Earth-abundant next generation materials for solar energy - III

Resume : Lead halide perovskites show impressive optoelectronic properties as demonstrated by their high radiative efficiencies and power conversion efficiencies in photovoltaic devices surpassing 20%. Since this is rather exceptional for materials synthesized directly from precursors in solution with simple coating techniques, the quality of semiconducting lead halide perovskites has sometimes been attributed to their “defect tolerance”. However, substantial improvements of radiative efficiency can be obtained by various post-deposition treatments. The effectiveness of such passivation techniques demonstrates clearly that these materials do suffer from defects and, moreover, that these defects are likely to be localised at the surface of grains in polycrystalline films. In this contribution, we review the different passivation strategies, compare them and also combine them for the model MAPbI3 perovskite as well as for high performing mixed cation, mixed halide perovskite such as FAxCs1-xPb(IyBr1-y)3. We focus our discussion on the peculiar form of passivation achieved via light -soaking, also known as photo-brightening. From a thorough analysis of the literature and our own experiments, we propose a comprehensive mechanism for this effect and discuss the parameters influencing its occurrence. Finally, based on this knowledge, we devise a new passivation route that mimics the photo-brightening effect without exposure to light or humid and/or oxygen containing atmosphere. We present thin films with improved photoluminescence efficiencies and solar cells with enhanced Voc and power conversion efficiencies.

Resume : Over the past decade, the hybrid lead halide perovskites have been perhaps the most enticing materials family among emerging photovoltaics, demonstrating long carrier lifetimes, scalable solution-based syntheses, and optimal light absorption, especially when the halide and A-site are tuned. Nevertheless, problems relating to device stability and the toxicity of lead remain significant barriers to widespread manufacture that are yet to be overcome. Bismuth, however, has access to the same ns2 electronic configuration as Pb2+ - highlighted as a key driving force behind the hybrid perovskites’ exceptional properties - while also being non-toxic. Its favoured 3+ valence means that insertion into the AMX3 perovskite framework requires compensation by a 1+ cation in a double perovskite – at a cost to optical and electronic properties – or a vacancy on every third metal site to form a 2D defect perovskite structure. In this presentation, we have accurately examined the electronic, vibrational and optical properties of the Cs3Bi2X9 (X=Br, I) family using hybrid density functional theory, in collaboration with experimental partners, to assess whether these materials can be applied in single junction or tandem cells, as well as other optical or electronic applications. Particular focus will be paid to the excitonic behaviour of the defect perovskite structure, as well as tuning electronic properties while maintaining a high dimensionality structure through halide alloying.

Resume : Halide Perovskites (HaPs) have remarkable electronic and optical characteristics, but much is still unknown regarding the connection between their physical and chemical properties. Cation or anion substitution can change the optical absorption edge, with or without change of structure. In this work I explored the halide exchange reaction in methylammonium lead tri-halides single crystals (SCs) in order to understand the process of exchange and the stability of the product(s). I demonstrate halide exchange in mm-sized MAPbX3 SCs, achieved by diffusion. Using the Boltzmann-Matano method and diffusion profiles obtained by electron dispersive spectroscopy it is possible to evaluate the halide diffusion coefficients, which are not constant and depend on the mixture of halides. For all permutations the change in composition as result of the diffusion, strongly affects the optical and electrical properties and especially the band gap of the semiconducting crystals, as seen in cathodoluminescence measurements in the scanning electron microscope. While these gradients cause a lattice parameter change and may cause a symmetry change, X-ray diffraction measurements show that if the interchanged halide pair is such that their sizes are relatively similar (e.g., Br- and Cl-, Br- and I-, but not Cl- and I-) the resulting material remains surprisingly single crystalline. These findings, are valid, no matter which one of the two halides is being exchanged. These results suggest that for these similar-sized halide pairs, this exchange occurs through a solid-state chemical reaction such that the resulting crystal orientation is determined by that of the initial crystal.

Resume : SnS is a promising photovoltaic absorber material because of its low cost and less toxicity. However, various types of point defects exist in this material, affecting the electronic defect states and thereby its electrical properties. In this work, SnS films were fabricated first by varying the substrate temperature (Ts) in the range of 303-623 K on the soda lime glass (SLG) substrate in radio frequency (RF) magnetron sputtering, which are all found to be of Sn-rich and p-type. However, concentration of point defects (i.e., Sn vacancies, S vacancies, Sn antisites and/or Sn interstitials) is found to vary in these films, thereby affecting their overall bulk and surface electrical behavior. In a second case, SnS films were deposited by varying the deposition time from 15 to 90 mins at Ts of 573 K (Ts at which SnS film on the SLG substrate yielded the lowest electrical resistivity) on a stack consisting of a buffer (ZnO) and transparent conductor (Al-doped ZnO (AZO)); i.e., ZnO/AZO/SLG stack configuration. Whereas, the local surface electrical properties measured using Scanning Tunneling Spectroscopy (STS) from the SnS films deposited only on the SLG substrates demonstrated semi-metallic behavior, those deposited on the ZnO/AZO/SLG stack configuration showed ohmic behavior, because of an increase in the overall conducting domains from the substrate and/or films.

Resume : Currently, there is a considerable interest in the design of new low cost and high efficiency semiconductor materials to be applied in photovoltaic devices. Among different approaches, the power conversion efficiency (PCE) can be increased through the in-gap bap (IGB) concept. Using this concept, the efficiency could be improved through two extra sub-bandgap absorptions across the IGB: from the valence band (VB) to the IGB and from the IGB to the conduction band (CB). Using this approach the theoretical efficiencies could reach up to 63%. In this work, solid solutions of group 14 nitrides with spinel structure are selected as host semiconductor material. In this sense, those semiconductors are thermally stable materials and suitable for optoelectronic applications. Concretely, Sn3Ge3Ni8 spinel, with a band gap around 2 eV, presents optimal electronic features to host an IGB. A Vanadium doped spinel is here proposed as a new and promising in gap band material. A detailed analysis of this new material as well as of the host spinel has been carried out using Density Functional Theory (DFT) usign a GGA functional and a HSE06 screened hybrid functional. Our results, including structural and electronic properties, point out that Vanadium 3d orbitals introduce new electronic states within the gap of the host spinel, which form the intermediate band leading to improved absorption features through new sub-bandgap transitions.

Resume : The field of photovoltaics is undergoing a surge of interest following the recent discovery of the lead hybrid perovskites as a remarkably efficient class of solar absorber. Of these, methylammonium lead iodide (MAPI) has garnered significant attention due to its record breaking efficiencies, however, there are growing concerns surrounding its long-term stability and toxicity. Compounding these problems is a more fundamental limitation: the hybrid perovskite structure shows a severe lack of compositional tunability. Beyond minor changes to the halide and A site composition it has not been possible to significantly supersede the original MAPI formulation, because the hybrid perovskite structure is not accommodating to significant changes to composition of the A or B sites. Many of the excellent properties seen in hybrid perovskites are thought to derive from the 6s2 electronic configuration of lead, a configuration found in a range of post-transition metal compounds. In this presentation I will outline our computational and experimental efforts to unearth new earth abundant absorbers containing ns2 cations. Our results highlight the importance of electronic and geometric connectivity of the lone pair cations for achieving suitable electronic structures for PV applications, and we computationally pinpoint some new compounds with high efficiency.

Resume : Halide perovskites have attracted widespread attention for their reported defect tolerance, in stark contrast to traditional solar absorbers. Using synchrotron X-ray nanoprobe techniques broadly applicable to studies of next-generation absorbers, we present investigations of the relationship between the defects and microstructure that do appear in thin film hybrid perovskites and optoelectronic performance and stability. Using a series of model thin film materials, we reveal a wide-ranging heterogeneity in local chemistry and strain and link these nanoscopic variations to impacts on charge collection and luminescence. We also share insights from our investigation of non-stoichiometry and second-phase formation in triple- and quaternary-A-site cation perovskite absorbers toward improving performance. By elucidating and ultimately controlling the compositional landscape of halide perovskites, we aim to systematically accelerate the development of these materials for photovoltaics.

Resume : Perovskite materials are a great example of systems with a multitude of structure-property relationships. For example, metal insulator transitions in the nickelates and manganites can be linked to breathing and Jahn-Teller distortions which allow for charge and orbital ordering, respectively. This study examines the relationship between atomic structure and electrical band-gaps in the promising photovolatics1 material A2Au2X6 (A = Cs, Rb, K, X = I, Cl), through first principles calculations based on DFT. We find that there is a complex competition between various structural degrees of freedom in the material, which can be manipulated through chemistry and pressure, and that each of the structural modes can strongly tune the band gap. For example, in the Rb2Au2I6 double perovskite, we predict that, contrary to expectation, hydrostatic pressure produces a polar phase. We argue that this is due to a) the cooperative coupling of the Jahn-Teller and tilt modes2 which are both reduced with pressure, and b) the competitive coupling of tilting and polar modes. We finally discuss the effect of each of these modes and chemical changes on the band gap, and how the polar mode could be helpful to separate photo-excited carriers via the photoferroic effect. [1] Debbichi, L., et al. 2018. adv. Mater, 30, 1707001 [2] Mercy, A., et al. 2017. Nature Communications, 8, 1677

Resume : Organic-inorganic perovskite thin-film photovoltaics has experienced startling boost of power conversion efficiency (PCE) by virtue of various studies for fabricating dense, uniform, and large grain sized perovskite film with desired crystal orientation. In accordance with such progress, scaling up for large-area solar cells is also highly demanding to realize the commercialization of perovskite solar cells. Herein, we introduce simple temperature-tuned antisolvent bathing method compatible for large-area fabrication. By utilizing low temperature antisolvent, retarded solvent extraction resulted in decreased number of nuclei, thereby leading to large grain sized perovskite film. Furthermore, highly oriented crystal growth was obtained because delayed crystallization promoted perovskite intermediates to grow into (110) and (220) direction in a perpendicular direction to the substrate. Such an alignment in crystal orientation were found to be beneficial to charge extraction. In addition, temperature-tuned antisolvent bathing based slow crystallization technique allowed less inter- and intra-grain defects generation inside the perovskite films. Owing to morphological and crystallographic benefits in which charge transport/extraction properties were enhanced, 20% improvement in PCE was observed. The 6×12 cm2 sized highly uniform and large-area PSCs was successfully demonstrated, clearly verifying the scale-up capability.

Resume : The development of cost-effective hole-transporting materials (HTM) that are capable of providing stable perovskite solar cells (PSC) under variable environmental conditions is of paramount importance to prospect an effective commercialization of this skyrocketing photovoltaic technology. We present here an organic-inorganic ternary composite HTM based on a semiconducting poly(3-hexylthiophene) (P3HT) matrix used to disperse copper thiocyanate nanoparticles and semiconducting single-walled carbon nanotubes (SWCNT) for integration in direct-architecture PSC. The choice of this particular materials combination comes from rational design of the HTM, based on exploiting the ease of processing a polymer-based HTM as P3HT, the electron-blocking ability of a high-band gap semiconductor as CuSCN and the fine-tuning of the layer electronic properties through the addition of SWCNT. Physico-chemical and electrical investigations on the bulk HTM, coupled to photophysical probing of the perovskite/HTM interface, unravel the contributions and limitations of each component in the composite. A synergic action of the three species occurs, ensuring the maintenance of a stable power conversion efficiency in PSC stored un-encapsulated for almost one month in a high-moisture content atmosphere.

Resume : Semitransparent perovskite solar cells are of great interest for future photovoltaic applications, such as power-generating window in buildings or automobiles, which would raise the usage of solar energy without occupying additional space. In order for these techniques to be practically applied, scale-up studies including large area semitransparent solar module must be developed. However, studies on a semitransparent solar cell is still limited to a small area, since it is difficult to accomplish uniform and large area semitransparent perovskite films by a solution process. Here we suggest the use of anodized aluminium oxide (AAO) template as a scaffold enabling uniform and thin large area perovskite layer through a spin coating method. This perovskite layer is consisted of vertically aligned one-dimensional structure in AAO pores, thereby having the advantages of low reflectance due to periodic nanostructures. Also, transient photocurrent/voltage study revealed that this nanostructured device showed better charge collection efficiency compared to the planar structure. In addition, the nanopillar perovskite structure passivated by AAO walls exhibited significantly improved long term stability against humidity. Based on AAO derived methodology, a solar module with active size of 40.8 cm2 was fabricated, demonstrating a power conversion efficiency of 7.5% with an average visible light transmittance of 30.2%.

Resume : Effective mass production method leading to flexible perovskite solar cells (PSCs) should be developed. In this regard, low-cost and highly flexible transparent conductive electrodes (TCE) is highly required to fully exploit the advantage of cost-effective perovskite absorber. Copper nanowires (CuNWs) are the promising alternative to ITO as a bottom electrode in PSCs due to low sheet resistance and flexibility. However, instability against thermal oxidation and chemical corrosion of CuNWs poses major hurdle for its application to PSCs. In this work, we suggested a conformal protective AZO (Al-doped ZnO) layer for CuNWs by utilizing atomic layer deposition (ALD). The AZO/CuNWs showed excellent stability even after storage at 150°C for 60 min or direct exposure to perovskite precursor solution. The AZO/CuNWs electrode with optimized condition showed the average visible light transmittance (AVT) of 89.4%, which presented a slight decrease in transparency compared with bare CuNWs (AVT of 90.1%). In addition, the conductivity of AZO/CuNWs maintained after 400 bending cycles. This result suggests that the AZO/CuNWs composite electrode has high flexibility sufficiently enough to be applied as flexible TCEs. The flexible PSCs fabricated on the AZO/CuNW TCEs exhibited a power conversion efficiency of about 11% with long term stability. Our results propose that transparent electrodes based on CuNWs protected by ALD derived compact AZO could be a promising candidate for high performance flexible PSCs.

Resume : In recent years, nanostructures, such as silicon nanowires (Si NWs), have received much attention due to their several unique features (e.g., light trapping, effective charge collection and high aspect ratio), which are crucial for the realization of high performance opto-electronic devices, including solar cells and photodetectors. Under the present study, Si NWs were successfully synthesized by a simple and cost-effective technique, electroless etching (EE), for the fabrication of high efficiency next-generation (one-dimensional nanostructures based) n-Si Nanowire/ p-CZTS structured solar cells on inexpensive, lightweight, and amorphous surfaces using a fracture-transfer technique. Since the physical properties of Si nanowires (e.g., radius, length, density, and different elements) can be adjusted by the growth parameters, it is possible to fabricate high performance solar cells. In this study, therefore, nanosphere lithography (NSL) technique was combined with metal-assisted chemical etching of Si to fabricate well-ordered, high-aspect-ratio Si nanowires. Si NWs were synthesized using the polystyrene spheres in desired diameters and densities. The synthesized Si nanowires were embedded into a thermoplastic polymer (PMMA, polymethyl methacrylate) coated onto a glass substrate (carrier substrate), which was pre-coated with a transparent conductive oxide (TCO) layer. In order to transfer the nanowires embedded into PMMA onto the carrier substrate, a horizontal force was applied to the mother substrate for the fracturing-printing process. In the end, Si NWs were successfully transferred onto ITO coated glass substrates. The Si NWs transferred on TCO substrates were then deposited with CZTS absorber layer using e-beam method to construct n-Si NWs/ p-CZTS core/shell structured solar cell. The performance of the fabricated solar cell was tested under standard test conditions (AM 1.5G).

Resume : BaZrS3 is one of a very few chalcogenide compounds that crystallises in the Perovskite structure, and is potentially interesting as a wide band gap solar cell material for tandem cell applications. Based on the limited experimental and theoretical work so far available, BaZrS3 has seemingly similar opto-electronic characteristics to leading halide perovskites such as MAPI, but with the significant advantages that it is free of rare elements and heavy metals and furthermore has exceptional chemical stability. The latter characteristics are favourable for a next generation of sustainable, long-lived solar cells that deliver maximum lifecycle power output per unit of resource and energy consumption. To examine the feasibility of using chalcogenide perovskites such as BaZrS3 in solar cells, and to study relevant opto-electronic properties in detail, a synthesis route for high-quality thin films at relatively low temperatures is an important pre-requisite. In this work, we outline our progress towards production of BaZrS3 thin films using a combinatorial sputtering and thermal processing route. We present the synthesis process characteristics, basic optical and structural properties of the obtained thin film samples, and a discussion of the challenges faced in developing single phase and high-quality crystalline films of this interesting material.

Resume : Pyrite (FeS2) is receiving a growing and renewed attention as a low cost material for photovoltaic and solar fuel applications. The use of FeS2 thin films deposited on glass is very frequent. In this communication, the presence of Na proceeding from the substrate, when glass is used, and their influence in the properties of FeS2 thin films are investigated. Pyrite thin films have been grown by sulfuration of Fe layers thermally deposited on sodalime glass substrates. Sulfuration temperatures (Ts) have been ranged from 200°C to 500°C for 20h. Morphology and structural properties of the films have been characterized by, profilometry, Scanning Electron Microscopy and X-ray diffraction. The presence of Na and its distribution through the films have been investigated by X-ray photoelectron spectroscopies (XPS). From these results, it is concluded that the Na atoms diffuses from the substrate through the formed pyrite thin film and reaches its surface. The Na surface concentration increases as sulfuration temperature is increased up to 350°C, at 400ºC the Na surface concentration goes down drastically and at Ts > 400ºC Na surface concentration slightly increases. By comparing these results with the structural properties of the films (grain and crystallite size, mainly) it has been concluded that a change of the diffusion mechanism of Na through the film (from diffusion through the grain boundaries to diffusion through the bulk grain) takes place at the critical sulfuration temperature of 400 ºC. The consequences of this conclusion are discussed in relation with the films´ properties such as electrical conductivity and majority carrier density.

Resume : Group IV clathrates are open-structured Si, Ge, and Sn cage-like compounds. The space in the cage is usually occupied by guest species such as Na. The clathrate obtained by removal of the gust, i.e., the guest-free clathrates has attracted attention as a new semiconductor because of a predicted direct wide band gap. However, most of their optical and electronic properties still have not been investigated because we did not obtain high quality thin films suitable for optical and electronic measurements. We prepared thin films of type II Ge clathrate slightly doped with Na, i.e. NaxGe136 (x ~ 0) on sapphire substrates. The produced films with submicron thickness were characterized by X-ray diffraction (XRD), scanning electron microscopy, energy-dispersive spectroscopy (EDX), Raman scattering measurements, and visible-IR-transmission measurements. Rietveld analysis of the XRD, Raman, and EDX data verified the formation of a polycrystalline NaxGe136 film, in which the Na content x was almost zero. The optical transmission spectra was firstly clarified on the well-characterized film, and the fundamental absorption as well as the free carrier absorption was clearly observed in the near infrared region.

Resume : The influence of air-annealing of CdS/Cu(In,Ga)Se2 heterojunction at temperatures between 150–300 oC for different time periods on the performance of monograin layer solar cells was investigated. XPS was used to study the air-annealing effect on the chemical composition of the surface of the CdS and on the heterojunction interface. According to XPS measurements, some oxygen compound contaminants are present in the CdS films. Moreover, XPS depth profiles have shown that there is some Se interdiffusion to the CdS layer during the CBD process. During annealing at elevated temperatures, the concentration of Se increased in the CdS layer. On the top of CdS layer a thin layer of SeO2 was formed, even on the top of the non-annealed sample. But after 30 sec Ar+ ion sputtering, XPS core level spectra of selenium showed that Se (+4) was removed and only Se (-2) was remained, presumably forming a CdS1-xSex. RT-Raman and RT-PL were used to identify any change in structural or optical properties of CdS buffer layer. Temperature dependence of Voc showed that CdS annealing at moderate temperatures reduces interface recombination losses. The efficiency of Cu(In,Ga)Se2 monograin layer solar cells improved from 6.9% to 13.8% after air-annealing of the heterojunction at 180 oC for 1 hour. Further increase of annealing temperature has a detrimental effect on performance of solar cell that could be caused by too high S substitution by Se in CdS and/or appearance of S defect phase.

Resume : Nitrides are an interesting family of inorganic compounds for various applications. Zn3N2 is one of the attractive n-type nitrides with special interest, as it is composed of only earth-abundant elements. A direct band gap of 0.84 eV predicted by theoretical calculations indicates that it could be promising candidate for thin film photovoltaic absorber with the characteristics of being “ultra-low-cost” [1,2]. In addition, the computedly effective mass of electron is as small as 0.08 mo (mo is the free electron mass), implying that the high mobility is expected [2]. However, most of the reported results show the degenerate properties of Zn3N2 thin films with the room temperature carrier concentration above 10E18 cm-3. In this work, Zn3N2 thin films have been grown on glass and MgO (100) substrates by UHV magnetron sputtering. Their electrical and optical properties have been investigated. It is shown that the substrate temperature during the deposition can influence the carrier concentration remarkably. Heating on the substrate will increase the carrier concentration, while no intentional heating is beneficial to suppress the carrier concentration, giving rise to non-degenerate properties. Room temperature carrier concentration of 1.5E17 cm-3 has been achieved in Zn3N2 thin films grown without intentional heating. The mechanism behind this tunable properties will be discussed. Finally, the optical properties of Zn3N2 thin films will be presented.

Resume : CuSbS2 and CuSbSe2 have great potential for being an earth-abundant absorber material in efficient thin film solar cells. In this study, we explore the growth and optical as well as structural properties of CuSbS2 and CuSbSe2 polycrystalline powders in dependence of the growth conditions. Different precursor materials were used including binaries Cu2S/Cu2Se and Sb2S3/Sb2Se3, or elementary Cu, Sb, and S/Se. The synthesis temperature was varied in the range from 450C to 900C. Synthesis conditions for the formation of single phase CuSbS2 and CuSbSe2 were determined. The synthesized powders were investigated using Raman scattering, X-ray diffraction, Energy Dispersive Spectroscopy and temperature dependent photoluminescence spectroscopy (PL). It was found that the low-temperature (T=10K) PL spectra of both, CuSbS2 and CuSbSe2, consist of two emission bands - edge emission and deep PL emission. Temperature and laser power dependent PL measurements were performed to determine the dominating radiative recombination mechanisms in the studied materials.

Resume : Polycrystalline Cu2ZnxFe1−xSnS4 (CZFTS) thin films have been deposited by single source vacuum thermal evaporation method on Si (001) substrates and an annealing process in sulfur atmosphere, which can improve the crystallinity and morphology of the thin films. Structural, morphological, optical and electrical properties of the sulfurized layers were studied. The effect of zinc content on these properties has been studied in detail .X-ray diffraction study indicates that a phase transition from stannite to kesterite occurred with increasing the Zn content. From the morphological and optical studies an evolution of size and thickness of the grain according to the zinc content was observed. Frequency measurements, in the range from 5 Hz to 13 MHz, were performed in order to evaluate the effect of Zinc content on the conduction mechanism. The complex impedance diagram for the different layers showed a single semicircle, implying that the response originated from a single capacitive element corresponding to the grains. It has been shown from the experimental data that the AC conductivity in thin films of CZFTS is proportional to ωs (s<1). The temperature dependence of both AC conductivity and the parameter s is reasonably well interpreted by the correlated barrier hopping (CBH) model.

Resume : Time- related degradation effects have been rarely investigated to develop accurate current–voltage characteristics of photovoltaic devices. The development of new models including time- related degradation effects is crucial to predict the device behavior as function of stress time and provide the possibility to estimate the degradation behavior of the solar cell operating under stress conditions such as: prolonged radiations and long term reverse biasing. In this paper, we propose new electrical modeling approach including time-related degradation effects to predict the CZTS solar cell behavior working under stress conditions. The obtained results demonstrated that the accurate modeling of solar cells must be considered as a time-related degradation effect instead of static mechanism since the defect generation and annihilation is a dynamic process. Moreover, the results show the importance of the time-related degradation effect-based modeling in investigating the device physics of solar cells under prolonged radiations for spatial applications.

Resume : High quality Cu2CdGeSe4 micro-crystals have been successfully synthesized by molten salt method at 700 oC in closed quartz ampoules using elemental Cu, Ge, Se and binary CdSe as precursor materials and KI as flux material. The effect of initial Cu and Cd content on the bulk composition of grown crystals was investigated. Cu-rich composition resulted in the formation of Cu2-xSe secondary phase on the surface of Cu2CdGeSe4 crystals and with Cd-rich composition, separate crystals of CdSe were obtained. The quaternary compound crystals had slightly Cu-poor and Cd-rich composition ([Cu]/([Cd]+[Ge])=0.97 and [Cd]/[Ge]=1.05-1.08) irrespective of the initial composition. Raman analyses revealed the orthorhombic crystal structure of all studied Cu2CdGeSe4 crystals showing characteristic Raman modes at 162, 187, 203, 271 and 277 cm−1. Although, the orthorhombic Cu2CdGeSe4 has p-type conductivity, suitable band gap of 1.27 eV for solar energy conversion, and high absorption coefficient (< 104 cm- 1), there is relatively little known about the implementation of Cu2CdGeSe4 as absorber material in solar cells. In this study, the post-growth surface treatments– wet chemical etching and annealing in different ambient were implemented resulting in improved performance of the solar cell from 4.2% up to 5.6 % (Voc= 452mV; jsc= 24.8 mA/cm2; FF= 50 %)

Resume : Solar cells based on thin film Cu(In,Ga)Se2 (CIGS) present efficiency of up to 22.9%1 and is highest under thin film category. Although, considering Shockley-Queisser limit of 33.7% indicates there is still a sufficient room for improvement. Even with its small grain size of 2 μm, its maximum efficiency is achieved in its polycrystalline form, the reasons of which are debated over the years. Previous studies have shown an important role of grain boundaries (GBs) 2–4 towards its output efficiency. Here we have used electron beam induced current (EBIC) technique and have found that most of the GBs are indeed beneficial for the device, however a small percentage (15%) of GBs are found detrimental. Hence, in this work we have isolated GBs with different traits and have performed correlative EBIC-EBSD and atom probe tomography (APT) on them to identify its type, structure and composition. Results show that detrimental GBs are mainly enriched in Cu and O and depleted in Ga. Whereas characteristics of a beneficial GB are Cu depletion, no O and no change in Ga. With sufficient statistics, we have obtained characteristics of ‘detrimental’ and ‘beneficial’ GBs5. This information will be helpful towards future developments of CIGS solar cells and will assist in optimization of deposition parameters and post deposition treatments. References: 1 https://www.pv-magazine.com/2017/12/20/solar-frontier-hits-new-thin-film-cell-efficiency-record/ 2 C. Persson and A. Zunger, Phys. Rev. Lett. 91, 266401 (2003). 3 O. Cojocaru-Mirédin, T. Schwarz, and D. Abou-Ras, Scripta Materialia (2017). 4 M. Raghuwanshi, E. Cadel, P. Pareige, S. Duguay, F. Couzinie-Devy, L. Arzel, and N. Barreau, Applied Physics Letters 105, 13902 (2014). 5 M. Raghuwanshi, B. Thöner, P. Soni, M. Wuttig, R. Wuerz, and O. Cojocaru-Mirédin, ACS Appl. Mater. Interfaces 10, 14759 (2018).

Resume : The absorption coefficient and bandgap are two critical properties of sensitiser’s in next generation solar cells. The quantum confinement effect in nanocrystals has allowed the exploitation of the optoelectronic properties of quantum dots (QD) in a variety of applications. QD nanocrystals have the potential to avoid the current limitations surrounding thin-film solar cells. In addition, quantum dots provide a possible pathway to breaching the Shockley-Queisser limit through the generation of multiple excitons from a single photon.1 CZTS has been identified as a candidate material for non-toxic, earth abundant absorbers for use in thin-film solar cells, with research cells reaching PCEs of 12.6%. By using CZTS nanocrystals as a sensitizer, the optoelectronic properties can be tuned to absorb in the near-IR/IR region where a large proportion of the energy of natural light lies. One limiting factor for CZTS is its quaternary structure, which is difficult to synthesize without the formation of binary defects that degrades cell performance. In this paper, we report our recent work in size-controlled synthesis, characterization and elucidation of the charge transfer mechanisms involved in the nanocrystals to further the understanding and progress in to breaking the Shockley-Queisser limit. Techniques ranging from absorption spectroscopy and XPS to transient absorption spectroscopy, and device implementation are employed to determine the scope of CZTS nanocrystals as sensitizers and multiple exciton generation from quantum dots.

Resume : Sodium doped Sb2S3 materials was synthesized by a solid-state reaction using earth-‎abundant antimony and sulfur elements. Sodium was used as dopant element (3 wt %) for the first time. ‎The thermal evaporation technique was used to prepare SbS:Na thin films. Then, the films were annealed at different temperatures (150, 200, 250, 300 and 400 °C) in a vacuum pressure of 10^-3 Torr. ‎The Raman and XRD ‎characterization revealed the amorphous to crystalline transition behaviour depending on temperature. Atomic force microscopy was used to estimate the films roughness. A strong change of the surface morphology of the films was observed and it ‎depends on the annealing temperature and the weight percent doping. Optical constant were calculated from ellipsometric measurement.The films exhibit high absorption ‎coefficients (104 - 2×105 cm-1) in the visible range. Surface Energy Loss Function (SELF) of all the samples were estimated. The direct band gap (Eg dir) was in the ‎range 1.61-2.06 eV. The refractive indices show an anomalous behavior in the optical gap region. All the dispersion curves of refractive index ‎were analyzed using Wemple-DiDomenico model. The real dielectric constant and relaxation time were ‎calculated.‎

Resume : The two dimensional (2D) materials present an emerging field of research and promising candidates for many novel applications. The aim of our study is to determine the physical properties of Cu doped ZnS nano-sheet on the basis of first principles simulation in order of their applications for optoelectronic, solar cells and photonic devices. The Density Functional Theory (DFT) is used according to Full Potential Linearized Augmented Plane Wave (FP-LAPW) method as implemented in Wien2k code. The structural properties are studied using PBE-sol functional then the other physical properties are investigated using the modified Becke-Johnson (mBJ) to describe the exchange correlation potential. The structure of Cu doped ZnS nano-sheet is obtained by substituting zinc by copper at different concentrations. The optimized Cu doped ZnS nano-sheet gives a planar structure with small decreases of the lattice parameter. According to the band structure, a p-type Cu:ZnS nano-sheet semiconductor is obtained with a direct band gap decreasing with increasing Cu concentration. This is reflected by the Fermi level pinning at defect states, as revealed by the comparison of DOS for different Cu contents. A significant absorption and a decrease of the transmittance were noted for highly Cu concentration. More importantly, Cu:ZnS presents a net total magnetization revealed in the projected density of states associated to unpairing of neighboring S-orbitals induced by the partially saturated Zn atoms. These results open the way to use the Cu doped ZnS nano-sheet in the field of transparent spintronics, the photovoltaic and optoelectronic devices.

Resume : Copper zinc tin sulfide (CZTS) thin films were deposited by thermal evaporation procedure of a powder prepared by solid state reaction synthesis from elemental materials (Cu, Zn, Sn and S). The prepared thin films were annealed under different atmospheres (N2 gas, N2 + sulfur powder, vacuum). The effects of different annealing atmosphere on the structural, morphological, electrical and optical properties of the CZTS thin films were investigated. The X-ray diffraction study showed higher intensity peaks and crystallite sizes for the film annealed under sulfur atmosphere followed by vacuum annealing. SEM study revealed that the surfaces of the annealed films are uniform and densely packed. The optical parameters of the films were calculated from the analysis of the transmittance and reflectance data in the spectral range 300-1800 nm. All annealed films showed absorption coefficient exceeding 104cm-1 in the visible region. The optical band gap (Eg) of the samples were found to be in the range of 1.46 - 1.53 eV. We also studied the effect of post annealing atmosphere on electrical resistivity of CZTS films

Resume : Cu(In,Ga)Se2-based thin films are examined as the most promising photovoltaic materials due to its high efficiency, which has reached 22.6% with use CdS buffer layer deposited by chemical bath deposition (CBD). However, CdS exhibits problems as high toxicity and relatively low bandgap so an earth-abundant and non-toxic ZnS as a buffer has been studied as potential material use. Until now, many investigators studied to form ZnS buffer but forming pure ZnS material using CBD process was quite hard because of similar solubility constants of Zn compounds, such as ZnS, ZnO and Zn(OH)2. Moreover, the role of ZnO is important with ZnS because of band gap alignment with CIGS. Therefore, controlling ratio of ZnO and ZnS without Zn(OH)2 is the key for ZnS buffer to make operating solar cells. Here, we studied band alignment of CIGS/Zn(O,S,OH) interface using two different precursors to control the ZnO/ZnS ratios. The prepared materials are characterized using SEM, TEM, and XPS. The results will be presented and discussed in detail in order for better understanding of deposition mechanism.

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. However, forming pure CTS is quite hard because of the oxidation states of Cu and Sn. To get pure CTS phase, 1 of Cu and 4 of Sn oxidation states are needed but Cu2 is naturally stable in air. Therefore, the control of Cu oxidation state is important to get pure CTS phase. In this study, the chelating effect in the hybrid ink was used to control Cu oxidation state. Also, 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 : Solar cells with the structure ZnO/CdS/Cu2ZnSnSe4/Mo were fabricated on Mo/glass substrates. The Cu2ZnSnSe4 (CZTSe) absorbers in the cells were produced by selenising metallic precursors. The cells were irradiated by 10 MeV electrons with a dose of 10^15 cm-2. Photoluminescence (PL) spectra and their dependences on temperature and laser (532 nm) excitation intensity of the cells were measured in the spectral region from 0.75 eV to 1.8 eV before and after irradiation. At low temperatures the PL spectra before and after irradiation contained three broad bands: high intensity dominant band at 1.06 eV (P1), low intensity and low energy band at 0.99 eV (P2), high energy bands at 1.32 eV (P3) and 1.7 eV (P4). Their shape, temperature and excitation intensity dependencies suggested that they all are associated with band tails induced by spatial potential fluctuations due to high concentrations of charged defects in CZTSe and probably in CdS. Irradiation did not change the shape of these bands however their intensity became lower. The most significant reduction in the intensity (by 50%) was observed for the P3 and P4 bands whereas the observed reduction of the P2 band intensity was of 20%. The least change in the intensity of 10% was observed for the P1 band. These changes were explained by radiation induced defects.

Resume : Low-dimensional nanostructures, targeting low cost and high-performance devices, have a great potential to be used in optoelectronic devices like photodetectors (PD) and solar cells (SC). Due to their abundancy, chemical stability, facile production and fast response to the light, metal-oxide semiconductive nanostructures are promising candidates for photoconversion devices. In this study, we propose a new n-p junction nanostructure, based on conformal TiO2-Co3O4 core-shell nanowires system. The suitable energy gap of Co3O4 (two optical direct bandgaps in the visible region, 1.5 and 2.1 eV) and its electronic band structure in terms of positioning of conduction and valence bands with respect to vacuum, matches very well the position of TiO2 conduction band to build-up an efficient p-n heterojunction. The samples were prepared by atomic layer deposition of Co3O4 thin film on TiO2 NW arrays grown by hydrothermal method. Morphological characterization has been carried out by SEM and TEM, showing the conformal core-sell TiO2-Co3O4 structure. Structural characterization, by XRD and Raman spectroscopy, revealed both anatase and rutile phases of TiO2. The photoresponse of the TiO2-Co3O4 at zero bias is less than 0.1 s. A clear photovoltaic effect has been observed in the J-V solar cell characteristic, with a Voc 0.34 V, Jsc 50 μA/cm2 and FF 45%.

Resume : Gallium-doped Tin Sulphide (SnS:Ga) thin films have been grown by Chemical Bath Deposition (CBD). For comparison, also some samples with small amounts of Fe, Cu and In were investigated. We studied the intrinsic defect density and doping-induced defects by optical and electrical characterization of the films with respect to absorption coefficient, refractive index, and band gap energy, using Optical Transmission and Spectral Ellipsometry. The electrical properties like photo- and dark conductivity and estimates for trap energy levels were studied by contactless Microwave Transient Reflection (MWTR) and by coplanar electrical measurements. A substantial increase in grain size, a decrease in the initial decay time, and a strong increase in the photogenerated microwave signal is observed for thermal annealing above 400 oC. The results are discussed considering different model assumptions for the role of defects at crystallite surfaces and within bulk regions, respectively. SnS has several advantages for solar cell application due to its ideal band gap, the simplicity of thin film deposition, and the abundance of source materials.

Resume : Antimony triselenide (Sb2Se3) has attracted intense attention as a low-cost light absorbing p-type semiconductor for photoelectrochemical (PEC) water splitting. To achieve high PEC performance, it is necessary to suppress the charge recombination by facilitating the rapid charge extraction. Most of previous studies have focused only on the improvement of photogenerated electrons through the formation of p-n junction. However, these strategies are unsuitable for preventing un-expected recombination at the interface at bottom electrode/absorber layer, leading to photocurrent loss. Here, we introduce copper doped nickel oxide (Cu:NiO) as a hole selective layer (HSL) that requires wide band gap as well as high hole conductivity to readily extract the photogenerated holes and simultaneously block the photogenerated electrons. By applying the optimized Cu:NiO HSL to Sb2Se3-based photocathode in a configuration of FTO/Cu:NiO/Sb2Se3/TiO2/Pt, we achieved high photocurrent density of 18 mA cm-2 at 0 VRHE. In-depth studies of incident photo to charge carrier efficiency measurement and intensity modulated photocurrent spectroscopy suggested that our remarkable performance is originated from the improved charge separation capability at the bottom contact/absorber with an aid of HSL layer, enhancing light harvesting ability in long wavelengths (550 ~ 1000 nm). We believe that introduction of HSL opens up the possibility to significantly improve the performance in Sb2Se3 photocathode.

Resume : Tin monosulfide (SnS) is a promising candidate for high-efficiency solar cell using earth-abundant materials. However, the efficiency of SnS solar cells is low. Some reasons for this discrepancy is the unidentified defect levels and species of SnS thin films. Other chalcogenide solar cells, such as CIGS and CZTS, can be used to control the carrier properties through intrinsic defects. To realize high efficiency solar cells, the defect properties of SnS must be understood. An investigation into the defect levels of the SnS layer and compensation mechanisms of the defects by fabrication of a SnS layer with various p-n junctions will be described in this presentation. A SnS precursor was deposited by conventional RF sputtering onto a Mo/SLG substrate. A CdS or ZnO film was deposited on SnS by CBD and ALD, respectively. To investigate the defect properties of the SnS thin film or related pn junction, PL, Raman spectroscopy, and admittance measurements were carried out. For example, typical low temperature PL spectra were obtained using the CdS/SnS structure with post annealing. On the other hand, no emission was observed from the SnS thin film and CdS/SnS without annealing because of the number of non-radiative defects. These results indicated that the Cd and S atoms diffused into the SnS layers. The non-radiative defects were passivated by the chemical bath deposition of CdS and post annealing, due to the Cd related extrinsic defects and/or Sn and S related intrinsic defects.

Resume : Transparent Conducting Oxides (TCO’s) has an important role in information technologies and the solar energy generation. Many devices like solar cells, organic light-emitting diodes, flat or flexible displays and low-emissivity coatings can be achieved with using TCO’s. In this work, nucleation and TCO properties of Earth-abundant ZnO and AZO (ZnO-Al2O3 2%wt) polycrystalline films growth by RF magnetron sputtering at room temperature have been investigated. In order to study the growth evolution, different ZnO and AZO samples with thickness ranging from 30 to 650 nm were deposited. Optical transmittance was performed using UV-Vis-NIR spectrometer (Lambda 750, Perkin Elmer) ranging from 190 nm to 2500 nm. All samples show average transmittance greater than 80% in the visible region. Electrical resistivity was measured by linear four-point probe method and Hall effect (ECOPIA-3000). The electrical resistivity decreases monotonically and stabilizes around 0.001 Ωcm for films thicker than 100 nm. Surface morphology was measured with atomic force microscopy (XE-100, Park Instruments) operating in air. All AFM images of the films were analyzed using scaling concepts. The surface roughness shows up from non-local shadowing mechanism and surface diffusion competition. The orientation and size of crystallites were estimated using X-ray diffraction (D/MAX-2100/PC, Rigaku). The chemical composition was obtained by Rutherford backscattering spectroscopy (RBS) with 2.2 MeV He ions. Our results indicate the formation of an Al-rich amorphous layer during the early growth stages, followed by an evolutionary selection of crystal orientation. For 100 nm-thick films, columnar structures with preferential orientation in (002) planes take place.

Resume : High-efficiency planar perovskite solar cell was developed using a solution-processed NiO hole-transporting layer (HTL). Nickel acetate tetrahydrate dissolved in 2-Methoxyethanol solution was spin-coated on the indium tin oxide (ITO) substrate in an atmosphere. The effects of annealing conditions and precursor concentration on the photovoltaic performance were systematically investigated. The post-annealing process was progressed in vacuum and atmosphere, and then the annealing temperature was scanned from 150 oC to 450 oC. The 400 oC annealed NiO thin film at atmospheric condition demonstrated the best performance at same precursor concentration. This NiO thin film with an amorphous phase and thickness of about 40 nm was found to be optimal for hysteresis-free high photovoltaic performance. Also, the photovoltaic performance and hysteresis were influenced by the precursor concentration. The NiO thin film at 0.2 M concentration showed high efficiency among the concentrations range of 0.1 M to 0.25 M (interval 0.05 M) owing to a larger and faster photoluminescence quenching. Consequentially, the planar CH3NH3PbI3 perovskite that was formed on the 40 nm thickness, 0.2 M based and 400 oC annealed NiO thin film delivered a power conversion efficiency (PCE) of 15% averaged out from the forward scan PCE of 15.02% and the reverse scan PCE of 15.1%.

Resume : Up to now range of methods have been reported for the synthesis of CFTS NCs of different shapes and sizes.1-3 The hot injection method is the most commonly used method for synthesis of size selective QDs with controlled shape and reproducibility. The method often use organic capping agents such as trioctylphosphine (TOP), oleylamine (OA) and other long chain amines, which increase the complexity of reaction, causes impurity in the products and enhances the toxicity of NCs. Therefore, it is mandatory to have an alternative facile approach for synthesis of CFTS NCs. The melt method utilized in the present study has many advantages and has been used to prepare CFTS nanocrystals in large quantities in the lab. The technique is a straight forward, inexpensive and single step utilizing xanthates as single source precursors for the fabrication of CFTS NCs. Phase pure quaternary chalcogenide nanocrystals of Cu2FeSnS4 from Sn(II) and Sn(IV) have been synthesized using solvent-less, simple and inexpensive melt method using a mixture of Cu2+, Fe3+, Sn2+ and Sn4+ O-ethylxanthates and annealed at different temperatures. The as-synthesized nanocrystals were characterized by (p-XRD), Raman spectroscopy, UV–Vis absorption, scanning electron microscopy (SEM) and energy dispersive X-ray (EDS). The average particle size of the nanocrystals calculated using Scherrer’s formula was found to be 13 nm each for both synthesised. Optical measurements show that the Cu2FeSnS4 nanocrystals which have synthesised from Sn(II) and Sn(IV) exhibit strong visible light absorption with direct band gap values of 1.35 eV and 1.37 eV, suitable for photovoltaic applications.

Resume : Abstract: Ternary zinc oxy-selenide Zn(O,Se) is a promising material for the replacement of the toxic CdS buffer layer in thin film solar cells. Herein, we report a successful formation of polycrystalline, dense, uniform and electrical conductive Zn(O,Se) layers onto PV glass substrate at 500 ℃ by pulsed laser deposition (PLD) technique in high vacuum. XRD investigation revealed that single phase of Zn(O,Se) films formed at 500 ℃ in high vacuum. UV-Vis analysis showed the transparency of all deposited Zn(O,Se) films in visible part of the spectrum with the significant shift in optical band gaps from 3.48 eV to 2.85 eV with the increase of film thicknesses. The Hall effect measurements illustrated n-type conductivity of Zn(O,Se) layers. Moreover, fluctuation in the carrier concentration, electron mobility and electrical conductivity observed with the increase of film thickness.

Resume : In this study, we report on the formation mechanisms of the double layer, voids, and ZnSSe layer in the CZTSSe which was shown high efficiency of 12.5% certified by the Korea Institute of Energy Research (KIER). Detailed formation mechanisms are reasonably proposed based on phase evolution tracking at various temperatures by means of TEM analysis. We proposed formation mechanisms for the CZTSSe double layer, voids and ZnSSe layer, which were observed in the CZTSSe using metal precursor. Due to the persistent dezincification from the metal precursors and preferential reaction between the Zn and chalcogens such as S and Se, almost all Zn is consumed to form the ZnSSe layer; as a result, large voids are produced first under the ZnSSe layer. Cu2Se and SnSe are grown on the ZnSSe layer via migration of the Cu and Sn through the grain boundaries of the ZnSSe layer. Thus, additional small voids are expected to form due to the mass transfer of Cu and Sn. Because of the preferentially formed ZnSSe layer and the chalcogenation of Cu and Sn after the mass transfer, a CZTSSe double layer can be formed, and ZnSSe can exist between these CZTSSe layers. This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012980), the Technology Development Program to Solve Climate Change of the National Research Foundation (NRF) funded by the Ministry of Science and ICT, KOREA (2016M1A2A2936781), and the DGIST R & D Programs of the Ministry of Science, ICT & Future Planning of Korea (18-BD-05). We thank Mr. Cheon and Mr. Eun in CCRF for the STEM measurements.

Resume : Transparent Conductive Oxide (TCO) films, usually of indium tin oxide (ITO), have been widely used as transparent electrodes for various applications, such as organic light-emitting devices, flat-panel displays and solar cells. At present, as good alternative to ITO, many research groups have studied Al-doped ZnO (AZO) films which are non-toxic and derive from readily inexpensive source materials. Since polymer substrates are cheaper, lighter and more flexible than glass substrates, they could be used to manufacture flexible displays and solar cells. Polymer substrates are more heat sensitive, however, which limits the temperature and energy during the TCO process. Furthermore, poor adhesion between TCO and polymer substrates and the development of cracks need to be overcome. In this work, AZO thin films (around 200 nm thick) were grown on polyethylene terephthalate (PET) at room temperature. The plasma was activated using a 13.56 MHz (rf) or a 15kHz pulsed (PMS) source at a power of 60 W. Optical reflection and transmittance were measured using a UV-Vis-NIR spectrometer (Lambda 750, Perkin Elmer) over the wavelengths from 190 nm to 2500 nm. All samples show average transmittances greater than 83% in the visible region. Electrical resistivity was measured by the linear four-point probe method and by the Hall effect (ECOPIA-3000). The electrical resistivity was measured to be around 0.001 Ωcm for 200 nm-thick AZO films grown by PMS. Surface morphology and composition of AZO thin films were characterized by secondary electron images and x-ray dispersive energy spectroscopy (EDS). For this, an EDS coupled to the scanning electron microscope (JEOL JSM-6010) was used. No cracks were observed only for films grown by PMS.

Resume : The powdered photocatalysts and semiconductor photoelectrodes for water splitting have become a more and more attractive study in the field of solar energy utilization. Development of a novel photoelectrode material with high electrical properties and visible light response has been sought for efficient utilization of solar energy. BiCuSeO, an oxide thermoelectric material, has attracted great attention due to its excellent thermoelectric device performance. Due to its layered structure, the BiCuSeO can also form nanoflake structures easily and has a larger specific surface area. Such morphologies would be beneficial for the surface catalyst. In this work, BiCuSeO thin films were obtained by evaporating precursors following with selenization treatment. The annealing temperatures and oxygen concentrations were optimized to obtain high-quality films. A great number of nanoflakes could be found in the thin film surface, which can help to be used as the catalyst. Optical absorption measurement indicated the high absorption coefficient in the range from 300 nm to 700 nm. Hall effect measurement indicated that the optimized film had the mobility of 3.2 cm2/Vs and carrier density of 2.9*1017 cm-3. Photo- electrochemical analyses showed that the photocurrent onset potential of BiCuSeO was about 0.4 V to RHE potential and achieved the photocurrent density of 10 mA/cm2 at relatively low bias, 0.55 V vs RHE. Those results indicated that BiCuSeO was a potential material for photocatalyst application.

Resume : Turnover of new materials into high-performance devices is often slow and inefficient, hampered by chemical incompatibility between the junction partners, unfavorable band evolution as the junction forms, Fermi level pinning, or large band discontinuity. If not properly addressed, these phenomena can lead to underperforming devices regardless of how good the bulk properties of the absorber material are. In the first part of this talk, we will review the state-of-the-art of some emerging absorber materials, such as ZnSnN2, Zn2SbN3 and BiOI. The potential of these compounds as solar absorbers will be assessed by comparing their properties to those of industrially-adopted materials, giving a perspective on the future of these materials for photovoltaic (PV) applications. In the second part of the talk, we will discuss the pivotal role of interface characterization by X-Ray Photoelectron Spectroscopy (XPS) in accelerating device fabrication and optimization. In particular we will present the evolution of the operando-XPS characterization method, a technique which consists of coupling traditional XPS experiments with light bias, to determine PV-relevant properties such as VOC. Finally, we will present examples of the application of this technique to study passivation mechanisms, junction candidates as well as assess the stability of the interface in new or already established PV absorbers.

Resume : Elemental substitution of either Cu or Zn in kesterite Cu2ZnSnS4 (CZTS) thin films has revealed exciting properties, such as increased grain size, tunable carrier concentration and band gap. In particular, alloying of CZTS with Ag has shown to improve the open circuit voltage (Voc) of the device because of reduced CuZn anti-site defects, improved carrier lifetime, reduced carrier density and band-tailing. An increase in the efficiency of the (Cu1–xAgx)2ZnSnS4 (ACZTS) devices by nearly 40% as compared to pure CZTS has been reported1. In this paper we study the effect of Ag on the electronic structure of the ACZTS surface and CdS/ACZTS interface using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to gain a better understanding of the band alignment at the CdS/ACZTS interface. Simultaneously, the position of the conduction band minimum of CZTS and ACZTS was determined by the leading edge of the S L2,3 X-ray absorption spectroscopy (XAS) spectra and it allows for a direct measurement of the conduction band offset. To access the interface, a mild sputtering of the CdS top layer was performed. Our findings reveal a reduction in the conduction band offset of ACZTS compared to that of CZTS by ~60 meV, which suggest that Ag alloying of kesterites can be promising approach for improving efficiency in these emerging thin-film solar cells. (1) Chagarov, E.et al., J. Chem. Phys. 2016, 144 (10), 104704.

Resume : Abundant amorphous transparent conducting oxide (a-TCO) materials possess great potential for impact in the field of optoelectronic devices due to their high conductivities, comparatively low deposition temperatures and their innate resistance to degradation upon bending. The high indium content of the current preeminent a-TCO material, amorphous InGaZnO4 (a-IGZO) [1], raises concerns about scarcity, supply and rising costs. In order for a sustainable low cost electronics industry to exist In-free alternatives must be developed. One potential abundant replacement a-TCO system is a-ZnSnO (a-ZTO)[2] . In this work we present extensive Zn/Sn composition maps of co-sputtering grown ZnSnO. Using Hall, X-Ray Diffraction/Reflection (XRD/XRR), X-ray Photoelectron Spectroscopy (XPS) and post deposition heat treatment with in-situ monitoring of the electrical properties we directly link the mobility, conductivity and carrier concentration of ZTO to its composition. We thereby establish parameters required to optimise its electrical properties. Conductivities of up to 225 S/cm2 and mobilities of 16 cm2/Vs are achievable via both reactive and non-reactive magnetron deposition. Furthermore, this analysis reveals that the relationship between the Zn/Sn ratio and the electrical properties of the material can vary depending on the deposition technique utilised, despite the overall conductivities achieved remaining similar in magnitude. 1. Kamiya, T., K. et al., Science and Technology of Advanced Materials, 2010. 11(4). 2. Fernandes C., et al, Advanced Electronic Materials, 2018, 4, 1800032

Resume : Multi-junction (MJ) solar cells and concentrator photovoltaics continue to achieve efficiencies way beyond the traditional technologies. Apart of tandem solar cells dominance in the ever-growing space industry, terrestrial utility-scale levelized cost of electricity (LCOE) for this technology is already competitive in sunny areas, and the LCOE is predicted to decline faster than for the traditional photovoltaics. The development of new generation thin-film MJ solar cell AlGaAs/GaAs/GaAsBi, which includes the sought-after 1 eV junction, is presented here. This configuration offers a higher power conversion efficiency and larger power-to-weight ratio than current state-of-art 3-junction solar cells that use Ge as the bottom cell. Epitaxial GaAs substrate lift-off and multiple reuse is employed here in the inverted-growth approach, which allows to further lower costs of the technology and deploy it on flexible carriers. Structural and chemical atomic-resolution scanning transmission electron microscopy of the 1 eV GaAsBi subcell exceeding the layer's diffusion length, as well as its characteristic photovoltaic measurements are presented. Molecular beam epitaxy synthesis, secondary-ion mass spectroscopy, and structural/optoelectronic characterization results of the constituent AlGaAs/GaAs/GaAsBi subcells and tunnel junction inter-connects are also shown and discussed in regards to the development strategy of this device.

Resume : Owing to its elemental composition, ideal 1.5 eV band gap, and low effective masses, ZnSnN2 is a candidate for earth abundant photovoltaics (PV) [1,2]. Computational studies of defects and doping are of high interest for PV applications, but the conventional dilute-defect model does not capture the effects of disorder and off-stoichiometry. Here, we present a predictive defect model that includes these non-idealities. The composition can be described as Zn1+xSn1-xN2-2xO2x and the atomic arrangement structure is sampled by Monte Carlo (MC) simulation with a model Hamiltonian. The change of the electronic structure with composition and due to disorder is taken into account in the defect thermodynamics to obtain the doping properties. Non-equilibrium growth conditions are modeled via the chemical potentials. The MC simulations also reveal specific “magic” compositions (here, x=0.25), at which the system can exhibit perfect short range order (SRO) conserving the local octet rule despite the absence of long range order. These structures have a much reduced mixing enthalpy, increased band gap, and a pristine electronic structure without charge localization effects. In a separate materials design effort [3], we explored novel ternary nitrides, and discovered and experimentally realized Zn2SbN3 with a slightly larger gap of 1.6 eV. [1] S. Lany et al, Phys Rev Mater 1, 035401 (2017) [2] A.N. Fioretti et al, Mater Horiz 5, 823 (2018) [3] W. Sun et al, arxiv/1809.09202 (2018)

Resume : Zinc tin nitride (ZnSnN2) has received considerable interest recently as a potential earth-abundant element based compound semiconductor for photovoltaic device applications. These elements are inexpensive, readily available in high-purity form, non-toxic, and both zinc and tin benefit from mature recycling infrastructure. ZnSnN2 has been synthesized using a variety of techniques, although there remains much that we do not understand regarding its fundamental properties. Using the technique of plasma-assisted molecular beam epitaxy (PA-MBE), we have studied the growth of crystalline thin films of this ternary heterovalent compound with a view to understand both optical and electronic properties. We have demonstrated that the optical band gap energy exhibits a strong correlation with the ordering of the cation sublattice, and used this to determine the full range of possible band gap energies (which span the optimal values for a terrestrial single junction solar cell). Cation ordering is quantified through the long-range order parameter, which can be measured by x-ray diffraction, Raman spectroscopy, or in-situ reflection high-energy electron diffraction. We are able to control the cation ordering over essentially the entire range of possible long-range order parameter values through systematic variation of PA-MBE growth parameters, and have demonstrated reproducibility of a few percent, making it possible to repeatedly synthesize the material with the desired band gap energy.

Resume : In this work, we present a combinatorial study of sputtered ZnGeN2 thin films, with cross-cutting applications in both fundamental materials science and novel device development. The II-IV-N2 materials, which are structural analogs to the III-N materials, offer the possibility of controlled disorder of the cation sublattice which would allow tunable properties at fixed composition. ZnGeN2 is analog and closely lattice-matched to GaN and exhibits a direct bandgap with predicted strong absorption, but experimental studies to date report inconsistent optical properties. Additionally, little work has explored variation with cation composition, which has been shown to greatly impact properties of other II-IV-N2 materials such as ZnSnN2. In this work, we present a study of combinatorial ZnGeN2 grown by RF co-sputtering in both a high- and low-oxygen background environment. X-ray diffraction reveals phase-pure films in the expected cation-disordered wurtzite structure for cation compositions from 30% to 60% Zn/(Zn+Ge) and synthesis temperatures from 200C to 600C. Changes in crystallinity are explored as a function of cation composition, synthesis temperature, and in-situ and ex-situ annealing. Finally, ellipsometry and photoluminescence are performed to investigate optical properties as a function of incident energy. This study re-affirms the tunability and potential of thin film ZnGeN2 as a direct- and wide- bandgap optoelectronic material.

Resume : Cu2ZnSnS4 (CZTS) is considered a promising candidate as an absorber material in solar cells. CZTS devices are, however, limited by a large voltage deficit. In this work, we examine correlations between the voltage deficit observed in devices and the material quality measured with other characterization techniques. Cu-Zn-Sn-S precursors were deposited by co-sputtering from binary targets. Precursors were annealed using a wide range of annealing times, background pressures, and sulfur quantities. These annealing experiments resulted in samples with a wide variation in absorber quality. A clear correlation between the voltage deficit of the devices and properties of the samples measured by Raman spectroscopy, quantum efficiency (QE), and photoluminescence (PL) is observed. The poorest devices are dominated by a large contribution of tail states near the band edges, while these states are reduced in better devices. A red shift of the A1 mode observed in Raman spectroscopy likewise correlates with an increase in voltage deficit. From PL measurements it is observed that the luminescence intensity of the devices with high voltage deficit was much weaker than better performing devices. It is argued that the high density of tail states formed under non-ideal annealing conditions causes the increased voltage deficit. The changes in the Raman signatures observed at the same time could have the same origin, but more research is needed to verify this.

Resume : Cu2ZnSn(S,Se)4 (CZTSSe) and Sb2Se3 are attractive candidates to be used as absorber for thin-film solar cells. These materials are characterized by a low toxicity and earth-abundant nature. One of the most crucial parts of high efficient thin film heterojunction solar cells is the absorber/buffer-layer interface. The highest CZTSSe-based device performance has been achieved by using CdS as buffer layer. However, the bandgap of 2.4 eV of CdS is quite low, observing losses in short circuit current density (Jsc) due to the absorption of low wavelength photons. In a recent study, the possibility of a ZnS/CdS hybrid buffer layer was investigated to combine the advantages of a wider bandgap buffer with the adequate absorber/buffer band alignment. This work is extended to a mixed Cd1-xZnxS buffer layer deposited by chemical bath deposition in a single deposition run. Earth-abundant CZTSSe and Sb2Se3 solar cells with Cd1-xZnxS buffer layer are compared to reference cells. The Cd1-xZnxS buffer resulted in solar cells with significant improved Jsc and Voc values and comparable fill factor. That fact led to achieve efficiencies of 9.5% and 5.31% for CZTSSe and Sb2Se3 respectively. Strategies for incorporating different Zn/Cd ratios using bath modifications will be presented, together with a full devices and materials characterization using C-V measurements, Raman spectroscopy, SEM/EDX and SIMS depth profiles.

Resume : Kesterite compounds offer a CRM-free alternative to traditional thin-film CIGS solar cells. However, the requirement of heavy out-of-stoichiometry compositions for high efficiency also promotes large inhomogeneities at the microscale level, both in composition and point defects formation that can limit the performance of the devices. This issue is especially relevant in two-step sputtering-based processes where the inhomogeneities are already present in the metallic precursor stacks. In this work, we explore the enhancement of the micro-homogeneity of Cu2ZnSnSe4 absorber layers by comparing standard DC-sputtered elemental Cu/Sn/Zn metallic precursors with bronze-based ones. Compositional, structural and morphological analyses (AES, XRD, Raman and SEM-EDX) confirm a high homogeneity improvement when employing bronze precursors that allows obtaining highly compact absorbers with large crystals (≈4μm). In addition, by stopping the selenization process at different stages, we demonstrate that bronze-based precursors induce a ternary phase-based (Cu2SnSe3+ZnSe) reaction path allowing a more controllable selenization process, minimizing Sn-loss and reducing the formation of inhomogeneities. This approach has allowed us to obtain a promising 8.2% efficiency device using the same precursor atomic ratio than the standard elemental stack. However, the optimization of the composition and the selenization process opens the possibility of substantial efficiency improvement.

Resume : Kesterite material Cu2ZnSn(S,Se)4 (CZTSSe) has been shown as a promising candidate to be used as absorber for thin-film solar cells. This material is characterized by a tunable band-gap energy between 1.0 and 1.6 eV adjusted via S/Se-ratio, low toxicity and earth-abundant nature. Wide band-gap absorber layers are attractive for top cells of a cost-efficient tandem device and for semitransparent solar cells. Another possibility to increase the band-gap energy of the absorber layer is via the partial or total substitution of Sn with Ge, reaching band-gap values between 1.4 eV (Cu2ZnGeSe4, CZGSe) and 2.25 eV (Cu2ZnGeS4, CZGS). Contrary to the S inclusion on kesterite, the introduction of Ge improves Voc-deficit, the main limitation of these devices. In this work, Cu2ZnSn1-xGeSe4 (CZTGSe) thin films have been grown by co-evaporation onto Mo/soda-lime glass and FTO substrates. In a second step, some co-evaporated samples are sulfurized. All absorber films are systematically characterized with SEM, EDX, GIXRD and the processed solar cells are analyzed with JV and EQE measurements. The effect of substrate temperature during co-evaporation process and Na diffusion from soda-lime glass into kesterite layer is investigated. A higher Na concentration implies a higher Ge incorporation into CZTSe lattice and leads to a bigger grain size, what could be a key parameter to enhance device performance. So far, CZTSSe-based solar cells with efficiencies of 6.4 % and Eg = 1.17 eV and of 3.5% and Eg = 1.5 eV have been achieved for S/(S+Se) = 0.38 and 0.84 respectively. The addition of Ge has led to efficiency devices of 5.5 % and Eg = 1.28 eV using CZTGSSe, and of 4.0 % and Eg = 1.10 eV with CZTGSe as absorber layers. Solar cells with higher Ge contents are being investigated.

Resume : Photovoltaic devices have a large role to play in our transition from fossil fuels to renewable energy. To collect the maximum energy from the sun, and in order to make the energy transition feasible, photovoltaic devices must be as efficient as possible. Tandem junction devices have higher device efficiencies than single junction devices, but are not yet a commercial reality on a largescale, and thus should be researched. The bottom cell in a tandem junction device in either two or four terminal configurations must have a band gap of between 0.9 and 1.1 eV for maximum power generation. Current viable material choices for bottom cells either require energy and carbon intensive processing, i.e. silicon, or scarce elements, i.e. the indium in Cu(In,Ga)Se2. The search for alternative inorganic material systems has mainly focused on Cu2ZnSnSe4 and Cu2(Sn,Ge)Se3, which use low energy processing and abundant elements. As will be shown, both of these systems are relatively complicated due to the large number of polymorphs and secondary phases possible, and both exhibit low output voltages in comparison to their band gaps. One way forward is to reduce the complexity of the absorber system by using binary alloy compounds. Hence we report on the (Sb,Bi)2Se3 system to establish its suitability as a bottom cell absorber layer, showing that band gaps below 1.1 eV are possible. One particular challenge of all three proposed material systems is the high vapour pressure of their constituent components. We report on methods to control their relative composition.

Resume : Sb2Se3 has recently attracted great interest as an absorber layer for photovoltaic devices. It has a suitable band gap, high absorption coefficient and good stability. Efficiencies using Sb2Se3 in PV devices have increased rapidly, now exceeding 6.5% in our labs. Yet, fundamental properties are still not well understood or reported. Raman spectroscopy is often employed to characterise Sb2Se3, however the vibrational peaks have yet to be indexed to the respective phonon modes. We have also found that oxidisation effects within the synthesis or measurement process mean that Sb2O3 Raman peaks are often misidentified as Sb2Se3 modes. Here we present polarised Raman experiments on well-defined planes of single crystal Sb2Se3. This technique allows us to separate the individual Raman modes by varying the incident/scattered polarisation direction and analysing the relative mode intensities. A novel theoretical approach is used to simulate the Raman spectra, calculating the Raman tensor elements using hybrid density functional theory. Understanding the oxidisation effect as well as reporting the indexed Raman spectra is important to fundamentally understand the structure as well as characterisation of polycrystalline layers for devices. The orientation of the ribbon structure of Sb2Se3 films majorly affects device performance and could be inferred from Raman spectroscopy. This approach could be widely used in the characterisation of other anisotropic solar absorber materials.

Resume : Progress with Sb2Se3 solar cells has been rapid, the efficiencies rising from 2% to 7.6% in 5 years [1]. Further progress is likely to hinge on optimising the band alignments in the heterostructure. Presently the most popular window layer is CdS, but TiO2 has a slight performance advantage in our devices [2]. In this work we have measured the conduction band offsets (CBOs) between Sb2Se3 and both CdS and TiO2 using x-ray photoemission and optical gap measurements. In order to overcome the sampling depth limitations of laboratory x-ray sources (~5nm), the greater depth-profiling capability of the HAXPES facility at Diamond Light Source (> 15 nm) was used to examine buried Sb2Se3/TiO2 and Sb2Se3/CdS interfaces. The samples comprised device-quality close spaced sublimation deposited Sb2Se3 from which we have made ~6.5% efficient devices [3]. It was found that Sb2Se3/CdS cells had a spike-like CBO of 0.04 eV and Sb2Se3/TiO2 cells had a cliff-like CBO of 0.5 eV. Contrary to expectations based on device performance [2], Sb2Se3/CdS therefore has a more favourable band alignment than Sb2Se3/TiO2. It was therefore concluded that it is the intermixing at the Sb2Se3/CdS interface to form CdSe that degrades the photovoltaic performance (relative to that of Sb2Se3/TiO2 devices) [2]. We conclude with recommendations for device fabrication protocols. [1] Wen et al., Nat Comms, 9 (2018) 2179 [2] Phillips et al., IEEE J PV, Early Access (2018) [3] Hutter et al., SOLMAT, 188 (2018) 177

Resume : Sb2S3 has excellent properties for solar cell applications – α>10E5cm-1 at 450 nm, Eg ~1.7 eV. Sb2S3 is made of Sb, and S, both abundant in the Earth’s crust. Efficiencies up to 5.5% at AM1.5 have been achieved in hybrid solar cells based on sprayed Sb2S3. However, ρ>10E6 Ωcm in spray deposited Sb2S3 films annealed in N2 at 300°C, coupled with limited grain size presents a challenge to further improve solar cell efficiency. Annealing in Se vapor could increase grain size, and reduce electrical resistivity in chemically deposited Sb2S3 films, enhancing power conversion efficiency [1]. High purity amorphous Sb2S3 layers were deposited by in-air area-scalable ultrasonic spray pyrolysis on glass/(ITO)/TiO2 substrates at 200°C from a solution of SbCl3 and SC(NH2)2 [2]. Samples were either annealed in flowing N2 at 200-400°C for <30 min, or, selenized by heating Se powder in a second zone to 500°C, guiding Se vapor by mass transport in flowing N2 to the sample zone. Morphology, chemical composition, structural, and optoelectronic properties of Sb2S3 layers were characterized by SEM, Raman, XRD, UV-Vis, and 4-pt-probe. Post-deposition annealing in Se vapor atmosphere formed Sb2(S,Se)3 as confirmed by XRD study, and red-shift in Eg. After selenization, ρ of Sb2(S,Se)3 films decreased to >5*10E4 Ωcm. Solar cell development is in progress, preliminary results refer to efficiencies of 6%. [1] Deng et al., Mater. Today Energy 2017, 3, 15-23 [2] Beilstein J. Nanotechnol. 2019, 10, 198-210

Resume : Among the emerging materials for the next generation of solar cells, thin film Cu2ZnSn(S, Se)4 (CZTS) appears as a promising candidate composed of non-toxic and earth abundant elements. Over the past years, CZTS solar cells have reached an efficiency peak of 13% with short-circuit current density and fill factor above 35 mA/cm² and 70 %, respectively. The latter two values are comparable to those reported for the parent Cu(In,Ga)(Se,S)2 (CIGS) technology that reached power conversion efficiency of 22.9 %. However, in CZTS the open circuit voltage (VOC) remains relatively low and is the main cause for the half efficiency. It thus appears urgent to bring it at the same level of efficiency as CIGS. In this context, the quick VOC assessment of bare absorbers (i.e. without the need to fabricate the complete device) is of high importance. This is possible with photoluminescence (PL), via the measurement of quasi-Fermi level splitting (QFLS). Here, we describe the PL setup we build and discuss its advantages and drawbacks as compared with other reported setups. Then, we present some results obtained on CZTS layers fabricated by colloidal-based ink method, like the possible contribution of the surface or interface recombination on the QFLS. The sample preparation allowing to improve QFLS extraction will be discussed. Finally, sprayed thin films with kesterite-derived materials with cation substitution will also be shown to provide useful insight on the VOC loss.

Resume : We present novel solution-based synthesis method enabling the morphology variation of Sb2Se3 light absorbers, which are nontoxic, low bandgap and earth-abundant materials. The morphology of Sb2Se3 films varies from dense particulate planar film to 1-dimensional nanowire-stacked film by modulating the Sb and Se molar ratio in the precursor ink. We carefully characterized the effects of morphology and crystallographic orientation on the optoelectrical and photoelectrochemical (PEC) properties of Sb2Se3 photocathodes after the surface modification with TiO2 and co-catalyst Pt. Furthermore, the hierarchical Sb2Se3 structure was fabricated by depositing the nanowires on the top of dense planar film not only to improve the optical properties but also to avoid direct contact between the conductive substrate and TiO2 protection layer, which induces severe recombination of charge carriers and shunts the device. IPCE at long wavelength increased dramatically compared to single-layered film, indicating that the wider range of the light is more efficiently utilized with the hierarchical structure. In addition, by its great capability of light harvesting, the record high photocurrent of 22.5 mA cm–2 was achieved at 0 V versus reversible hydrogen electrode. Therefore, we successfully demonstrated highly efficient Sb2Se3 based photocathode with a hierarchical structure, which clearly illustrates the potential impact of our Sb2Se3 device as a promising candidate for practical PEC water splitting.

Resume : Up to date, kesterite based thin-film solar cells have reached power conversion efficiency (PEC) of 12.6% by hydrazine process. Though several issues ought to be resolved, the importance for researching light-weight and flexible Cu2ZnSn(S,Se)4 (CZTSSe) has increased because their broad applications. In this study, flexible and high-efficient CZTSSe thin-film solar cell were successfully fabricated by a sputtering method. Especially, in order to avoid forming unwanted secondary phases, we varied the number and the order of the precursor layers; 3- and 7- precursor stacks. Interestingly, we were able to high PCE of 10.34% with the 7-precursor stack sample. To figure out the carrier behavior in the materials, we introduced photo-assisted atomic force microscopy (AFM). Photo-assisted conductive AFM and Kelvin probe force microscopy (KPFM) can measure the current flow induced by the lights, showing them on the thin-film surface. Especially, the surface photovoltage (SPV) and photocurrent decreased with 3-precursor stack sample. This can be because of non-uniform phase formation within the materials, detected by confocal micro-Raman scattering. Consequently, varying precursor stacking process would give different material properties; particularly optical and electrical ones. Through this research, we can figure out how the deposition condition can affect the thin-films’ structural and optoelectrical properties, thus solar cell performances.

Resume : In this presentation I will discuss the influence of elemental composition on opto-electronic properties and efficiency of kesterite solar cells, and introduce novel strategies for optimization of the reaction pathway and compositional evolution of the CZTSe absorber. The presented fabrication process includes the preparation of stacked elemental and alloyed layers (SEAL), which are annealed in Se containing atmosphere. Avoiding non-alloyed elemental Sn in the SEAL structure suppresses the formation of SnSe2-x phases throughout the selenization process and therefore effectively prevents Sn loss from the film during the high temperature annealing stage. The selenization process is designed to be a facile one step procedure, which however, due to its slow heating ramp and high temperature dwelling time, reveals consecutive reaction regimes. These include i) the formation of binary and ternary compounds, ii) the formation of a stable CZTSe phase, and finally iii) a shift of the chemical composition of the CZTSe film from initially Cu-rich towards Cu-poor (similar to CIGSe processing). It will be shown, that the obtained material characteristics are not only governed by the final elemental composition, but also are influenced by the compositional pathway during the process, which can be illustrated in a time dependent pseudo-ternary phase diagram. Furthermore, strategies for stabilization of the Mo back contact for reduced formation of MoSe2, and a novel approach for forming a band gap grading by fast sulphurization via a H2S post-treatment will be presented.

Resume : To achieve further cost reductions in thin film solar cells, efficiency must be increased beyond the single-junction limit. Therefore, there is an increasing interest in semiconductor materials which are potentially suitable for wide bandgap applications such as absorber layers in the top cell of multi-junction solar cells. Quaternary chalcogenide semiconductors, like Cu2ZnGeS4, Cu2ZnGeSe4 or Cu2ZnSiSe4 contain only earth abundant, non-toxic elements. Cation and anion substitution offer the possibility of flexible bad gap tuning between 1.5 eV and 2.2 eV. The focus of our research is on the investigation of structure-function relations in the solid solution series Cu2Zn(GexSi1-x)Se4 and Cu2ZnGe(SxSe1-x)4. Polycrystalline samples were grown by solid state reaction of the elements in evacuated silica tubes. As already visible from X-ray diffraction data, within the series the crystal structure changes between the end members from a tetragonal (Cu2ZnGeSe4) to an orthorhombic structure (Cu2ZnSiSe4). Cu2Zn(GexSi1-x)Se4 mixed crystals within 0.45≤x≤0.55 show the coexistence of an orthorhombic and a tetragonal phase, both with the same chemical composition. Cu2ZnGeS4 crystallizes in a tetragonal (-phase) as well as in an orthorhombic structure (-phase). Also the Cu2ZnGe(SxSe1-x)4 mixed crystals show the coexistence of tetragonal and orthorhombic phases. The differentiation between the isoelectronic cations Cu+, Zn2+, and Ge4+ and thus between the tetragonal structure types kesterite and stannite is only possible by neutron diffraction. Detailed simultaneous Rietveld analysis of neutron and X-ray diffraction data revealed the complex structural transition from the orthorhombic to the tetragonal crystal structure within the studied solid solution series.

Resume : A promising approach to overcome the Zn-Cu antisite disorder problem in kesterite Cu2ZnSn(S,Se)4 while retaining earth-abundance of the constituting elements is the replacement of Zn by Ba – despite its high atomic number, Ba is even more abundant than Cu, Sn and Se. The large size of Ba forces the Cu2BaSn(S,Se)4 phase and also Ba-containing secondary phases into completely different structures compared to the Zn-containing system. While in the recent past promising solar energy conversion efficiencies of more than 5 % have been achieved, little is known about the phase diagram and the stability of this material and hence also little is known about ideal reactions paths for the synthesis of solar cell absorbers. In this contribution, we present a detailed study of the formation of Cu2BaSn(S,Se)4 via chalcogenization of sputtered precursors by means of synchrotron based real-time diffraction and fluorescence analysis as well as ex-situ analysis of the microstructure of the resulting material. In particular, we investigate issues that are known to be crucial for the synthesis of kesterite films, such as stability against loss of Sn and the formation of secondary phases. We compare the phase formation and stability of Cu2BaSn(S,Se)4 with those of Cu2ZnSn(S,Se)4 and suggest strategies for improved and robust synthesis routes of Cu2BaSn(S,Se)4 absorber films.

Resume : Kesterite solar cells have a record efficiency of 12.8%. If we compare this efficiency of Kesterites with other thin film technologies like chalcogenides and perovskites 12.8 % might not be pleasing. However, this material is still very promising. The only step that is needed is a key idea to boost the performance of this thin film solar cell without scarce materials. Finding this pathway starts with understanding what is limiting the kesterite solar cell performance. Characterization and modelling of devices help us in understanding the mechanisms that are present in the structure. Capacitance spectroscopy and profiling are a commonly used technique to determine input parameters for these models. Unfortunately, this technique is often not able to scan the whole energy range of the bandgap nor the whole spatial region of the absorber and buffer. Therefore recombination centers that often have a strong impact on the performance can be invisible. In this work, we estimate these parameters from a regression analysis on the temperature and light intensity dependence of the performance parameters Voc, Isc and FF. It is shown that acceptor defects in absorber and buffer can explain the large cross-over effect and Voc-deficit which is typically present. Sensitivity analysis with SCAPS demonstrates that with the presence of these lifetime killers in bandgap material optimizing the doping concentration in line with the generation profile is necessary to have decent efficiency.

Resume : Recently quaternary chalcogenides Cu2ZnSn(S1-xSex)4 (CZTSSe) have gained a lot of attention. These kesterite-type compounds[1], which consist mostly of earth abundant and non-toxic elements, are promising low cost alternative absorber materials for thin film solar cells. The absorber band tailing caused by the exceptionally high density of Cu/Zn disorder is believed to be one of the reasons for the limited open-circuit voltage in CZTSSe devices. A differentiation between the isoelectronic cations Cu and Zn2 and consequently quantification of Cu/Zn disorder within the kesterite type structure is not possible using X-ray diffraction. Neutron diffraction can solve this problem; the coherent scattering lengths are sufficiently different for these cations [2]. A detailed structural analysis of CZTSSe powder samples, grown by solid state reaction of the elements, was performed by neutron diffraction giving insights into the cation distribution within the crystal structure of CZTSSe, and consequently the Cu/Zn disorder. The values of the order parameter Q [3], which is a quantitative measure of Cu/Zn disorder, were determined. The correlated information about changes in lattice parameters and cation site occupancies, details on the existing intrinsic point defects and their amounts as well as a comparison of these parameters to the ones from stoichiometric CZTSSe with the same S/S Se ratios will be presented. [1] S.Schorr, Solar Energy Materials and Solar Cells, 95 (2011)1482. [2] V.F. Sears, Neutron News 3(3) (1992) 26. [3] D.M. Többens et al. Phys. Stat. Sol. B 253 (2016), 1890 The research leading to the presented results has been partially supported by the STARCELL project as well as INFINITE-CELL project. These projects have received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreements No 720907 and 777968 respectively.

Resume : In this work, the effect of chemical etching treatment in Cu2ZnGeSe4 based solar cells is investigated using electrical characterization techniques, e.g., current-voltage, capacitance-voltage, admittance spectroscopy and EQE. The samples were etched with 12 wt% HCl solution at different bath temperatures and different durations. An increase in the open-circuit voltage from 500mV to up to 600mV is reported after the HCl etching, but a noticeable degradation in the other solar cell parameters is observed. Detailed transmission electron microscopy analysis performed on samples before and after the etching revealed the presence of several secondary phases, like ZnSe, Cu2-xSe and Cu2GeSe3, and that the chemical treatment removes mainly the ZnSe secondary phases located at the top part of the Cu2ZnGeSe4 absorber. The capacitance-voltage measurements show an increase in the doping profile in accordance with the increase reported in open-circuit voltage after the chemical etchning. We propose a device model to explain the solar cell behavior before and after the chemical treatment based on the parameters extracted from the in-house characterization techniques.

Resume : Relatively wide bandgap and earth abundant Cu2ZnSnS4 (CZTS) kesterite absorbers are excellent candidates for advanced photovoltaic applications, such as semi-transparent solar cells or top devices for tandem concepts. Nevertheless, CZTS has mainly been developed onto opaque substrates, non-compatible with these technological applications. In this work we present a complete optimization of CZTS solar cells prepared onto transparent substrates. CZTS was synthesized onto glass/FTO substrates (and glass/Mo as control samples) using a sequential process based on the sputtering of metallic stacks followed by a reactive annealing under S atmosphere in a graphite box. A complete optimization was performed including composition (through a compositionally graded sample), reactive annealing conditions, surface etching and passivation with Al2O3. The layers were fully characterized in regards of their morphology, composition, structure and opto-electronic properties. The obtained layers exhibit very similar properties and quality than those deposited onto conventional glass/Mo substrates and after a first optimization we report a record glass/FTO/CZTS/CdS/i-ZnO/ITO device with an efficiency of 7.7% (VOC = 677 mV), the highest reported for this absorber onto a transparent substrate to the best of our knowledge. A comparison with devices obtained under the same conditions onto opaque glass/Mo substrates will be presented, as well as strategies to further improve the conversion efficiency.

Resume : In solar cells, photogenerated carriers (electrons and holes) can recombine, limiting the performance of devices. Under standard solar illumination, non-radiative recombination is often a dominant mechanism as described by Shockley-Read-Hall (SRH) statistics. However, the conventionally theoretical maximum efficiency of solar cells, so-called Shockley-Queisser limit, has mainly focused on the thermodynamics of light - the radiative limit - due to the difficulty in estimating the non-radiative recombination rate a priori. In this work, we predict the theoretical maximum efficiency of kesterite solar cells (Cu_2Zn(Ge,Sn)(S,Se)_4) including non-radiative carrier recombination mediated by the native point defects. We calculate the thermal equilibrium concentrations of native defects and their capture cross-sections. We find that the inert-pair effect in Sn-related defects induces both deep levels and large lattice distortion after the carrier captures. Hence, the sulfur vacancy (V_S), sulfur vacancy-donor complex and Sn antisites (Sn_Zn) produce deep levels and large capture cross sections resulting in the low open-circuit voltage and the solar-to-electricity conversion efficiency. The predicted upper limit is compared to the current generation of best performing kesterite solar cells. [1] S. Kim, J.-S. Park, and A. Walsh, ACS Energy Lett. 3, 496 (2018) [2] S. Kim, J.-S. Park, S. N. Hood, and A. Walsh, arXiv:1810.11259

Resume : In the way achieving solar cells with efficiency higher than 30%, multi-junction cells already demonstrated their potentialities. Nevertheless, their production cost is prohibitive for terrestrial development and they do not achieve a sustainable model. Then, the best trade-off is using tandem cells where only two junctions are used. In this approach, the hybrid integration scheme mixing organic/inorganic materials, i.e. a silicon bottom cell and a perovskite top cell, has already shown nice demonstrations but the attained efficiency is right now not exceeding the one of high efficiency silicon cells and is facing the inherent problematics of perovskite materials. Another way consists in a full inorganic material approach using an inorganic thin film as top cell material. Targeting a sustainable strategy, such a thin material shall be composed of abundant raw materials. This was the driving idea investigating the ZnSnN2 material line. Bearing terrestrial development of such a technology in mind, an up-scalable material fabrication method has been used. We chose physical vapour deposition, i.e. sputtering technique, since it could moreover minimize waste in production and provide a low environmental footprint process. First experiments were made varying the respective proportion of Zn and Sn in the layer. High Sn containing layers exhibited an amorphous structure. The characterization of the electrical properties revealed a high carrier concentration, between 1019 and 1020 cm-3 , and a carrier velocity in the 0.1 to 1cm²/V.s range. Absorption measurements show bandgap value in the order of 1.85-2 eV. All those parameters are dependent on Sn concentration. Parallel to those electrical measurements, solar spectrum measurements exhibited no, or at least no significant, photo-carrier generation. A second set of material properties tuning was investigated incorporating hydrogen during the material deposition. Hydrogen content was varied from 0% to 5%. Its effect was recorded as to drastically decrease the carrier concentration, 1016 can be achieved, without affecting drastically the carrier mobility. Some post-doping was also tried using ion implantation. B, C, F, Si, As, P, were implanted in thin films. Although some increase in photoconductivity has been observed, particularly for As and P, no drastic increase of photo-carrier generation capabilities has been highlighted.

Resume : ZnSnN2 (ZTN) is an element-abundant n-type semiconductor analogous to the traditional III-nitrides InxGa1−xN with great potential as a photovoltaic absorber, due to its direct bandgap, steep absorption onset, tunable properties by various approaches (like doping and cation disorder) and environmentally friendly [1-3]. Despite the intriguing possibilities of ZTN for a thin film absorber, its degenerate n-type carrier density and low mobility seriously frustrate the application of ZTN. In this work, ZTN thin films with various cation compositions have been successfully synthesized on glass substrates by reactive magnetron co-sputtering. It is found that both the cation composition and the heat treatment have significant influence on the electrical properties of ZTN thin films. Non-degenerate ZTN thin films with rich Zn contents have been achieved by non-equilibrium deposition without intentional heating on the substrates. Room temperature carrier concentration and mobility are 9E17 cm-3 and 19 cm2/Vs, respectively. The electrical transport mechanism of ZTN thin films will be discussed via the electrical properties as a function of temperature. Finally, the performances of photovoltaic cells using non-degenerate ZTN thin films as absorbers are presented. References: [1] L. Lahourcade, et al. Adv. Mater. 25, 2562 (2013). [2] A.N. Fioretti, et al. J. Mater. Chem. C 3, 11017 (2015). [3] F. Alnjiman, et al. Sol. Energy Mater. Sol. Cells 182, 30 (2018).

Resume : Discovery of a high performance p-type transparent conductor (TC) is critical to advancing photovoltaic technologies, e.g. as transparent contacts and window layers. Combined high-throughput computation and combinatorial experiments offer a promising pathway to achieve such materials, yet properties comparable to n-type ITO have not yet been found due to low hole mobilities and dopability. TC ternary chalcogenides (AxByChz), where A and B are earth-abundant metal cations and Ch is anion S or Se, offer advantages to oxides due to lower ionization potentials and potential p-type dopability. This talk will explore wide-gap p-type ternary chalcogenides that we have predicted computationally as TCs, with a focus on the role of cation chemistry, hole dopability, and stability. Case studies will be presented of combinatorial sputtering to synthesize such compounds, investigate chemical phase space, and characterize structural and optoelectronic properties within a combinatorial framework. Although these TCs are predicted by density functional theory to reside in a thermodynamically stable ground state, synthesizability remains a major challenge. In particular, decomposition in the presence of O2 and water vapor is a prominent issue, which we will explore using thermodynamic Pourbaix calculations and experimental assessments. The end goal is to implement these materials as contacts in photovoltaic devices to complete the computationally-guided materials discovery framework.

Resume : Sequential DC magnetron sputtering and rapid thermal processing appear very promising to fabricate CZTS-based thin-film solar cells considering environmental and industrial issues. However, their state-of-the-art efficiency remains limited to 10% to date. In this work, we aim at optimizing their optical and electrical properties by an extensive screening of their correlation with processing conditions. We assess the impact of absorber formation on the bandgap energy, absorption coefficient, p-type carrier concentration and mobility. The most important results are shown to be related to the analysis of the carrier concentration versus mobility trend. Our conclusions point the optimal composition ratios towards Cu-poor and Sn-rich range. In addition, the annealing conditions under hydrogen gas show the shift of the carriers concentration (CC) – mobility (µ) pair toward target values (CC = 1e15-1e16cm-1 with µ > 10 cm²V-1s-1). As a result, a potential roadmap is drawn up based on presented experimental results and previous SCAPS simulations for reaching more than 10 % cell efficiency with the target technology.

Resume : Thin film CdTe is a promising, low cost material for a solar cell that can be deposited using pulsed DC Magnetron Sputtering. It is an ideal candidate for a solar cell, however, after sputtering, analysis shows ~ 4% of the working gas, Ar, is incorporated into the film. This can cause large surface blisters, ~10 micrometres in size, during post annealing at 450oC combined with a CdCl treatment to remove stacking faults. This is a barrier which stands in the way of efficient CdTe cell production via this low-cost method. The aim is therefore to use computational modelling to explain the mechanisms by which these bubbles form and to determine how to minimise this effect. To achieve this we use Molecular Dynamics (MD) to find threshold energies for which the incident Ar will deposit into the lattice. Calculations also show that Ar can diffuse through the lattice with an energy barrier of 0.6eV which is reduced when 4% Ar is included. MD simulations of the high temperature annealing process show that Ar bubbles grow through diffusion and trap mutation mechanisms. Bubble size distributions are determined as a function of annealing times and a comparison is made to other inert working gases, such as Xe. Finally, results using adaptive Kinetic Monte Carlo, which simulates CdTe growth over realistic time scales, demonstrates how Ar might be incorporated into the thin film during deposition. As a result of the simulations, some suggestions for improved thin film quality are presented.

Resume : Antimony selenide (Sb2Se3) has great potential for low-cost photovoltaics due to its excellent optoelectronic properties and low processing temperatures. This study presents detailed photoluminescence (PL) analysis of Sb2Se3 thin films deposited by RF magnetron sputtering onto the Mo-coated soda lime glass substrate. Raman scattering, X-ray Diffraction and Energy Dispersive Spectroscopy analysis confirmed the formation of uniform single phase material. Temperature and laser power dependent PL measurements were used to reveal the dominating radiative recombination mechanisms in the studied Sb2Se3 thin films. Narrow edge emission band at 1.075 eV and two deep PL bands at 0.82 eV and 0.94 eV were detected in the low-temperature (T = 10 K) PL spectra of Sb2Se3 thin films post-annealed at T = 623 K in Ar atmosphere in the presence of additional Se source for 30 min. From the temperature dependence of the PL spectrum, the thermal activation energies for the quenching process were obtained ranging from 10 to 20 meV for all three observed PL bands. In addition, the three detected PL bands shifted towards lower energies with increasing laser power. The origin of these PL bands is discussed.

Resume : Antimony selenide (Sb2Se3) solar cells are a comparatively recent addition the thin film device canon but efficiency values have climbed rapidly and are now approaching 10%. Despite this rapid growth the devices remain voltage limited, with the vast majority of devices reported having Voc values of < 440mV. Developing an in-depth understanding of limiting defects and recombination pathways is therefore key to continued performance improvement. This paper will discuss deep level transient spectroscopy (DLTS) analysis of Sb2Se3 solar cells to identify key deep levels within the material and their links to performance. By comparison to single crystal Sb2Se3 samples we will demonstrate the source of these defects and identify routes to control the defect composition and doping of Sb2Se3 solar cells.

Resume : Sensitized solar cells (SSC) are the best known of the third-generation type of solar cells. Some types of SSC are fabricated on solid basis, presenting the advantage of a better stability. A promising type of SSC is the extremely thin absorber (ETA) solar cell. Sb2S3 has received increasing attention recently as ETA material due to its unique properties (α ≈ 1.8 × 105 cm–1; Eg = 1.7 eV). Typically, Sb2S3 is synthesized by chemical bath deposition methods on TiO2 based electrodes. This method causes oxygen incorporation within the Sb2S3 layer which implies the formation of carrier traps in the bandgap. In this work, we use atomic layer deposition to grow Sb2S3 layers. We study pure Sb2S3 and the incorporation of oxygen. We observe an unexpected dewetting effect during annealing of pure Sb2S3 while changing the phase from amorphous to stibnite. This effect is not observed in oxygen-contaminated layers. Their compositions are studied by XPS and Raman spectroscopy exposing the differences between the two kind of Sb2S3 layers. Interfacial ZnS layers solve the dewetting problem in pure Sb2S3 and, in addition, it helps to block backwards recombination. Transient absorption spectroscopy reveals that the kinetics of charge transport and recombination in p-i-n organic-inorganic heterojunctions with an interfacial ZnS layer are ruled by hole transfer-driven processes. These structures present an enhancement on the performance of the solar cell devices with efficiencies of 4 %.

Resume : The chalcogenide semiconductor SnS has attracted recent attention as a promising candidate for solar cells made from earth-abundance materials. Based on the experimental results of other chalcogenide solar cells, such as CIGS and CZTS, sulfurization following Sn-S precursor deposition shows promise as an effective method for obtaining high quality SnS. However, there are several issues with obtaining high quality SnS thin film due to its high vapor pressure and low formation energies resulting in unwanted compositional films and/or extra phases. Moreover, the morphology and crystal structure of SnS strongly depend on the sulfurization process. In fact, only a few experimental results have been reported regarding the reaction pathway during sulfurization. In this presentation, we will clarify the reaction mechanism of Sn-S thin films during sulfurization for the purpose of obtaining high quality SnS thin films. A mixed Sn-S precursor was prepared by RF sputtering on Mo/SLG substrates. The sulfurization was carried out in a quartz tube in an N2 and S vapor atmosphere. Conventional SnS solar cells were fabricated. The key issues when obtaining the single-phase orthorhombic SnS were (1) the Sn-S precursor atomic ratio of Sn/S before sulfurization, (2) the heating rate before the sulfurization, and (3) the cooling down condition after sulfurization. The details of the growth mechanism of SnS and the associated solar cell properties will also be investigated.

Resume : Copper sulfide (Cu2-xS) is a low-cost, environment-friendly p-type semiconductor, with optical band gap varying with respect to its stoichiometry. Their electronic structure is complicated because of the changes in the positions of the valence and conduction band edges. Out of all the stable stoichiometries, i.e., x = 0, 0.04, 0.2, 0.4 and 1.0 in Cu2-xS, Cu2S (i.e., at x = 0) has the right band gap to be used as a solar absorber material. However, its proximity in stoichiometry with other Cu2-xS stable compounds, could lead the formation of a mixed phase while device fabrication, hence altering the electronic structure. To this end, varying stoichiometry of Cu2-xS films were grown here using a room temperature molecular solution based deposition method, followed which a wide range of characterization tools were used to understand the microstructure and optoelectronic properties of these films. The hole concentration of these films are found to vary from 3.32 × 1019 cm-3 to 2.54×1022cm-3 as Cu2-xS stoichiometry changes from Cu2S to CuS. This increase in hole concentration with increasing Cu deficiency in Cu2-xS is because of the formation of copper vacancy, which pushed the Fermi level deep into the valence band (as observed in X-ray Photoelectron Spectroscopy). Optical and transport gap are found to increase from 1.36 eV to 2.23 eV and 1.31 eV to 2.02 eV respectively with increasing copper vacancies from Cu2S to CuS, which is because of the vacancy induced reduction in Cu d-band width. Moreover, in this work, the valence and conduction band edge positions are measured and then related with their stoichiometries.

Resume : Cuprous oxide (Cu2O) is an earth-abundant and low-cost material that is attractive for photoelectrochemical water splitting or as p-type semiconductor in photovoltaic applications [1]. The material can be deposited by several low-cost and up-scalable solution-based deposition techniques, including electrochemical deposition, chemical bath deposition or spray pyrolysis. In this study, Cu2O was deposited by spray pyrolysis from aqueous solutions with addition of acetic acid and without organic solvents. An important aspect of our investigation was the exclusive use of non-toxic and abundant low-cost raw materials. Spray pyrolysis of Cu2O proceeds firstly through the reduction of the Cu2+ with a reducing agent followed by the oxidation of copper clusters to Cu2O [2]. Deposition temperature and precursor solution constituents are crucial parameters to obtain only the Cu2O phase and avoid the formation of CuO or Cu. A main aspect was to understand how the solution composition influences the properties of Cu2O films. The influence of the solution pH value, alkali dopants, copper precursor concentration and type of reducing agent is shown for films deposited on glass substrates. Sprayed Cu2O films with thicknesses between 500-1000 nm were then applied in semi-transparent solar cells, based on In-doped ZnO electrodes and Ga2O3 buffer layers. All layers of the all-oxide solar cells were entirely fabricated by spray pyrolysis from aqueous solutions. References: [1] Wick, R. & Tilley, S. D. Photovoltaic and Photoelectrochemical Solar Energy Conversion with Cu2O. J. Phys. Chem. C 119, 26243–26257 (2015). [2] Kosugi, T. & Kaneko, S. Novel Spray-Pyrolysis Deposition of Cuprous Oxide Thin Films. Journal of the American Ceramic Society 81, 3117–3124 (1998).

Resume : Cu2O is a p-type semiconducting metal oxide with a direct bandgap of about 2.1 eV. Copper being earth-abundant and presenting a low toxicity, Cu2O is widely studied for various potential applications. In particular, Cu2O is attracting a lot of interest in optoelectronic applications due to the high theoretical conversion efficiency for Cu2O-based solar cell devices, which could reach 20% according to theoretical predictions. But in order to harness the full potential of Cu2O, the deposition of high-quality thin films through low-temperature, scalable approaches still must be improved. In this communication we present the effect of carrier gas (CG) humidity on the texture and the resulting electronic properties of Cu2O thin films deposited using aerosol assisted chemical vapor deposition (AA-CVD) at low temperatures (<365 °C) is reported. By increasing the CG humidity, the preferred orientation of the films can be tuned from [110] to [111]. By studying the initial stages of film deposition, a different growth mode is found for dry and humid conditions, which in turn directs the final texture of the films. The analysis of the electric properties of the films by Hall effect shows that carrier concentration remains in the order of 1015 cm−3 when using both dry and humid conditions. Conversely, Cu2O samples deposited with humid CG generally present a higher mobility, up to 17 cm2 V−1 s−1. [111]-textured Cu2O films with high mobility were used to fabricate a diode by depositing a ZnO layer on top using atmospheric pressure spatial atomic layer deposition (AP-SALD). The diode shows an excellent rectifying behavior with a high asymmetry close to 104 between −1 and +1 V.

Resume : Cu2SnS3 has been studied as a rare metal-free solar cell material with a narrow band gap less than 1.0 eV. Recently, we studied crystal structures, optical properties and electronic structures of Cu2SnS3, Cu2GaS3, Cu2(Ge,Sn)S3 and Cu2(Ge,Sn)Se3 by experimental and theoretical approaches [1-3]. The Cu2(Ge,Sn)S3 and Cu2(Ge,Sn)Se3 solid solutions have a monoclinic crystal structure. The band-gap energy (Eg) of the Cu2(Ge,Sn)S3 increased from 0.87 eV of Cu2SnS3 to 1.53 eV of Cu2GeS3 with increasing Ge content. In this study, we synthesized Cu2(GexSn1-x)(S1-ySey)3 solid solution (x=0.0, 0.1, ,0.9, 1.0 and y=0.0, 0.25, 0.5, 0.75, 1.0) by planetary ball milling and post-heating at 600 oC for 30 min in an N2 gas. The Eg of the Cu2(GexSn1-x)(S1-ySey)3 solid solution increased with increasing both Ge and S contents. The energy level of the valence band maximum (VBM) from vacuum level was determined from the ionization energy measured by photoelectron yield spectroscopy (PYS). The conduction band minimum (CBM) was calculated by adding the Eg to the VBM. The energy level of the VBM of Cu2(GexSn1-x)(S1-ySey)3 system was almost constant with Ge content but decreased with increasing S content. On the other hand, the energy levels of the CBM increased with increasing both Ge and S contents. The results are compared with those in Cu2Zn(Ge,Sn)(S,Se)4 system. [1] M. Morihama, T. Maeda, I. Yamauchi, and T. Wada, Jpn. J. Appl. Phys. 53, 05FW06 (2014). [2] H. Nishihara, T. Maeda, A. Shigemi and T. Wada, Jpn. J. Appl. Phys., 55, 04ES08 (2016). [3] Q. Chen, T. Maeda, and T. Wada, Jpn. J. Appl. Phys. 57, 08RC20 (2018).

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). Usually, orthorhombic CAS phase has been used as an absorber material but there are other phases, such as Cu3SbS4, Cu3SbS3, Cu12Sb4S13 and etc. which has different structures. Originally we tried to fabricate orthorhombic CAS phase to fabricate solar cell, hoewver, psedo-phase transition behavior was examined depending on the control of S flux. Therefore, in our study, using non-vacuum hybrid inks through the sulfurization process, we investigated the effect of S flux on the structural and electrical properties of the CAS thin films while Cu/Sb were kept constant. The different sampls with variious S fluxes 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 : Cation substitution in CZTSSe offers a path to improved solar cell device performance through the modification of defect structure of the absorber material. In the present study, the effect of partial substitution of copper by silver in Cu1.85(Cd0.2Zn0.8)1.1SnS4 monograin powders was investigated. (Cu1-xAgx)1.85(Cd0.2Zn0.8)1.1SnS4 monograin powders with different x values (0 < x ≤ 0.05) were synthesized from CuS, Ag2S, ZnS, CdS, SnS and elemental sulfur in the liquid phase of KI as flux material in evacuated quartz ampoules at 740°C. EDX analysis of the as-grown materials in the entire series showed that the Ag concentration was different in the precursors mixture and in the synthesized materials. Furthermore, the Ag concentration at the surface of crystals was found to be higher than in the bulk of crystals. The EDX and Raman investigations revealed that in addition to the desired compound, powders contained ZnyCd1-yS crystals as secondary phase. The SEM images showed that Ag incorporation to Cu1.85(Cd0.2Zn0.8)1.1SnS4 had no effect on the surface morphology and the shape of crystals. Raman spectra showed a shift in the A1 peak position from 336 to 334 cm-1 as the Ag content increased from x = 0.01 to 0.05 in the (Cu1-xAgx)1.85(Cd0.2Zn0.8)1.1SnS4. In addition, changes in the defect structure were studied by photoluminescence spectroscopy. The incorporation of Ag into Cu1.85(Cd0.2Zn0.8)1.1SnS4 monograin powders improved the efficiency of solar cells from 5.7% to 8.6%.

Resume : Yolk-shell plasmonic metal-semiconductor photo-catalysts have been reported to show significant enhancement from individual semiconductor nano-materials. It is widely accepted that the photo-catalytic activity is closely related to the amount of photo-generated electron-hole pairs, which is strongly dependent on the capability of the light absorption of the photo-catalyst. It is also known that in the hollow structure, light can be reflected and scattered then redirected to nearly all directions, leading to a better utilization of incident light due to an increased light path length. To date, plasmonic-semiconductor yolk-shell photocatalysts are rare in the literature. Since the multiple light reflection and scattering in-between the yolk and shell are claimed to be responsible for an enhanced local-field of plasmonic metal, a boosted photocatalytic performance can be expected by optimizing the space in-between and size of plasmonic core in this hetero-nanostructure. However, plasmonic metals such as Au, Ag are precious materials, and semiconductors such as titanium oxide, cadmium sulfide suffer from demerits such as weak photo-response from wide bandgap (above 3 eV) and toxic nature, respectively. A plasmonic metal of earth-abundant element and green semiconductors with a suitable bandgap for greater light harvesting are therefore needed. Herein, Cu-CuxS (1≤ x ≤2) with yolk-shell structure has been constructed to introduce multiple light reflections and scattering, enabling light collision with the Cu yolk and the CuxS shell. This structure not only increases the light path length but also allows for surface plasmon resonance (SPR) coupling between the plasmonic Cu and plasmonic CuxS, provides a platform to investigate their photocatalytic activities. Moreover, a series of work on varying the space in-between by using different size of plasmonic core has been carried out to optimize the photo-catalytic performance. Thus, the plasmonic coupling of Cu with CuxS can suggest an avenue for SPR-promoted photocatalysis not only due to increased efficiency of charge carriers generation from enhanced light harvesting but a basic understanding of SPR couplings of two plasmonic components.

Resume : SnS is an earth abundant semiconductor with orthorhombic crystallographic structure that possesses near-optimal direct band-gap of 1.3eV, high absorption coefficient and high density of charge carriers. Consequently, this material is widely used in environmentally-friendly applications, such as photovoltaic and photocatalytic devices for clean and renewable energy production. Recently, a new cubic binary phase of SnS was discovered in our group – a crystallographic structure of this material that was hitherto unknown. Preliminary research demonstrated unique properties, including exceptionally high lattice parameter (11.6Å), increased band gap of 1.5-1.7eV, mechanical stability and p-type conductivity of the cubic SnS phase. In the current work, we explored the electronic properties of the cubic SnS and compared them with those of its orthorhombic-phase counterpart. First, we deposited orthorhombic and cubic SnS thin films on Fluorine doped Tin Oxide (FTO) and p-Si substrates. This was attained using the chemical bath deposition method, which is an inexpensive, simple and environment-friendly growth technique. We characterized the structure of the resulting films using Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy and X-ray Diffraction. After achieving a complete and adherent coverage of the substrate in glove box environment, preliminary contact potential difference (CPD) measurements by the Kelvin probe method, aimed at characterizing the film's work function, showed unexpectedly low values suggesting the presence of tin oxide on the surfaces of both of the films. Ultraviolet Photoelectron Spectroscopy, aimed at characterizing the ionization potential, showed similar trends. X-ray Photoelectron Spectroscopy was then used to confirm this. Future work will include sputtering (milling) of the surface, in order to remove the oxide layer and to characterize the properties of the SnS phases.

Resume : Cu10(Zn,Cd)2Sb4(S,Se)13 compounds of the tetrahedrite structure (tetrahedrites) could be new p-type semiconductor candidates as absorber materials in solar cells. In this work the tetrahedrites were synthesized as monograin powders (MGP) in molten salt environment (CdI2 as flux). Influence of different technological parameters (temperature, initial composition and the amount of CdI2 flux material) on the elemental and phase composition, powder particle size distribution and shape of crystals was studied. MGPs were synthesized from Cu2S, CdS, Sb2S3 (initial ratio of Cu : Cd : Sb : S = 10 : 2 : 4 : 13) in molten CdI2 in closed vacuum ampoules heated at different temperatures (400, 440, 480, 495, 510 and 550 oC) for two weeks. Mainly single phase (99,7 %) tetrahedrite with composition close to the stoichiometrical Cu10Cd2Sb4S13 was formed at 480 and 495 oC (based on XRD and EDX investigations). Raman spectra of MGPs revealed three main peaks at 109, 354 and 362 cm-1. The peak at 362 cm-1 is characteristic to the Cd containing tetrahedrite [1]. MGP grown at 495 oC was used as absorber material in monograin layer (MGL) solar cell with a structure of ZnO/CdS/Cu10Cd2Sb4S13/graphite. The effective band gap energy value 1.3 eV was observed by using external quantum efficiency measurements. The efficiency of the first MGL solar cell was 0.11 %. [1] S. Bera, A. Dutta, S. Mutyala, D. Ghosh, N. Pradhan, 2018, Journal of Physical Chemistry Letters, 9(8), 1907-1912.

Resume : Recombination at Molybdenum (Mo) back interface is reported by several studies to be hindering the kesterite performance. Main culprits are MoSe2 formation [J. Li et al., Adv. En. Mat., (2015), 5] and a detrimental decomposition of kesterite during thermal processing [J. Scragg et al., J. Am. Chem., (2012), 134]. To evaluate the effect of back contact recombination on the performance, a dedicated series of samples was prepared and characterized by absolute PL imaging. Mo back contact was structured by laser scribing to obtain Mo-free lines of 1mm alternated with Mo lines. Half of the sample was then covered with a 10nm layer of Al2O3 by ALD to examine the passivation of the Mo contact. A solution-processed kesterite was spin-coated on top of the whole sample and annealed in a rapid thermal annealing (RTP) oven in selenium atmosphere. This design allows to compare quasi Fermi level splitting (QFLs) of kesterite absorbers with and without the back contact, passivated with Al2O3 or not. Full solar cell devices obtained with the same kesterite growth process achieve efficiencies up to 11.6%. From preliminary experiments, the QFLs is 30meV larger for the kesterite absorber on Mo contact than on bare glass substrate. This indicates that recombination at the back interface is not the limiting factor. The result is also supported by a series of full devices processed with different RTP annealing profiles showing a comparable performance, despite a varying MoSe2 thickness.

Resume : Cuprous oxide (Cu2O) is a p type semiconductor with a direct band gap of about 2.0-2.7 eV and high absorption coefficient. Due to its outstanding photoelectronic properties, natural abundance, non-toxic nature, and simple preparation procedures, Cu2O has generated interest in various applications, such as the conversion of solar energy into electrical or chemical energy, photochemical decomposition of water into O2 and H2 under visible-light irradiation, photocatalyst for degradation of organic contaminants, as hole transporting material in photovoltaic technology, for gas sensing and magnetic storage. Cu2O thin films are commonly deposited by thermal oxidation of copper thin films, which previously are deposited by sputtering or evaporation. Thermal oxidation is an expensive technique because of high temperatures and consequently the wasting of energy. In the other hand CBD is an economic, easy and scalable deposition technique, which is not well studied for Cu2O thin films deposition. We report an easy and direct CBD method for Cu2O thin films. We have studied the effect of thickness on the optical, structural, morphological, electrical and photoelectrochemical properties. We found a direct relation between strain and Urbach energy (disorder) with the electrical properties, which explain the increase of resistivity with the increase with the thickness. The thin films were analyzed by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), energy dispersive spectroscopy (EDS), UV-vis spectroscopy, electrically and photoelectrochemical characterized 1.- Qi, H., Wolfe, J., Fichou, D., & Chen, Z. (2016). Cu2O Photocathode for Low Bias Photoelectrochemical Water Splitting Enabled by NiFe-Layered Double Hydroxide Co-Catalyst. Scientific Reports, 6.. 2.- Moniz, S. J., Shevlin, S. A., Martin, D. J., Guo, Z. X., & Tang, J. (2015). Visible-light driven heterojunction photocatalysts for water splitting–a critical review. Energy & Environmental Science, 8(3), 731-759.

Resume : CdS is well-known buffer material in CIGS solar cell which reported 22.6% efficiency. CdS has a band gap of 2.4 eV so shows a good band alignment between CIGS and TCO in solar cell structure. Usually, as a buffer, CdS is deposited around 30~60 nm thickness by chemical bath deposition (CBD). However, until now, although there were many researches about changing thickness, bath temperature for reaction rate, etc., there is no research for finding right time to deposit CdS by tracking mechanism. Normally, two mechanisms of forming thin layer during chemical deposition were proposed: (i) heterogeneous reaction, associated with the ions are attached one by one on the surface in order to form films, and (ii) homogeneous reaction, with appearing agglomeration of colloidal particles pre-formed in the bulk of the solution which leads to adsorption on the surface to make particulate films. The homogeneous deposition is highly undesirable because it leads to non-adherent films resulting in a porous layer structure. However, in practice, both mechanisms may occur and/or interact at the same time during the reaction, leading to deposition of films in which colloidal materials are included. Therefore, in our study, the best time to deposit CdS layer with heterogeneous reaction was investigated. The samples were characterized using XRD, XPS, SEM, TEM, and UPS. The optimum point to deposit CdS buffer will be presented and discussed in detail.

Resume : Binary metal chalcogenides have attained growing interest in various device applications due to their promising optoelectronic properties. Antimony selenide (Sb2Se3) is perspective material in this group as an effective absorber for thin film solar cells. In the present investigation, the influence of post deposition thermal treatment in different atmosphere (vacuum, Se and Ar/Se) on compositional, morphological, structural, optical and electrical properties of sputtered Sb2Se3 thin films was studied. The antimony selenide thin films were deposited by RF magnetron sputtering with a film thickness around 1um onto glass, FTO and Mo - coated soda lime-glass substrates and then annealed in different ambient in the temperature range 250-450 ºC for 30min. The as-grown and annealed films were analyzed using HR-SEM, AFM, UV-Vis-NIR and Raman spectroscopy, XRD, and photoluminescence. It was found that post–deposition annealing leads to significant growth of the Sb2Se3 grains and to improvement in the density of films in comparison with as–deposited layers. EDX data showed nearly stoichiometric composition for all as-deposited and annealed Sb2Se3 films. Also, XRD and Raman analysis confirmed the orthorhombic crystal structure of prepared Sb2Se3 films. Photoelectrochemical and Kelvin probe measurements showed that Sb2Se3 films annealed in Ar /Se and Se atmosphere at high temperatures are highly photosensitive and exhibit p-type of conductivity. The findings are discussed and special account is taken to practical application of obtained structures in complete optoelectronic devices.

Resume : Cu2SnS3 (CTS) is a I-IV-VI ternary sulfide material composed of environmentally benign and abundant elements, principally explored for photovoltaic and optoelectronic applications. It is considered as a chemically simply derivative of Cu2ZnSnS4 and Cu(In,Ga)S2 absorbers. However, its current poor performance overshadows all the above mention advantages. Factors responsible for the low performance of CST solar cells have yet to be clearly been identified, however, Cu-rich (Cu4SnS4, Cu2S, CuS) and Sn-rich (Cu4Sn7S16, SnS, SnS2) impurities coexistence with stoichiometric CTS adversely alter the optical and electronic of the films and inevitable affect the photovoltaic performance. In addition, low energy defects and anisites located deep within the band gap can also act as recombination centers that shorten carrier lifetime and eventually limit performance. It is therefore essential to growth CTS films with high purity for photovoltaic applications. In this study, we explored the deposition of highly stoichiometric CTS thin film using thermal atomic layer deposition process. The growth characteristics, phase stability, optical and electronic properties were observed to depend on the Cu:Sn ratio. For instance, the growth rate per cycle of CTS significantly decreased in comparison to its constituents due to the etching of SnS by the Cu2S sub-cycle and the retarded growth of Cu2S on the SnS terminated surface. The absorption edge of the CTS films was also observed to red shift with increasing Sn composition.

Resume : ZnO with hexagonal wurtzite structure is a promising material for solar-energy conversion and photocatalysis due to its excellent carrier mobility and natural affluence. However, its broad application limited by optical absorption ability. In this work, hydrogenated ZnO core-shell nanowire-array was designed and fabricated on FTO as photoanode, depicting excellent electrochemical activity and light-harvesting capabilities such as current density of 4.0 mA/cm2 and the decrease of the band gap (~ 2.9 eV). In addition, the mechanism of interfacial charge transfers, electronic structure and local environment of zinc and oxygen vacancies in the hydrogenated ZnO and the correlation with the photocatalytic performance was systematically investigated with synchrotron-based in-situ XAS.

Resume : In this contribution, we report on Sn-doped indium sulfofluoride thin-films exhibiting semiconducting properties. The films were deposited on fused silica and Corning glass substrates using a radio-frequency plasma-enhanced reactive thermal evaporation system. The deposition was performed evaporating indium-tin alloy in SF6 plasma at substrate temperatures ranging from 373 to 423 K. The films are highly transparent in the visible-infrared range due to an indirect bandgap of 2.6-2.8 eV. The resistivity of undoped material varies in a wide range of 100 MΩ-cm to 2 TΩ-cm depending on deposition conditions. Sn-doping leads to the decrease in the resistance down to 10 MΩ-cm at 300 K. Moreover, the doped material is highly photosensitive in the blue - UV range. The light-to-dark current ratio reaches a value of 300 at 1E15 photons/cm2-s flux. Photoconductivity kinetics under various excitation conditions was also studied. The synthesized material is a promising candidate for a buffer layer in chalcogenide-based solar cells.

Resume : CZTS is an earth-abundant p-type material having high absorption coefficient and optimum band gap (1.4 eV) which can straddle the water oxidation-reduction potential (1.23 eV) but lies its band edge position close to water reduction potential. CZTS can be used as a photocathode in PEC but is vulnerable to (photo) corrosion in an aqueous solution and therefore studies have been carried out to enhance its stability by applying a protective layer. Moreover, the suitable band gap for water splitting photoelectrode material should be =1.9 eV, which is a higher value than CZTS. To enhance the PEC water splitting performance graphene oxide can be a suitable material for heterostructure PEC device due to its suitable optical band gap (2.1-2.4 eV) and stability. Stoichiometric CZTS thin films have been prepared on Glass/Mo substrates by co-sputtering of Cu, Zn, Sn precursors and followed by close reactor sulphurization process. Further, the GO was prepared in DMFO (0.1 wt percent) and drop cast on CZTS surface to make Mo/CZTS/GO photocathode. PEC water splitting measurements were carried out in sodium sulfate electrolyte (0.5 M) where sample active area is 1cm x 1cm. The photocurrent density observed for Mo/CZTS/GO sample is 2.98 mA/cm2 than that for Mo/CZTS 1.82 mA/cm2. The Nyquist plot shows that charge transfer resistance decreases due to CZTS/GO interface. The graphene on CZTS layer is found to enhance the photocurrent density during water reduction reaction.

Resume : So called all-oxide photovoltaics is gaining attention and interest among the community due to the low cost, abundance and long term stability. Copper oxides based heterojunctions are the one of the key family being intensively investigated. The efficiency of such heterojunction solar cells surpassed the threshold of 6 %. In this work we demonstrate the material studies aimed to provide methodology and components necessary to fabricate complete cost effective solar cell. The proposed structure is Cu/CuOx/ZnO/AZO. The p-type CuOx was obtained by simple oxidation method of copper substrate, while n-type ZnO and transparent front electrode (AZO) was deposited by Atomic Layer Deposition method. The obtained copper oxide has been characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and optical techniques. Moreover, ohmic contact on Cu/CuO junction was confirmed. The energy band gap (Eg) was estimated from Kubelka – Munk method as 1.6 eV indicating the presence of CuO. XPS analysis confirmed the presence of CuO Surface topography mapping revealed moderate roughness and surface extension about 10-15 %. Optical properties of ZnO and AZO were obtained from ellipsometry and UV-vis (Tauc plot). The Eg of ZnO was found as 3.5 eV. For AZO Eg varied with the Al content according to Moss-Burstein effect. Resistivity of AZO was of the order of 10-3 Ωcm as deduced from 4-point probe and ellipsometric measurements. Furthermore, the influence of Al concentration in ZnO on free carriers concentration and their mobility, was investigated. The interface of the heterojunction between the CuO/ZnO layer, having the key significance for the cell structure was characterized by transmission electron microscopy (TEM).

Resume : When InP capping layer is deposited on InGaAs active layer, few nm of unintentional interfacial layer is formed between InGaAs and InP layers. Since the interfacial layer modulates band gap and structural defects, various growth techniques have been developed to suppress the layer. There are two main origins generating interfacial layer between InGaAs and InP. One is residual arsenic in a growth chamber which is carried over to the successive InP layer. The other is phosphorus in InP layer exchanged to arsenic in InGaAs layer since chemical bonding between Ga-P is more stable than Ga-As. In this study, by using high resolution X-ray diffraction (HR-XRD) measurement and simple selective etching, the interfacial layer between InGaAs and InP has been analyzed. In order to obtain the thickness and compositional information of the interfacial layer, InP capping layer was etched by steps of 100 nm and HR-XRD measurements of rocking curves and reciprocal space maps (RSMs) were carried out at each etching step. Lattice expansion of InP capping layer along growth direction originated from interfacial layer containing arsenic was observed. The lattice expansion due to carried-over arsenic is saturated when the thickness of remaining InP layer is around 170 nm. The thickness of the remaining layer indicates the interfacial layer thickness between As-free InP capping layer and InGaAs active layer. In addition, the thickness and compositional results from HR-XRD were compared with those of TEM, SIMS and EDS, and the results were consistent with each other.

Resume : A systematic study of changes in the properties of Sb2Se3 thin films induced by post-deposition treatments (PDTs), including annealing in evacuated ampules (with and without Se over pressure) and in the process tube under N2 atmosphere at temperatures between 300 and 500 oC, was carried out in order to understand the physicochemistry of the processes and to optimize the properties of the absorber. The 3-4 µm thick Sb2Se3 films were grown by close-spaced sublimation at temperatures between 300 and 450 oC on glass substrates. The as-grown films exhibit a (221) preferred orientation, a p-type conductivity with a dark resistivity of ~107 Ω·cm and a high dark-to-light resistance ratio. A band gap of ~1.1 eV and an absorption coefficient of 105 cm-1 were obtained from UV-Vis measurements. Ampule and N2 annealing conditions do not affect the grain size of the Sb2Se3 layers but impact the concentration of intrinsic point defects and carrier density. The dark resistivity decreased up to 103-104 Ω·cm as a result of annealing at 400-450 oC and a hole density of 1013-1014 cm-3 was achieved. By creating Se-rich conditions in ampule annealing step, the hole density was further improved to 1015 cm-3, indicating to the great potential of the absorber for solar cell applications. Annealing at 500 oC restored the high resistivity of the layers. Changes in the properties of Sb2Se3 films induced by PDTs are explained by the dynamic behavior of Sb vacancies prevailing under Se-rich conditions.

Resume : Cu2ZnSnS4 (CZTS)-based solar cells are a promising candidate among various thin film solar cells due to low cost, high efficiency, non-toxic, abundant in the earth's crust. In additon, CZTS is a p-type material with a direct band gap energy of ~1.50 eV, which is exactly matching the optimal value required for absorbing the most intense part of the solar spectrum reaching on the surface of the earth. In this study, CZTS thin film as an absorber layer have been succesfully deposited on soda-lime glass (SLG) substrates by both sol-gel and thermal evaporation technique. Structural, optical, electrical and morphological properties of the deposited CZTS films obtained by these two techniques were compared with each other and discussed in detail for the first time in this study. The thickness of films was measured about 500 nm by Veeco 150 profilometer. While Cu/Zn+Sn and Zn/Sn ratios were calculated 0.5 and 2.36 for the thermal evaporated as-grown CZTS thin films, the same ratios are 1.09 and 1.06, namely, they are stoichiometric for the as-grown CZTS thin films prepared by sol-gel method. Neither sulfurization was applied in both techniques. Structural analysis has revealed that the as-grown CZTS films deposited by these two technique have an amorphous matrix and it transform into a polycrystalline form (the preferred orientation of (112)) with a kesterite phase after post-annealing process in the 300 oC under the inert gas flow. From the XRD analysis, it was clearly seen that there was a clear trend of an increase in peak intensities with increasing the annealing temperature. The secondary phases were investigated with Raman measurements and no secondary phase was observed in the CZTS structure. The morphological analysis showed that the thermally evaporated films have comparatively smoother surfaces. The transmittance and reflectance measurements of deposited films were conducted in the wavelength range of 400-1000 nm. The band gaps of the films were found to be 1.45 eV for thermally evaporated films and 1.50 eV for CZTS films prepared by sol-gel method after annealing at 500 oC.

Resume : Magnesium half silicide (Mg2Si) having anti-CaF2 structure and an indirect energy gap of Eg = 0.61 eV, is an interesting material for the thermo-photovoltaic (TPV) application covering in the near infrared region because both constituent elements in Mg2Si are earth-abundant and non-toxic, and its Eg is varied between 0.3 eV and 0.61 eV by making Mg2Si1-xSnx alloy compound. The good oxidation resistance and thermal stability are also suite for the TPV cell. Until now, we have reported a near infrared photoresponse from the Mg2Si pn-junction photodiode that fabricated by a simple thermal diffusion process of Ag acceptor into n-type Mg2Si substrate. However, precise Ag diffusion profile and the pn-junction depth in the Mg2Si photodiode have not been clearly understood, so far. In this paper, we investigated the relationship between the Ag concentration profile and the pn-junction depth in Mg2Si pn-junction photodiodes fabricated by thermal diffusion of Ag acceptor. The Ag concentration profiles and pn-junction depth in the samples annealed between 400 and 550 °C for 10min were studied by SIMS and EBIC, respectively. The Ag diffusion profiles, C(w, t), were fitted well by assuming the sum of two kinds of lattice diffusions with two different diffusion coefficients. This result could suggest the interstitial-substitutional diffusion of Ag acceptor in Mg2Si.

Resume : In order to enable terawatt scale-up of thin film photovoltaics more Earth abundant elements than tellurium (from CdTe cells) are required. An example of a successful Earth-abundant material investigated in recent years is antinomy selenide, an anisotropic material with an orthorhombic Pnma space group. It has risen to an efficiency of 7.6% [1] in the last four years. One alternative material that has the same crystal structure and similar band gap is germanium selenide; an initial study has reported a GeSe cell with efficiency of 1.48% [2]. To understand the material better, hard x-ray photoelectron spectroscopy of a single crystal GeSe sample has been done to study the valence band spectra and compare it to density functional theory calculations. Additionally, thin films of GeSe have been deposited to assess optical properties. The suitability of this material for photovoltaic application will be assessed. [1] X. Wen and et al. Vapor transport deposition of antinomy selenide thin film solar cells with 7.6% efficiency. Nature Communications, 9:2179, 2018. [2] D. Xue and et al. GeSe thin-film solar cells fabricated by self-regulated rapid thermal sublimation. Journal of the American Chemical Society, 139:958-965, 2017.

Resume : CZTSSe is one of the most promising PV materials in terms of being constituted of low-cost, earth abundant and non-toxic elements. Though intense research has been carried out on it, device efficiencies have not been able to surpass those of the more established material CIGS. Even then, the CZTSSe based devices reported to have the best efficiencies are those made by sputtering which in itself is an expensive process requiring high vacuum equipment. Therefore it is essential to find a solution to this problem (of low device efficiencies), which doesn’t compromise on the synthesis technique being simple, scalable and economically viable. In this work, a low-cost, wet chemical technique (by CBD) to grow CZTSSe films has been optimized. The effect of variation of elemental ratios on the band structure has been studied and used to established a direct correlation between the starting chemical precursor ratios and important material properties (band gap, work function and valence band edge position). The process gives repeatable CZTSSe films with minimal secondary phases. Following this basic optimization procedure, zinc and tin have been partially replaced by novel elements (Cd, Ni, Sb) and detailed material characterization carried out to study the effect of this elemental replacement on the film's properties. It has been found that there are clear changes in the band gap, x-ray diffraction pattern and Raman spectroscopy data. Additionally, x-ray photoelectron spectroscopy has also been carried out to analyze the band structure which together with the basic characterization data, has provided a complete picture of the changing material properties as elemental doping is carried out

Resume : Lately, the demand for low-cost and efficient micropower sources has rapidly increased. This concerns for instance autonomous micro-systems in the fields of IoT and can be related to both energy harvesting and energy storage. Numerous devices examples can be cited, the conversion of mechanical kinetic energy into electrical energy to power electronic devices is one of them. In parallel the interest for flexible devices has also imposed new constraints. Several technologies related to micro-power sources (piezo-electric, micro-batteries, photovoltaics…) need electrodes. This creates new challenges for electrodes which should be either transparent or flexible or both. Indium Tin Oxide has so far dominated the field of transparent electrodes (TE) but indium’s scarcity and brittleness have prompted a search into alternatives. For instance metallic nanowire networks appear to be one of the most promising emerging TE. Indeed randomly deposited silver nanowire (AgNW) networks present very good electrical properties (sheet resistance values below 10 Ω/sq), good optical transparency (90%) as well as high mechanical stability under bending tests. Still, for efficient integration into micro-power sources, several additional requirements have to be considered. In this brief review we will discuss the main features related to electrodes in micropower sources, including their stability (electrical, thermal and mechanical aspects), ageing and chemical degradation.

Resume : The efficiency of pure-sulfide Cu2ZnSnS4 (CZTS) photovoltaics is on a steady raise, and solution-processing methods are needed for upscaling technologies in the future. Among them, spray-coating offers large-area deposition, minimal material losses, and an automated setup. In this work, we synthesize organic ligand-free CZTS nanoparticles (NPs) according to our earlier work [1], and formulate an ink consisting of water and ethanol. This allows inclusion of new elements e.g. Ag, Ba, and alkali elements in the form of Cl-salts, and we have seen particularly beneficial effects with K [2]. A Nadetech spray-system is used, which utilizes ultrasonic atomization and includes an ultrasonic syringe pump. The ultrasonic waves ensure well-dispersed NPs, which leads to a more uniform film. Moreover, forming a flat film depends on NP size distribution as well as a complicated relation between substrate temperature, droplet size, and nozzle height. The as-deposited films are annealed and when appropriate completed into full solar cell devices. The morphology is characterized by scanning electron microscopy and profilometry, and film quality via photoluminescence. [1] N. Mirbagheri, S. Engberg, et al., Synthesis of ligand-free CZTS nanoparticles via a facile hot injection route, Nanotechnology, 27 (2016). [2] S. Engberg et al., Liquid phase assisted grain growth in Cu2ZnSnS4 nanoparticle thin films by alkali element incorporation, RSC Advances, 8 (2018).

Resume : To minimize interfacial recombination losses and optimize the charge extraction in photovoltaic systems, thin nickel oxide (NiO) nano network films as a p-type hole transport layer (HTL) are prepared by chemical bath deposition (CBD) at room temperature. The prepared NiO films shows 2-dimensionally networked porous structures and their morphology and thickness were controlled by addition of aluminum sulfate. With increase aluminum sulfate, thickness of NiO nano network was decreased from 720 nm (pure) to 65 nm (1.5 atomic % of Al) without change the network structure. In addition, conductivity enhancement of NiO films was observed with increase the aluminum sulfate. Thickness controlled NiO films were inserted between MAPbI3 perovskite layer and ITO to promote the photovoltaic application. Optimized devices achieve the improved solar cell efficiency especially fill factors compare with dense NiO based perovskite solar cells.

Resume : By exploiting the analytical solution of the phonon Boltzmann transport equation under the gray relaxation time approximation, we study the bahavior of the effective interface thermal resistance (ITR) in both the steady-state and dynamical regimes. This is done by using accurate expressions for the temperature and one-dimensional heat flux propagating across thin films supporting a diffusive phonon scattering at their interfaces [1]. In the steady state regime, we show that the effective ITR between two layers can be asymmetric on their thermal properties, such that its asymptotic value in the ballistic regime is higher than that in the diffusive one. In the ballistic-diffusive regime, the effective ITR depends strongly on the ratio λ = L/l_1, between the layer thickness L and mean free path (MFP) l_1 of phonons. Our predictions for the effective ITR in the ballistic regime are in good agreement with those of the diffuse mismatch model, while they differ by about 16% in the diffusive regime [2]. In the dynamical regime, we mainly show that in the diffusive regime, the ITR reaches a maximum at the characteristic modulation frequency f_c =( (sqrt(10)/2π)(l_1/L)^2)f_1, where l_1 and L are the phonon MFP and thickness of the finite layer. This maximum thermal resistance is associated with the minimum of the modulated heat flux at the interface [3]. The obtained results can thus be applied for describing the modulated heat conduction in dielectric thin films and determining the dominant MFP and relaxation time of phonons through the comparison of our theoretical model with experimental data measured by thermoreflectance or other relevant photothermal techniques. [1] J. Ordonez-Miranda et al., J. Appl. Phys., 118, 075103 (2015). [2] K. Alaili, J. Ordonez-Miranda, and Y. Ezzahri. Int. J. Therm. Sci., 131, 40 (2018). [3] K. Alaili, J. Ordonez-Miranda, and Y. Ezzahri. J. Appl. Phys., 124, 245101(2018).

Resume : Since the noticeable progress of SnS thin-film solar cells made by Gordon et al.[1], tin monosulfide (SnS) has attracted much interest as a p-type absorber for photovoltaic devices. Thus far, the high-performance SnS absorber films have been fabricated mostly by high-vacuum methods such as thermal evaporation, ALD, and sputtering techniques. In this study, we fabricated SnS thin films by a non-vacuum process. SnS nanocrystals (NCs) were mechanochemically synthesized from elemental precursor powders without using any organic solvents and additives. To investigate the effect of Sn to S stoichiometric ratio on the crystalline phase of SnS NCs, the [Sn]/[S] ratio was systematically controlled from 0.95 to 1.05 by adjusting the mixing ratio of elemental powders. The morphological and crystallographic properties of the synthesized SnS NCs were analyzed by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The as-synthesized SnS NCs with a stoichiometric composition (i.e., Sn1.00S) were found to have the Sn2S3 impurity phase in a non-negligible amount. The Sn2S3 impurity phase could be eliminated from the synthesized Sn1.00S NCs by a post-heat treatment at 500 °C under a 1% H2 atmosphere. Interestingly, however, the formation of Sn2S3 during the mechanochemical synthesis was greatly alleviated by introducing Sn-excess composition (e.g., Sn1.05S). To demonstrate the SnS-based thin-film solar cells, SnS NCs inks were casted onto Mo-coated glass substrates by a doctor-blade method, and subsequently the SnS NCs layer was annealed under S-containing atmosphere. The SnS solar cells were fabricated with a configuration of Mo/SnS/CdS/ZnO/ZnO:Al/Ni/Al. The solar cell with a Sn1.05S absorber layer exhibited a power conversion efficiency of 1.7%, which is vastly superior to Sn0.95S or Sn1.00S-based devices. From this work, it is suggested that preventing the formation of unfavorable secondary phases is a major challenge to achieve the highly efficient SnS thin-film solar cells. References [1] P. Sinsermsuksakul , L. Sun , S. W. Lee , H. H. Park , S. B. Kim , C. Yang and R. G. Gordon, Adv. Energy Mater., 15 (2014) 1400496.

Resume : The one of the most accurate methods is Full Potential-Linearized Augmented Plane Wave (FP-LAPW) formalism [1] based on density functional theory (DFT) [2,3] has been made to discuss about the influence of sulphur substitution into Cu2ZnSn(SxSe1-x)4 on the enthalpy of formation. In this study we studied also the temperature dependence on volume and isothermal bulk modulus of this solid solution. In our work, a primitive 64 atoms super-cell in tetragonal structure was adopted, which corresponds to a 2ax2bx2c conventional cell. As result, our computed lattice constants for the pure compounds CZTS and CZTSe showed satisfactory agreements with the experimental result. The predicted mixing enthalpy is negative (exothermic) for our Stannite based structure implying that alloy formation is a thermodynamically favourable process. The increase in temperature can alter the minima of the free energy variation with the volume and the shift in the minima represents the change in volume with the temperature. The obtained results are used to give an important guideline to perform other electronic and optical properties to material’s design for optoelectronic devices. [1] P. Plaha, K. Schwarz, P. Sorantin, S. B. Trickey, Com. Phys. Comm. 59, 399-415 (1990). [2] P. Hohenberg, W. Kohn, Phys. Rev. B136 , 864-871 (1964). [3] W. Kohn, L. J. Sham, Phys. Rev. A140, 1133-1138 (1965).

Resume : Zinc phosphide (Zn3P2) is a promising earth abundant solar cell material. Zn3P2 has a direct band gap transition of 1.5 eV [1], which is close to the optimum solar energy conversion range. It has a large optical absorption coefficient of >10^4 cm-1 near its band edge [2], and a minority carrier diffusion length reported ~10 µm [1, 3]. Additionally, as both zinc and phosphorous are earth abundant elements it could potentially make solar cells more economical for large-scale deployment. However, since 1977 there has been no improvement in the reported efficiency of Zn3P2 based solar cell. One of the factor being the quality of the grown epitaxial film, which is influenced by the substrate. Mismatch between the substrate and overlayer introduces defects, which ultimately impairs the device quality. Here we report the influence of the substrate on the thin film growth using van der Waals epitaxy. Due to large lattice parameter and high coefficient of thermal expansion of Zn3P2, the choice of substrate for epitaxial growth is limited and this largely affects the thin film quality. In order to obtain Zn3P2 growth independent of substrate lattice parameter we use van der Waals epitaxy using commercial graphene as a substrate. We demonstrate the growth of single crystalline flakes and propose a growth mechanism. We investigate the influence of various growth parameters on the Zn3P2 growth on graphene, such as temperature, flux and V/II ratio. We use Raman spectroscopy to confirm the stoichiometry and crystalline quality of the material and to detect the presence of impurities. Scanning electron and atomic force micrographs of the crystalline seeds show their triangular structure, most of them keeping a preferred orientation with respect to the underlying graphene. TEM further confirms the single crystalline nature of the material. GIXRD is conducted to identify the phase of the grown material. Finally, we discuss the optical properties of the grown material and perspectives for solar cell devices. [1] G.M.Kimball, Applied Physics Letters, vol. 95, p. 112103, 2009 [2] E.A.Fagen, Journal of Applied Physics, vol. 50, no. 10, p. 6505–6515, 1979 [3] M.Bhushan, Applied Physics Letters, vol. 38, no. 1, pp. 39-41, 1981

Resume : In this work, we theoretically investigated and optimized the performance of spherical and cylindrical conductive thermal diodes operating between a phase-change material whose thermal conductivity depends on temperature and a phase invariant one with a constant thermal conductivity. Analytical expressions for the heat fluxes, temperature profiles and optimal rectification factor are derived and analysed for junctions between the two conductive diodes. It is shown that the diode geometry has a significant impact on the temperature profiles and heat fluxes, but not much on the rectification factor. Optimal rectification factor of up to 20.7% and 20.8% are obtained for the spherical and cylindrical diodes associating with a temperature difference between their terminals of 376-300=76 K and 376.5-300=76.5 K, respectively. These similar rectification factors could be enhanced with a material thermal conductivity exhibiting a faster phase transition and/or higher contrast than that of VO2. The obtained results can therefore, be useful to guide the development of phase-change materials able to optimize the rectification of conductive thermal diodes with different geometries.

Resume : Recently, kesterite based Cu2ZnSn(S,Se)4 (CZTSSe) solar cell has attracted many attentions due to several advantages of CZTS(e) such as earth-abundant, cheap, nontoxic, high absorption coefficient (~10-4 cm-1) and tunable bandgap (1~1.5eV). [1-4] So far, the highest efficiency of CZTS(e) solar cell is 12.6% achieved by IBM team but its power conversion efficiency (PCE) still cannot compete with the commercial thin-film solar cells, e.g., CIGS and CdTe. There are numerous issues need to be solved to enhance the PCE (e.g., back contact losses, interfacial losses, deep defects). Especially, back contact losses cause degradation of CZTS(e) solar cell due to interface defects between CZTS(e) and MoS(e)2 produced by sulfo-selenized Mo-coated soda lime glasses (SLG). In this work, the PCE of CZTSSe solar cells were greatly improved by a simple but effective method, wherein a CdS nano-layer was deposited between the Mo substrate and the metallic precursors prior to CZTSSe sulfo-selenization, producing p+-CTS(e) and ZnS(e), which helps reduce interface recombination, on the bottom of the CZTSSe absorber layer. The mainly improvement results from increasing fill factor (FF) and short circuit current due to less recombination at back interfaces. Moreover, a 9.61% PCE (~10.6% in the cell effective area) CZTSSe cell with improved FF from 44% to 64% has been attained. The morphology, elemental composition, and distribution of the absorber layers are being examined by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), Transmission electron microscope (TEM), and Raman spectroscopy. References [1] V. Tunuguntla, W.C. Chen, P.H. Shih, I. Shown, Y.R. Lin, C.H. Lee, J.S. Hwang, L.C. Chen and K.H. Chen, J. Mater. Chem. A, 2015,3, 15324-15330 [2] Y.R. Lin, V. Tunuguntla, S.Y. Wei, W.C. Chen, D. Wong, C.H. Lai, L.K. Liu, L.C. Chen and K.H. Chen, Nano Energy, 2015, 16, 438 [3] W.C. Chen, C.Y. Chen, V. Tunuguntla, S.H. Lu, C. Su, C.H. Lee, K.H. Chen and L.C. Chen, Nano Energy, 2016, 30, 762-770 [4] C.Y. Chen, B. S. Aprillia, W.C. Chen, Y.C. Teng, C.Y. Chiu, R.S. Chen, J.S. Hwang, K.H. Chen, and L. C. Chen, Nano Energy, 2018, 51, 597-603

Resume : Kesterite Cu2ZnSn(SxSe1-x)4 (CZTS) has been considered as a promising candidate material class for scalable photovoltaic applications, due to its elemental abundance and controllable bandgap. However, the CZTS-based thin-film solar cells have shown significant open-circuit voltage (VOC) deficit problem, largely due to deep trap states and band tailing. Theoretical calculations predict that the Ag-alloyed phase possesses shallower defect levels than pure Cu phase. We study various approaches to improve micro-structure of CZTS thin-films and to reduce their intrinsic defects by using a rapid thermal process and novel Ag-alloying processes. Photo-Hall measurements of the thin-films and Quantum efficiency and capacitance-based measurements of the devices indicate that significantly improved minority carrier diffusion length, resulting in a record VOC-deficit below 600 mV. Microstructural analysis and a comparative study of photoluminescence properties between the pure-Cu CZTS and Ag-alloyed phases are discussed.

Resume : The research of new absorbers for both photovoltaic and photoelectrochemical applications recently has produced great development towards kesterite-based thin-films, whose the most promising are CZTS and CZTSe thin films. Contrarily to CdTe and CIGS, they have the advantage of being constituted by earth-abundant, not toxic, and relatively low-cost materials. In this vein, a basic issue is the development of optimized growth procedures, to obtain high-quality layers with good homogeneity, and which exploit relatively easy and cheap configurations. Among these, electrochemical-based deposition techniques can be good candidates, although a deep optimization is still necessary to obtain layers with suitable features. We here present the results of a newly-conceived CZTSe deposition approach. Our procedure is composed by two steps: a first one where a CZT precursor material with Mo/Zn/Cu/Sn stack is grown by electrochemical procedure, and a second one where the sample undergoes an in-vacuum selenization, during which the film is also heated to obtain the requested crystalline phase. The optimized procedure of the first part allows to obtain stacks with a finely controlled stoichiometry, while the second one produces an efficient and homogeneous Se distribution. The features of the obtained material are shown and possible perspectives for application on photovoltaics and photoelectrochemistry are discussed.

Resume : Solar cells based on earth abundant Cu2ZnSnS4 (CZTS) semiconductor can find a very promising application as top-cell in CZTS/Silicon tandem devices. In addition to the development and optimization of CZTS absorber and single junction device, the evolution of tandem technology also requires the development of proper structures for the connection between the top and the bottom cell. This intermediate structure must allow a good electrical contact between the two cells, exhibit a high transparency in the infrared region of the solar spectrum, demonstrate a good chemical stability and must prevent silicon degradation during the thermal annealing treatment at high temperature in sulfur atmosphere, typically used for the growth of CZTS films. In this work, multilayer structures based on MoS2 and different TCOs are tested as possible candidates for intermediate contact in CZTS-Silicon monolithic tandem devices. Optical, electrical and chemical characterizations of different contacts based on i:ZnO, Al:ZnO, ITO and FTO were performed before and after the sulfurization treatment. The most promising structures have been used to produce complete tandem devices. A first working device with open circuit voltage higher than 1 Volt has been obtained using a MoS2/FTO/ZnO intermediate contact.

Resume : For three decades density-functional theory (DFT) has imposed itself as an accurate quantum method to investigate materials properties. In parallel, developments of density-based descriptors such as Bader’s quantum theory of atoms in molecules (QTAIM) brought new insights into the chemical bonding in materials. The ternary Cu-based chalcopyrite compound, CuInSe2 (CIS), is an interesting material as solar cell absorber layer due to its low cost, high absorption coefficient, excellent optical and electrical properties. Many approaches have been adopted to improve its energy conversion efficiency. However, its narrow band gap and the scarcity and expensiveness of indium constrain its large-scale development. Replacing indium by the abundant and inexpensive aluminum to form the quaternary CuIn1-xAlxSe2 (CIAS), has been considered as a promising alternative with few changes in physical and chemical properties. In this work, we investigated by DFT calculations the structural, electronic and optical properties of CuIn1-xAlxSe2, for various x from 0 to 1, and determined the optimal substituting percentage. Moreover, in current PV cells, strains originating from the lattice mismatch between the PV materials and the substrates inevitably influence the optical performances, we calculated the band gap and optical properties for the optimal alloy subjected to biaxial strains. In the aim to unravel the deep relationship between bond interactions and optical properties, a detailed investigation of topological properties based on the electron density has been conducted as strain is applied.