density of interface defects values under 1×1013 eV-1cm-2. This is a range of values that, according to the literature, allows for an effective interface passivation. The material with lower Dit values was Al2O3 deposited by sputtering, 0.8×1012 eV-1cm-2, which is a remarkable result taking into account that its deposition method, sputtering, causes CIGS surface modification.

Resume : Dye, quantum dot and perovskite sensitised metal oxides are a subject of intensive research. An alternative approach to sensitising surfaces is to use small band gap 2-D materials, such as chalcogenides where the band gap can be tuned by varying the number of layers (Brent 2015). In order for such devices to operate the relative positions of valence and conduction bands of the sensitiser and n-type material is important. Here we report on the measurement of band alignment of 2-D SnS deposited on Anatase (101) surface by x-ray photoelectron spectroscopy (XPS). The 2-D SnS was obtained by liquid-phase exfoliation and deposited directly onto an Anatase (101) single crystal surface, which had been cleaned under ultra-high vacuum conditions. To determine the alignment the valence band offset for the heterojunction n-TiO2/p-SnS was measured using soft XPS which gave an overlap of 0.55 eV. Literature values of the band gaps of 2-D SnS (1.6 eV) (Brent 2015) and TiO2 (3.2 eV) (Kaven 1996) were used to determine the conduction band position. Analysis shows that the interface between p-SnS and single crystal Anatase phase n-TiO2 has a type II offset. Under the same conditions Rutile (110) and ZnO (100), with bandgaps of 3.0 eV (Poumellec 1991) and 3.3 eV (Liang 1968) respectively, also demonstrated a type II offset interface with 2-D SnS. Rutile (110) and ZnO (100) showed larger overlaps with 2-D SnS of 0.9 eV and 0.7 eV respectively.

Resume : We report a systematic study of the impact of post deposition thermal treatments (PDT) on properties of SnS thin films and SnS/CdS solar cells prepared by closed-spaced sublimation. Results are analysed for different PDT including annealing in air, N2, vacuum and closed quartz ampoules in the range of 400-500 oC without, and in presence of alkali halide flux. The properties of SnS films were analysed by XRD, SEM, EDX and UV-vis, while the glass/FTO/CdS/SnS/Au solar cells were characterized by I-V and EQE. As deposited SnS films on glass and CdS/FTO/glass substrates had stoichiometric composition, large grains of orthorhombic structure and band gap of 1.4 eV. Hall measurements indicate p-type carrier concentration of 1018 cm-3 and mobility of 0.9 cm2 V-1s-1. Properties of SnS films strongly depend on the PDT conditions. Annealing in evacuated ampoules increases mobility to 2 cm2 V-1s-1 and decreases the density of holes to 5 × 1017 cm?3 which is beneficial for p-n junction formation. Strong activity of alkali halide flux was observed in the PDT, especially highlighted in the air - treated SnS films. The flux generates melted phase and promotes grain growth by mass transport, resulting in recrystallization and sintering processes in films. While as deposited SnS solar cells exhibit conversion efficiency of 0.5 %, then thermal treatment of glass/FTO/CdS/SnS structure in closed ampoules resulted in efficiency of 1.6 % with VOC of 250 mV, JSC of 12.7 mA/cm2 and FF of 50 %.

Resume : CuInSe2 (CIS) and Ga back-graded Cu(In,Ga)Se2 (CIGS) devices with efficiencies as high as 16.1 % are characterized by means of time resolved photo luminescence (TRPL). In particular the Ga back graded CIGS device consists of a Ga grading towards the back while the top part is CIS with sufficient thickness for absorption. TRPL measurements with different surface modifications are carried out and the extracted measured decay times are compared to obtain insights into surface recombination velocities, the minority carrier mobility and bulk lifetime. Experimental data is systematically analyzed based on analytical expressions for the CIS and the back graded CIGS absorber and corroborated by Sentaurus TCAD simulations. The presented approach will allow the estimation for upper and lower bounds of the surface recombi-nation velocities based only on the tail decay time of experimentally measured transients. Additionally, it will be shown that the back-graded CIGS device with an optical bandgap of 1.0 eV has a well passiv-ated back contact, which might be the reason for the performance leap to 16.1 % efficiency.

Resume : We present a numerical model to investigate spectrally resolved photoluminescence images acquired by hyperspectral imager. A common method is to consider the high energy part of the luminance spectrum and extract the temperature and the Quasi-Fermi Level Splitting using a linearized form of the generalized Planck?s law for semiconductors. On the contrary, our method considers the complete luminescence spectrum which is modeled following the work of J. K. Katahara and H. W. Hillhouse. This allows to extract absorption properties as well as thermodynamic properties of the device such as the temperature and the quasi-Fermi level splitting. A three-dimensional deconvolution algorithm has been developed to jointly deal with the spectral and the spatial response of the camera sensor and therefore improve accuracy of the results. The validity of the method is discussed to get rid of any ambiguity that could be inherent to fitting procedures with multi-parameters. We investigate the impact of the noise to signal ratio of the acquired data to properly access the material properties. Spatial correlations between some parameters are inherent to the absorption modelling and may induce wrong interpretation of the results. We apply the method to CIGS polycrystalline thin films solar cells. We show that the Quasi-Fermi Level Splitting is much less spatially varying than shown before and discuss the real link between luminescence mapping and the spatial variations of this material properties.

Resume : Thin ?lms of metal chalcogenides have been point of interest in their potential industrial applications. Among these materials, II?VI semiconductor group of materials with their remarkable optoelectronic properties as ideal direct band gap, high absorption coefficient and photo-sensitive behavior in the visible region of spectrum have attracted the research on the photovoltaic diode applications and investigated an alternative material system in the fabrication of low cost optoelectronic devices. In this study, cadmium selenide (CdSe) thin ?lms were deposited on a soda-lime glass substrates using direct evaporation of stoichiometric CdSe powder with purity 99.99% at room temperature. Following to the deposition, the effect of the annealing temperature on compositional, structural, morphological, optical and electrical properties of the films were discussed. From the analysis of energy dispersive X-ray spectroscopy, it was observed that the stoichiometric transfer of material from source powder to the deposited films was achieved, and additionally under the post-heat treatment this compositional homogeneity was preserved. The X-ray diffraction profile shows the characteristic diffraction peaks in a good agreement with the literature and crystalline quality was improved with increasing annealing temperature. No major peaks corresponding to secondary phases were detected in the diffraction pattern of both as-deposited and annealed films. Moreover, crystal structure of the films was detailed with Raman spectroscopy measurements. The surface imaging and analysis using scanning electron microscopy and detailed surface morphology and roughness analysis using atomic force microscope showed that the deposited CdSe thin ?lms are dense and compact in nature. From the optical transmission studies, films were found in direct optical transition characteristics and the band gap values of the samples were calculated using Tauc plots. The temperature dependent conductivity measurements were carried out to investigate their conduction mechanisms and investigate the change in the electrical conductivity depending on the illumination intensity.

Resume : We present our study about the diminishing impact of air-light exposure (ALE) of bare Cu(In,Ga)Se2 (CIGSe) absorber layers on the electronic properties of the complete solar cells. Former investigations with time-resolved photoluminescence revealed a strong degradation of the minority charge carrier lifetime of uncovered CIGSe layers after illumination in ambient air. The reason could be referred to an increased surface recombination velocity caused by a Na, O, and light induced modification of the CIGSe surface. In this regard for completed solar cells a reduction of open circuit voltage and power conversion efficiency in the range of 10-20% compared to solar cells from non-ALE absorbers is observed. With admittance spectroscopy under an applied DC bias voltage (-1 .. +0.75 V) we detect an additional defect signature in the case of cells with ALE absorbers. The voltage dependence of the attempt to escape frequency of this defect signature is an explicit sign of interface states. Simulations by SCAPS 1d reveal a deep acceptor like defect state Et ? EV ? 350 meV with a broad Gaussian energy distribution ?(Et) ? 100 meV. The high defect density Nt ? 10^10 .. 10^11 cm^-2 causes a reduced type inversion of the absorber at the interface and a drift of the Fermi level to an unfavourable position close to midgap. The critical role of the band alignment or the increased interface recombination velocity on the reduced performance values of ALE absorber solar cells will be discussed.

Resume : Transparent and conductive molybdenum oxide (MoOx) have become an attractive material in a variety of technological and industrial applications in the field of solar cells. In a variety of techniques, sputtering is a favorable deposition technique to obtain a wide range of electrical and optical properties of thin metal-oxide ?lms. Therefore, in this work, transparent and conductive MoOx thin film samples were deposited on the ultrasonically cleaned soda lime glass substrates by DC sputtering technique. A full understanding of the structural, morphological, optical and electrical properties of the as-deposited films were investigated under the effect of post-annealing treatments with different applied temperatures. The effects of deposition pressure and sputtering power were investigated and optimization process was achieved to obtain amorphous MoOx ?lms with low surface roughness and enhanced conductive behavior with low deposition temperature. From optical transmission measurements, band gap values were calculated in terms of annealing temperature. The optical and electrical characteristics were found to be strongly enhanced by post-deposition annealing. Improvement of these characteristics of the films was achieved as highly transparent in the visible wavelength range and conductive behaviors. An impact on the electrical conductivity was also observed under illuminated current-voltage measurements. The activation energy for electrical conduction were calculated and the photoconductivity behaviors of the films were discussed by applying two-center recombination model

Resume : In this contribution, we report on transparent semiconducting indium sulfur fluoride (ISF) thin-films exhibiting high sensitivity to UV radiation. The films were deposited on fused silica and silicon substrates using a radio-frequency plasma-enhanced reactive thermal evaporation system. The deposition was performed evaporating pure indium in SF6 plasma at a substrate temperature of 423 K. Rutherford backscattering (RBS) measurements were used to determine the chemical composition of the films deposited on silicon substrates. The film characterization includes electrical, optical, and photoconductivity measurements. The synthesized compound is highly-resistive (~700 M?-cm at 300 K) and exhibits an evident semiconducting behaviour. The activation energy of 0.88 eV is deduced from the temperature dependence of electrical resistivity. The indirect band energy gap of 2.78-2.9 eV is determined from transmittance spectra of the ISF films. The photoconductivity band is centred at 342 nm wavelength. The photoconductivity spectrum also shows the Urbach tail with a characteristic energy of 123 meV. ISF is a promising candidate for buffer layers in chalcogenide-based solar cells.

Resume : A narrow p+ layer between the buffer layer and the absorber is often assumed in order to interpret the electrical characteristics of Cu(In,Ga)Se2-based solar cells, in particular I-V and C-V. The model of the p+ layer can explain changes of the fill factor after light soaking and ? at least partially ? so called double-diode behavior. However, it is difficult to obtain the width and the acceptor density of such a layer from standard methods (like C-V or DLCP). Our goal was to analyze this model quantitatively in order to derive equations allowing for extracting parameters of the p+ layer from I-V characteristics. Starting from the electrostatic analysis we derived formulas describing the relationship between parameters of the p+ layer (width and density of acceptors) and the voltage at which the deterioration of the I-V curve begins. It is shown that the potential distribution in the buffer layer has a strong impact on analytical results. Then, we apply obtained equations for the analysis of experimental I-V curves in different metastable states: light-soaked and reverse-biased. Finally, we discuss calculated parameters of the p+ layer in the framework of the existing models, like (VSe-VCu) or InCu. (VSe-VCu) model can be a good explanation of the fill factor changes related to the p+ layer.

Resume : The structural and photoluminescence (PL) properties are studied for the ZnTe nanolamellar structures obtained by thermal annealing (TA) of the GaTe crystals, in Zn vapours, at 800°C, for 1 h up to 24 h. As a result of the TA, the external surface of the GaTe plates becomes reddish. The surface formations are oriented parallel to the GaTe plates and entirely cover the surface when treatment duration increases up to 24 h. The XRD analysis revealed the presence of the ZnTe crystallites. At the same time, there some phases of the Ga compounds with Te are formed (e.g. Ga2Te3). The Ga2Te3 phase is an intermediate phase in the formation process of the ZnTe compound, while Ga2Te5 and Ga7Te10 phases are obtained as a result of the TA. The presence of the ZnTe layer on the surface of GaTe plates is confirmed by Raman spectroscopy. SEM and EDX analysis showed that Ga atoms, released as a result of ZnTe compound formation, create microislands spread at both structure surface and Van der Waals space between elementary packings (Te-Ga-Ga-Te). The obtained material has PL features characteristic for ZnTe and GaTe compounds. Thus, at 80K, PL spectra contain two types of bands localized near to the fundamental absorption bands of the GaTe and ZnTe compounds. In the PL spectrum, the band of the localized excitons prevails by intensity in the fundamental absorption region of GaTe crystals. The emission band of the donor-acceptor type in ZnTe crystallites, is present in the region h?>Eg(GaTe).

Resume : Abstract ? Cu(In,Ga)Se2 (CIGS) polycrystalline thin-film solar cells with an area of 0.25 cm2 were prepared by a three-stage co-evaporation method. By applying ZnMgO secondary buffer layer, we obtained CIGS solar cells with an efficiency of 13.12% with only 20 nm CdS first buffer layer. The thickness of CdS buffer layer was greatly reduced by applying ZnMgO secondary buffer layer. The ZnMgO buffer layer reduced the optical loss and interface recombination, leasing to the enhanced performance of CIGS solar cells.

Resume : At present various simulation software for numerical modeling are being utilized to estimate the performance of group II-VI based solar cells. In this work, we modelled tin sulfide (SnS) as an absorber layer for solar cell and it belongs to group IV-VI. SnS is an environmental friendly earth abundant material and it is suitable for solar cell application. The structure of solar cell is SnS as absorber layer, Cd1-xZnxS as buffer layer and ZnO as window layer. The performance of this device is simulated numerically by changing the molar concentration of Zn in buffer layer by adjusting conduction band offset. After optimizing the molar concentration of Zn in buffer layer efficiency of solar cell achieved in this work is 26.414%.

Resume : Chemical bath deposition yields SnS thin films of cubic (SnS(CUB)) or orthorhombic (SnS(ORT)) crystal structure with optical band gap of 1.7 eV or 1.1 eV, respectively, and p-type conductivity. In the present work, we have developed solar cells incorporating these films of different crystal structures by sequential deposition on glass substrates coated with SnO2:F (FTO) and CdS. We used a chemical bath with thioacetamide as sulfide source for the cubic films and that with thiosulfate for the orthorhombic thin films. The superstrate solar cell: FTO/CdS/SnS(CUB)/C-Ag showed an open circuit voltage (Voc) of 0.368 V, a short circuit current density (Jsc) of 2.77 mA/cm2, a fill factor of 0.33 and a conversion efficiency (?) of 0.33%. The solar cell parameters became significantly better when an SnS(ORT) thin film is added, which leads to more light generated current through more effective optical absorption in it. Thus, the cell structure FTO/CdS/SnS(CUB)-SnS(ORT)/C-Ag presented Voc of 0.488 V, Jsc of 8.35 mA/cm2, FF of 0.40 and an efficiency of 1.65 %. The paper will also deal with results on our subsequent investigations on modifications in properties of SnS(CUB)-SnS(ORT), heating the cell structure, and application of different electrodes toward improving the performance of these solar cells.

Resume : Antimony chalcogenides, Sb2S3, Sb2Se3, and their solid solutions Sb2(S/Se)3, with optical band gaps (Eg) in the interval of 1.88 to 1.1 eV have prospects for application as absorber materials of low toxicity in solar cells. In the present work, we have prepared thin films of Sb2(S/Se)3 by chemical deposition as well as by thermal evaporation of the solid antimony sulfide/selenide, which settled as precipitate during chemical bath deposition of thin film materials. These thin films of Sb2(S/Se)3 have Eg of 1.45 ? 1.71 eV with a coefficient of absorption > 1E5 cm-1 in the visible region. The photoconductivity of the films is typically 1E-6 (ohm cm)-1. The films were integrated into solar cell structures, SnO2:F(TCO)/CdS/Sb2(S/Se)3/C-Ag. Solar cell using the absorber film of Sb2S0.78Se2.22 prepared by thermal evaporation showed a conversion efficiency (?) of 6 % with an open circuit voltage, Voc, of 0.503 V and a short circuit current density, Jsc, of 25 mA/cm2. In solar cells with chemically deposited thin film of lower selenium content Sb2S1.96Se1.04, a lower Jsc of 17.6 mA/cm2 and a conversion efficiency of 4 % were obtained. The paper will deal with these and results of further work on improving the solar cell parameters.

Resume : In this paper, we present a new approach based on the use of GaSe as buffer thin films for CIGSe solar cells with SCAPS simulator (Solar Cell Capacitance Simulator). In the contrast with the excellent proprieties of the thick CdS layer, the thickness seen to be lowest for a good match between performances and a thickness reduction of CdS was coupled to a dramatic decrease of device performance, due to homogeneity problems. This new approach allows a reduction of the thickness furthermore the parameters lattice are matching, these characteristics will keeping quality of the device and improving the surface coverage of the absorber layer with the buffer. The results of this study indicate that the buffer layer 10 nm thick we obtain value of 30 % efficiency, this is mainly explained by the current density of the solar cells increases and reaches the value of 42.68mA/cm². This opens the possibility to use ultra-thinbuffer layers for high efficiency chalcogenide based solar cells.

Resume : In order to understand the effect of stoichiometric change of Cu2-xS on the optoelectronic properties, a molecular solution based deposition approach for different composition of Cu2-xS films (x = 1, 0.4, 0.2, 0.04, 0) is proposed. To this end, different amount of Cu and CuS particles were dissolved in the binary thiol-amine alkahest solvent mixture due to its ability of room temperature dissolution and easy recovery at low-temperature solvent evaporation which helped the formation of desired Cu2-xS film compositions. A series of experimental techniques, such as XRD, FE-SEM, EDX, UV-Vis-NIR spectroscopy and Hall Effect measurement were used to understand the microstructure and optoelectronic properties of these films. This provides the systematic understanding of the stoichiometric dependent variation in optoelectronic properties of Cu2-xS films. Furthermore, the scanning tunnelling spectroscopy (STS) analysis provides location of band-edges of individual composition with respect to Fermi energy. This in-depth understanding of composition controlled electronic structure variation offers the guideline for application of identical composition to its respective place.

Resume : In searching strategies for improving photovoltaic chalcogenides it turns out that the material's properties are not well-known; not least this is because of the material's inhomogeneities. This problem led us to the development of a setup for two-dimensional electro-optical simulation of thin-film solar cells. The novelty of this program is that many simulation parameters are treated as random variables and their values are given by density functions rather than deterministic numbers. We believe this approach to be auspicious, since most simulations of macroscopic experiments lack of an exact mapping of the material's properties and probabilistic statements are the utmost information available. In the first part of this talk we focus on how the random number approach is realized in our simulation setup. Based on a few user requirements, the program calculates completely automatically a grain structure of the absorber, “deposits" conformally additional window layers on top, and distributes defect densities, doping densities and band gaps within the absorber. That way, the program forms the base for establishing a micro-structure model of the material under investigation. In the second part we demonstrate with several examples, how this program can be used to gain deeper insight into the influence of a microstructure on the photovoltaic parameters. We start with the effect of surface roughness, which is mimicked by a polycrystalline geometry with grain height distribution. The calculations reveal increased diode quality factors and saturation current densities, as a result of the enlarged space charge region. With the aid of raytracing it is shown that the optical incoupling in the present case is reduced against all odds. As a second example, we study the effect of grain boundary defects with additional grain size fluctuation. It turns out that the photovoltaic parameters decrease on average, if the grain size distribution is broadened while keeping the mean grain size constant, since the “worst" grain dominates the solar cell. The same applies to our last example of defect densities being distributed non-uniformally to the grains. Again, the photovoltaic parameters are reduced when the distribution is broadened and the mean value is kept constant.

Resume : The model of defects strongly correlated to the lattice and undergoing configurational changes upon illumination describes well persistent photoconductivity effect (PPC) observed in copper-poor Cu(InGa)Se2 compounds. Such model explains PPC as due to a transition between two configurational states of the defect which requires overcoming the potential barrier. Lany and Zunger relate this effect to one of the intrinsic defects, namely VSe-VCu. In our report we apply this model to evaluate quantitatively concentration of defects responsible for PPC. First we confirm experimentally that in agreement with theory the low-temperature conductivity in the relaxed state corresponds to a frozen non-equilibrium value depending on the cooling rate. Then we propose a method of calculation of the concentration of defect states responsible for PPC using the low-temperature ratio of conductivity in the relaxed and light-soaked state and under assumption that all defects convert from donor to acceptor state due after light soaking. We apply the procedure to a range of copper-poor thin films with different I/III ratio, selenium and sodium content and estimate the concentration of defects to more than 1017 cm-3 in most cases. The results indicate that defect concentration is constant within the range 0.75 - 0.99 of Cu/(In+Ga) ratio. Also sodium content is not the decisive factor while annealing in selenium decreases defect concentration only in case of the sodium-free sample.

Resume : The model for intrinsic defects of the Cu(In,Ga)Se2 chalcopyrite is still under debate. It is commonly agreed that there are at least one shallow donor and two shallow acceptors. A third acceptor A3 was proven to exist by spatially resolved luminescence for CuGaSe2, but is debated in CuInSe2. In the present study we show by low temperature spatially resolved photoluminescence (PL) that the peak at 0.95 eV in CuInSe2, that was previously attributed to the third acceptor, is a phonon replica and not a DA transition. But another pronounced peak at 0.9 eV together with its phonon replica was detected on polycrystalline CuInSe2 samples grown under high selenium excess and moderate copper excess. Intensity and temperature dependent PL reveal that this peak originates from a DA-transition with phonon replicas that have 28meV spacing and a Huang Rhys factor of 0.75. From temperature dependent PL measurements an activation energy of (120±20)meV is determined. It is concluded that there is a third acceptor in CuInSe2. From comparison with recent theoretical results and from growth experiments under various pressure conditions, we propose this might be the indium vacancy. Admittance spectroscopy on Cu-rich and Cu-poor CuInSe2 devices shows a step with an activation energy of 130meV, matching the A3 from PL. With the addition of small amounts of gallium to the composition (GGI up to 0.15) the activation energy determined by both methods increases, moving the defect deeper into the bandgap.

Resume : Deep level transient spectroscopy (DLTS) has a limited application for CIGS solar cells since the signals deviate significantly from typical textbook cases. In order to explain the origin of unexpected behaviour of experimental dependencies 4 mutually exclusive models were proposed and the discussion is still ongoing. In this work we simulate capacitance transients by solving a set of differential equations describing the response to a voltage pulse of a solar cell consisting of two junctions and one resistance representing the neutral region. We discuss how this extended two-diode model helps to understand previously unexplained distinctive features of N1 DLTS and RDLTS signals of CIGS solar cells: (i) the sign of signals and their amplitudes, (ii) the difference in the position and widths of RDLTS and DLTS peaks, and (iii) the increase of an amplitude at higher temperatures. We show that the expression for junction capacitance needs to be reformulated since the equivalent circuit and therefore the susceptance of a sample must be calculated differently. The simulation results are compared to the measurements of the samples with a modified surface after KF and NaF treatment. Contrary to literature data, the parameters used to achieve the best fit to experimental data suggest that the secondary barrier explaining the N1 signal is located in the buffer/absorber interface region rather than at the back contact.

Resume : Photoluminescence (PL) spectroscopy is a powerful tool to investigate defect states, potential and band gap fluctuations, as well as the quasi-Fermi level splitting of semiconductors. It can thus be used to analyze high-quality Cu(In,Ga)Se2 (CIGS) thin-film absorber layers and define properties leading to high power conversion efficiencies. However, CIGS absorbers that exhibit a double band gap grading, a common feature in high-end CIGS devices, are likely to show PL spectra dominated by interference effects. For example, if PL is measured at low temperatures, the interference fringes lead to the appearance of several peaks that could wrongly be attributed to individual defect transitions. Thus the presence of interference effects can lead to erroneous interpretation of data. In particular, interferences influence the low energy part of the PL spectrum, which is essential in the analysis of electrostatic potential fluctuations. Here we introduce a method for removing interference effects in CIGS by depositing an auxiliary surface layer on top of the absorber. This surface treatment consists of the chemical deposition of several monolayers of polystyrene beads. We thus create an additional scattering medium for the emitted photons that consequently lose their coherence. Hence, we can measure nearly interference-free PL spectra down to low temperatures and down to low energies. This approach allows to investigate the extent of electrostatic potential fluctuations in high quality CIGS absorbers. First results indicate that absorbers that underwent an alkali post-deposition treatment (PDT) show reduced potential fluctuations. We conclude that one of the benefits of the PDT seems to be an improved absorber quality by means of reduced non-radiative recombination as a result of smaller potential fluctuations.

Resume : Modulated photoluminescence (MPL) as a carrier lifetime measurement method was successfully applied during the past to silicon wafers characterization by several teams, for example at the GeePs laboratory. The goal of this technique is to perform a frequency study of semiconductor materials recombination dynamics by applying an intensity modulated lighting and monitoring the photoluminescence variations. Among other Photoluminescence techniques, Time Resolved Photoluminescence uses for example extremely fast light pulses as an optical excitation when the Quasi Steady-State Photoluminescence uses really slow pulses. The MPL technique is able to go from slow to fast varying excitation by changing the modulation frequency, and to work at a given injection level by adding a light bias, providing additional information on the carrier recombination dynamics. Our team recently built a new modulated photoluminescence setup, covering the [40 kHz, 4 MHz] modulation frequency range. The system was employed to record modulated photoluminescence response of a CdS/CIGS/Mo/Glass Stack. The experiment was performed at several lighting power. The obtained phase vs frequency curves will be analyzed by comparison with numerical simulation, including zero dimensional carrier densities rate equations and 1D drift-diffusion equations. The result will be compared to TRPL measurements and the presence and characteristics of carrier trapping centers discussed.

Resume : Application of a post-deposition treatment (PDT) of alkali metals to Cu(In,Ga)Se2 absorbers during processing has resulted in significant performance enhancements in this material system. Absorbers fabricated with such PDTs using RbF have resulted in world record-device performance for thin-film photovoltaics and enhanced minority carrier lifetimes reported for this material [1,2]. Detailed optoelectronic characterization is needed to understand the role these alkali PDTs have on the enhanced performance reported for this system. Accordingly, in this work we use optical spectroscopy to investigate the role that a RbF-PDT has on the defect properties of CIGSe grown by two-stage coevaporation. Here we find that a RbF-PDT results in a minority carrier lifetime approaching 500 ns, which is the highest value currently reported for this material. Quantitative photoluminescence analysis is used to verify the minority carrier lifetime measured from time-resolved photoluminescence. In contrast to previous reports regarding the benefit of KF-PDT, here we find that the benefits of the RbF-PDT mainly result from reduced interface recombination; the relative impact of interface recombination in samples with and without a PDT is measured using super-continuum excitation time-resolve photoluminescence. Lastly, the RbF-PDT is shown to significantly enhance stability of the Cu(In,Ga)Se2 absorber layer to photodegradation. [1] Jackson et al., RRL Solar, 8, 2016; [2] Jensen et al., JAP, 120, 2016, 063106

Resume : Kesterite solar cells based on pure sulfide CZTS (Cu2ZnSnS4) offer prospects of a cost effective and environmentally benign solution for photovoltaics. While promising on its own, CZTS is now receiving increased interest for applications in monolithic CZTS/Si tandem solar cells due to its high bandgap and low lattice mismatch with Si. However, integrating the Si and CZTS production steps to obtain an efficient monolithic cell is a challenging task. In this work, CZTS absorbers were produced by co- or sequential sputter deposition of the binary precursors SnS and ZnS and of the metallic precursor Cu. The characterization of a baseline single junction CZTS solar cell is presented, and some of the challenges and early results on the CZTS/Si tandem cell integration are reported. CZTS was grown on a Si substrate with a TiN diffusion barrier, to prevent the diffusion of some CZTS elements into Si during annealing – particularly Cu and S – that can degrade the photovoltaic properties of Si. The effects of annealing on the resulting structure were studied as a function of the initial CZTS precursor configuration and composition. The results of this initial integration and their implications for CZTS/Si tandem devices are discussed. The ideal thickness of the CZTS layer, in order to match the Si thickness for complete light absorption, was studied experimentally, in conjunction with optical simulations on complete CZTS/Si tandem structures.

Resume : Antimony selenide (Sb2Se3) is a promising photovoltaic material due to its suitable bandgap, strong light absorption, simple phase, nontoxicity, and earth-abundant constituents. Currently, most efficient Sb2Se3 thin-film solar cells are based on CdS buffer layer. But it is still a big problem that CdS is high-toxic and makes the device unstable due to the Cd diffusion. Hence, a new way using wide-bandgap and non-toxic oxide such as TiO2 and SnO2 as the buffer layer was explored to configure Sb2Se3 thin-film solar cells. For TiO2/Sb2Se3 solar cells, the TiO2 and Sb2Se3 films were fabricated by spray pyrolysis and rapid thermal evaporation (RTE) methods, respectively. Based on the high-throughput combinatorial approach (HCA), key parameters such as TiO2 thickness, post-annealing temperature and Sb2Se3 thickness are systematically optimized in a short experimental period. Finally, in combination with (NH4)2S back surface cleaning, TiO2/Sb2Se3 solar cells with 5.6% efficiency and decent stability were obtained.[1] For SnO2/Sb2Se3 solar cells, spray pyrolysis method was also used to fabricate SnO2 films. By controlling the interfacial defects and optimizing the quality of Sb2Se3 films, SnO2/Sb2Se3 solar cell with 3.05% efficiency was obtained, and the devices showed a superior light-soaking stability compared with CdS/Sb2Se3 solar cells.[2] The related experiments imply that it is possible to configure non-toxic Sb2Se3 thin-film solar cells, and further optimization and more studies are ongoing to improve device performance. Key words?Sb2Se3 thin-film solar cells; non-toxic buffer layer; high-throughput combinatorial approach Reference? [1] C. Chen , Y. Zhao , S. Lu , J. Tang* et. al. Adv. Energy Mater. 2017, 7, 1700866. [2] S. Lu , Y. Zhao , J. Tang* et. al. Adv. Electron. Mater. DOI: 2017, 10.1002/aelm.201700329

Resume : CuSb(S,Se)2 is emerging as a new absorber for photovoltaic (PV) applications, for its intrinsic p-type conductivity mainly due to Cu vacancies, tunable energy bandgap (1.0÷1.5eV), high absorption coefficient >10^5cm-1 and for the very cheap cost of Sb. Recently, some devices based on CuSbSe2 (CASe) reached a PV efficiency 𝜂 = 5% [1]. We studied structural, compositional and electro-optical properties of CASe films grown by Low-Temperature Pulsed Electron Deposition (LTPED) [2,3] vs. substrate temperature. CASe layers grown in a single stage from ternary targets between RT and 180°C exhibit a chalcostibite phase with Cu-poor composition (Cu/Sb≈0.55), while at higher temperature, a sort of “self-regulated” growth occurs. The Cu deficiency is compensated by the sublimation of Sb2Se3, leading to the formation of stoichiometric CASe phase, as confirmed by XRD/Raman analysis. By moving from solar cells based on Cu-poor CASe absorbers to stoichiometric ones, we found similar Voc (∼280÷320 mV), while the Jsc is boosted from ∼3 mA/cm2 to ∼20 mA/cm2. A FF of ~35÷40 limits the performances in all the cells, mainly due to the CIGS-like architecture and in particular to CdS. A post-growth annealing at 200°C improves all the electrical parameters in stoichiometric CASe cells, leading to a remarkable 𝜂 = 3.8%, very close to the state of the art. [1] A W Welch, et al. Adv Energy Mater 7, 1601935 (2017) [2] S Rampino, et al. APL 101 (13), 132107 (2012) [3] M Mazzer, et al. Sol Mat 166 (2017)

Resume : We report on the development of close-space sublimation (CSS) deposited antimony selenide (Sb2Se3) solar cells, and how the choice and modification of partner layers can enhance performance. CSS deposited Sb2Se3 thin films have large grains with preferred alignment along the (112) and (212) directions. However, intermixing between CdS-based window layers and the Sb2Se3 absorber layer has forced the use of more resistive oxides such as titania which has a detrimental effect on the fill factor. Here, we show that by doping the titania with niobium, charge extraction is significantly enhanced. Niobium doped titania window layers are deposited from solutions containing 0 to 10% niobium ethoxide : titanium isopropoxide in ethanol. Films are deposited via spin coating and annealed in air at 500ºC to create compact electron transport layers. The Sb2Se3 absorber layer is deposited on top using a CSS process at 450ºC without post-growth annealing or selenization. The cells are completed with a hole-transporting partner layer and a gold back contact. Performance improves, particularly via the current and fill factor, with cell parameters improving from η = 3.4%, Voc = 0.4 V, Jsc = 22.6 mA/cm2 FF = 37.3%, without Nb, to to η = 4.6%, Voc = 0.4 V, Jsc = 31.6 mA/cm2, = FF 40.8% with 5% Nb doping.