Thin film chalcogenide photovoltaic materials

Resume : Chalcogenide absorbers in thin-film solar cells render relatively high optical absorption of solar spectrum up to the photon energy determined by the absorber bandgap. By using sufficiently thick absorbers (> 1 um) the main concern related to light management is, in this case, decreasing optical losses in front layers (new electrode and window materials) and minimising reflectances at front interfaces (antireflecting nanostructures). When thinning down absorber layer thicknesses d < 0.5 um (ultra-thin devices), a highly reflective back reflector in combination with light scattering nano/micro structures have to be introduced. In this contribution we will make a short overview of main existing solutions of light confinement in different types of chalcogenide solar cells (CIGS, CdTe, CZTS). In the main part of the contribution we will present the results of advanced optical modelling, verified with selected realistic cases, which enables us to: - quantify optical loses in chalcogenide solar cells when thinning down absorber layers to ultra-thin devices - show the potential gains in photocurrent related to introduction of different light scattering and anti-reflective nanostructures (periodic and random) at different positions in solar cell and in combination with highly reflective back reflectors - show possibilities for tandem devices, such as combinations of chalcogenides with perovskite solar cell.

Resume : CIGS-based solar cell including an ultra-thin absorber layer (below 500 nm) gives an opportunity to reduce the deposition time as well as material consumption. As already reported, we can observe a drop in cell efficiency when the CIGS layer thickness decreases, predominately due to a decrease of Jsc. As this is caused by an incomplete absorption of light and a sharpened impact of the interfaces, a rear interface developed to reflect the remaining light and to readily collect majority carriers can improve the cell efficiency. In this work we analyze the potential of replacing the molybdenum back contact with a Transparent Conducting Oxide (TCO) coupled with reflectors. We have simulated the CIGS absorption with a Ray Transfer Matrix calculation method. The current density of 300-nm-thick-CIGS cell is improved up to 4 mA/cm² on a TCO back contact coupled with a Lambertian reflector. The Jsc improvement is also presented with various structures. Based on these results, solar cells with a thin coevaporated CIGS layer on ZnO:Al and SnO2:F substrates were fabricated. The difficulty to adjust the amount of Na in CIGS, which has to be externally supplied, and the impact of growth temperature will be discussed. An analysis of the CIGS/TCO interface is made, based on the interface composition, contact resistance and recombination characterization. Finally, we report in this work higher efficiency of 450 nm CIGS cells on SnO2:F/mirror (10.6%) than on Mo (9.6%).

Resume : Decoupling of the effects of optical and collection losses in solar cells based on graded Cu(In,Ga)Se2 (CIGS) absorber can be achieved by numerical simulation methods. However, accurate measurements of the complex refractive indices of the different layers of the solar cell structure are essential to calculate the light propagation and charge carrier generation in the absorber. In this contribution, we report spectra for the complex refractive indices of Mo, MoSe, CIGS, CdS, intrinsic and aluminum-doped ZnO, and MgF layers. Data from several characterization techniques, such as ellipsometry, optical reflectance and transmittance, AFM, SEM, and profilometry measurements, are combined. To account for gradings and changes in absorber composition, CIGS layers with different [Cu]/([In]+[Ga]) (CGI) and [Ga]/([In]+[Ga]) (GGI) compositions were grown by co-evaporation on Mo coated glasses to ensure comparable growth conditions as for actual devices. Discrepancies between our results and literature data are discussed. Comparison of the computed light absorption spectra of CIGS absorber layers with the corresponding device quantum efficiency measurements allows to differentiate effects of optical and collection losses. Based on this analysis, we suggest possibilities to improve cell performance in the near-infrared range.

Resume : The optical properties (i.e., dielectric functions, absorption coefficients and bandgap) of 500 nm and 800 nm-thick Cu2ZnSnS4 absorbers grown by reactive sputtering on bare and Mo-coated soda-lime glass (SLG) were determined using spectroscopic ellipsometry (SE) and spectrophotometry. X-ray fluorescence, X-ray photoelectron spectroscopy depth profiling, X-ray diffraction, Raman spectroscopy, scanning-electron microscopy and atomic force microscopy measurements were performed to characterize the composition, crystal structure, phase purity and morphology. The ensuing sample characteristics were utilized in SE analysis for the establishment of a multilayer stack optical model and the dielectric functions and refractive indices were determined. The SE-derived absorption coefficients from CZTS/SLG samples were compared with those derived from complementary spectrophotometry measurements and found to be in good agreement. The bandgap determined from Tauc plots was Eg =1.57±0.02 eV. The absorption coefficients with moderate sub-band tailing were found to be a few 10^4 cm^{-1} just above the bandgap and exceeding 10^5 cm^{-1} at energies above ~2.5 eV. Separate device characterization performed on identical samples showed that the optical properties determined in this work can be linked to 2.8% and 5.3% to the 500 nm and 800 nm-thick samples featured in this study. The same studies on the Cu2ZnSn(S,Se)4 samples are in progress.

Resume : Although the photovoltaic (PV) market is still dominated by the Si-based solar cells, thin film technology is considered to be a viable solution, in particular for unconventional applications such as flexible, transparent or bifacial solar cells, which represent an great opportunity in the field of the integrated PV (BIPV). Cu(In,Ga)Se2 (CIGS) represents a good compromise between conversion efficiency, stability and possibility to be grown on different substrates therefore is considered to be the most promising material for thin-film PV. We report on the fabrication of CIGS-based bifacial thin film solar cells by means of the Pulsed Electron Deposition (PED) technique, using transparent substrates (glass) and back contacts (based on In2O3:Sn, ITO). In the last years, PED has been developed at IMEM-CNR [1-2] for high-quality CIGS deposition; recently, small solar cells with PV efficiency exceeding 17% have been obtained by controlling the absorber doping and by optimizing the cell architecture and the device processing [3]. Interestingly, high-quality CIGS absorber layers can be deposited at much lower temperature (250-270°C) compared to alternative techniques, enabling the use of a large number of substrates, including polymeric thermo-labile ones. This makes the PED a very effective method to grow “complex” materials and to explore unconventional devices, such as flexible or transparent solar cells. The studied cells have the typical bifacial configuration [4]: substrate (glass)/back contact (ITO, by sputtering)/absorber (CIGS, by PED)/buffer layer (CdS, by CBD)/front contact (ZnO-ZnO:Al, by sputtering). We will report on the performance of PV cells with a total exceeding 10% of efficiency and significantly higher than recently published results [5] under different types of illumination (direct, diffused, back reflected) for thick and thin CIGS layers. REFERENCES: [1] S Rampino et al., “15% efficient Cu (In, Ga) Se2 solar cells obtained by low-temperature pulsed electron deposition”, Applied Physics Letters, vol. 101, p. 132107, 2012 [2] S Rampino et al., “Low-temperature growth of single-crystal Cu(In,Ga)Se2 films by PED technique”, Solar Energy Materials and Solar Cells vol. 133, pp. 82-86, 2014 [3] M. Mazzer et al. submitted [4] T. Nakada et al., “Novel device structure for Cu(In,Ga)Se2 thin film solar cells using transparent conducting oxide back and front contacts”, Solar Energy, vol. 77, pp. 739–747, 2004 [5] S. H. Moon et al., “Printable, wide band-gap chalcopyrite thin films for power generating window applications”, Scientific Report, vol. 4, p. 4408, 2014

Resume : Over the last few years, the development of ever more efficient thin-film chalcogenide solar cells has been steady, in spite of incomplete understanding of the physical mechanisms that determine and limit the cell’s performance. However, keeping the pace of efficiency improvement above the recently overcome 20% barrier toward 25% and beyond will likely require increased support from device-level physical modeling. 1D numerical modeling has been routinely used for many years and proven to be a valuable tool, especially for c-Si and III-V cells. Thin-film chalcogenide cells, however, have peculiarities that make 1D modeling inadequate in some respects: the effect of grain boundaries, for example, is a typical 3D effect, that can be treated with a 2D model but clearly eludes a 1D approach. In addition to that, the use of patterned front- or back-side passivation layers with point contact openings to reduce the negative effect of defective interfaces requires 2D/3D models for optimum choice of the contact layout. The aim of this paper is to give an overview of results of 2D/3D numerical simulation of thin-film chalcogenide cells including dark I-V curves, C-V curves, and illuminated characteristics; the role of defects, whether in the grain interior, at the grain boundaries, or at heterointerfaces, will be specifically addressed, together with the use of passivation and point contacts to mitigate the impact of defective surfaces.

Resume : A sulfur incorporation into the surface of sequentially grown Cu(In,Ga)(Se,S)2 absorbers is one of the approaches to optimize the efficiency of these devices. A sulfur gradient widens the bandgap in the space charge region without a significant impact on the bandgap in the bulk. As a result, a separation of the recombination and absorption processes takes place which leads to an improved open circuit voltage without a loss in the short circuit current. The question which is still open, and therefore will be addressed in this contribution, is an impact of the sulfur gradient on the minority carrier lifetimes in the absorber. In this work, solar cells with different sulfur gradients have been investigated. Photoluminescence decay measurements have been performed to study carrier dynamics and recombination mechanisms. Since the interpretation of these results is not straightforward, the temperature-dependent measurements of transient photoluminescence and current-voltage characteristics of these devices will be correlated in order to build a relevant recombination model to extract minority carrier lifetimes. Experimental results will be supported by simulations. Furthermore, the impact of a front grading on the temperature- and intensity-dependence of transient photoluminescence with respect to different sulfur contents will be discussed in terms of carrier trapping and re-emission.

Resume : Kesterite based photovoltaics is a promising thin film technology with the potential of future large scale production: Cu2ZnSnSe4 absorbers are made of cheap/earth-abundant elements and present optimal band gaps for solar cells (between 1 and 1.5 eV). Devices with efficiency higher than 11% have been demonstrated by different groups. Significant VOC deficit limits the performances of kesterite solar cells and the causes for this issue have not been clearly identified yet. Among the possible sources of VOC loss, some studies in literature suggested that the Grain Boundaries (GBs) inside the polycrystalline absorber can play a dominant role. We employ 2D modeling and the software Silvaco® to perform electrical simulations of Cu2ZnSnSe4 solar cells structures including GBs inside the absorber and a MoSe2 layer between the absorber and the Mo back contact. The GBs are modeled as thin vertical stripes presenting different properties and physical mechanisms than the surrounding Intra-Grain kesterite: lower bandgaps, higher recombination, presence of static charges (positive or negative) and n-conductivity: these conditions are considered both independently and combined. The analysis of the results gives an insight about the possible correlations between the electrical activity of GBs and their impact on the performances of the device. Experimental KPFM measurements and JV curves of real samples are showed to interpret and motivate our theoretical findings.

Resume : The decay of the room-temperature time-resolved photoluminescence (TRPL) on thin-film semiconductors such as Cu(In,Ga)Se2 and Cu2ZnSnSe4 often is bi- exponential. This is often traced back either to fluctuations of the electrostatic potential or to minority charge carrier trapping. We show by means of simulations that both effects can be discriminated by a measurement of the TRPL decay at different excitation intensities and temperatures.[1,2] A requisite is that the samples are stable under light exposure which is not always the case. Application of the standard semiconductor theory yields, that the bi-exponential photoluminescence decay in Cu(In,Ga)Se2 and Cu2ZnSnSe4 must result from a strong minority carrier trapping. By simulation of experimental TRPL decay curves we can determine the minority carrier lifetime, the trap energy, the trap density, and the doping density of these materials with values in the ranges of 1 − 10 ns, 200 meV, 10^15 − 10^16 cm^−3 , and 10^15 − 10^16 cm^−3 .[3,4] These yield reasonable solar cell parameters and they also explain the non-correlation of the open-circuit voltage and the luminescence decay time found by our and other groups. [1] M. Maiberg, T. Hölscher, S. Zahedi-Azad, and R. Scheer, Journal of Applied Physics 118, 105701 (2015). [2] M. Maiberg, F. Bertram, and R. Scheer, Journal of Applied Physics (2015), submitted. [3] M. Maiberg, T. Hölscher, S. Zahedi-Azad, W. Fränzel, and R. Scheer, Applied Physics Letters 107, 122104 (2015). [4] M. Maiberg, T. Hölscher, S. Hartnauer, W. Fränzel, and R. Scheer, Applied Physics Letters (2015), submitted.

Resume : Quantum efficiency measurements on Cu(In,Ga)Se2 (CIGS) solar cells are widely used as a non-destructive and easy to use method to extract the bandgap of the CIGS absorber layer. Information about the bandgap is of major relevance, e.g., for process control or parameter definition in device simulations. However, determining the bandgap from quantum efficiency measurements bear on the assumption that the quantum efficiency is solely given by the absorption of a homogeneous absorber layer. We therefore compared different bandgap extraction models on CIGS solar cells with different diffusion length, thus different bulk recombination properties, and with different bandgap grading profiles. The extracted minimum bandgap value is compared to that calculated from the composition measured with glow discharge optical emission spectroscopy. To study the effects of limited diffusion and grading in more detail, we created a one-dimensional optical simulation tool that calculates the optics by means of the generalized transfer matrix method. The tool includes models to account for roughness, thickness non-uniformity and graded layers. The quantum efficiency is given by the local generation rate multiplied with a local collection probability. The latter is a function of the diffusion length of the absorber material. We found that bandgap values that are extracted from layers with a bandgap gradient are overestimated by up to 200 meV depending on the slope of the grading. In contrast to this, the influence of the diffusion length on the extracted bandgap is negligible.

Resume : Increasing the chemical complexity of zinc-blende derived semiconductors through elemental substitution on the cation lattice has been a long-standing design principle for generating new candidate materials for photovoltaics. We have recently investigated the role of cation disorder in Cu2SnS3, Cu2ZnSnS4, and other zinc-blende derived materials using both theory and experiment. The underlying motif-based Hamiltonian goes beyond the conventional theoretical point defect model, and allows for performing Monte-Carlo simulations to create realistic models of disordered structures. The application of this model to stoichiometric phases has revealed an entropy-driven nano-clustering with impact on photovoltaics by the resulting potential fluctuations and band-gap variations [1], as supported by the experiments [2]. In case of off-stoichiometric materials, the disorder can dramatically impact the doping properties through the formation of extended anti-site defects with up to 1 eV lower formation energies compared to the conventional point defects [3], consistent with experimental observations [4]. The lessons learned from the disorder studies point towards those materials where disorder is suppressed by a high energy cost for formation of non-ground-state motifs, which, notably is the case in the chalcopyrites. However, a similar effect can be achieved in non-tetrahedral ternary compounds, where the disorder is suppressed by a different coordination environment of the cation. As an example of this class of materials, we considered CuSbS2 [5] and CuSbSe2 [6], and demonstrated the development of a solar cell with almost 5% efficiency within an unusually short time scale of less than 2 years. [1] P. Zawadzki et al Phys. Rev. Appl. 3, 034007 (2015). [2] L. L. Baranowski et al Phys. Rev. Appl. 4, 044017 (2015) [3] P. Zawadzki et al Phys. Rev. B 92, 201204(R) (2015). [4] L. L. Baranowski et al Chem. Mater., 26, 4951 (2014) [5] A.W. Welch et al Sol. En. Mat. So. Cel. 132, 499 (2015) [6] A.W. Welch et al Appl. Phys. Express 8, 082301 (2015).

Resume : The chalcopyrite Cu(In,Ga)Se2 alloy is today a rather well-established compound for thin film solar cells. Emerging Cu-based materials are explored to benefit from the energetically high-lying Cu d-state in combination with low effective mass of the minority carriers. In the present study, we explore the details in the optical properties of emerging Cu-based compounds, like for instance Cu2ZnSn(S,Se)4, Cu2SnS3, Cu(Sb,Bi)(S,Se)2, and Cu3(Sb,Bi)(S,Se)3, employing the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. We calculate the electronic structure and we analyze the optical properties in terms of the dielectric functions and absorption coefficients. By modeling the quantum efficiency of the compounds, we further discuss the optical response. The results help to understand fundamental physics of then emerging Cu-based materials in order to design and optimize solar cell devices. Refs.: R. Chen and C. Persson, J. Appl. Phys. 112, 103708 (2012); ibid, Thin Solid Films 519, 7503 (2011); M. Kumar and C. Persson, Appl. Phys. Lett. 102, 062109 (2013); ibid Energy Procedia 44, 176 (2014); S.G. Choi, et al., Appl. Phys. Lett. 101, 261903 (2012).

Resume : Recently, we reported the first-principles study on alkali-metal effect of Li, Na, and K in CuInSe2 (CISe) and CuGaSe2 (CGSe) [1]. The substitution energies of Li atom for Cu (LiCu) atom in these compounds are much lower than those of NaCu and KCu atoms in CISe and CGSe. Therefore, we predicted that a large amount of Li atoms can substitute for the Cu atom in CISe and CGSe. However, there is a few report on the lithium effect on the crystallographic and optical properties of CISe and CGSe. In 1992, German group reported the phase diagram of CuInSe2-LiInSe2 system [2]. The crystal structure changes from chalcopyrite-type CuInSe2 to NaFeO2-type LiInSe2. In this study, we prepared (Cu,Li)InS2 and (Cu,Li)GaS2 samples by a mechanochemical process and post-heating at 550 oC. We obtained the single-phase (Cu1-xLix)InS2 solid solutions with for (0.0 ≤ x ≤ 0.15) and (Cu1-xLix)GaS2 solid solutions for (0.0 ≤ x ≤ 0.30). The band-gap energies of the (Cu,Li)InS2 and (Cu,Li)GaS2 samples linearly increase with increasing Li content. The energy level of the valence band maximum (VBM) were estimated from the ionization energies measured by photoemission yield spectroscopy (PYS). [1] T. Maeda, A. Kawabata, and T. Wada, Jpn. J. Appl. Phys., 54, 08KC20 (2015). [2] U.-C. Boehnke and H. Neumann, and G. Kühn, J. Alloys Compounds 190, L17-L18 (1992).

Resume : Zinc-blende CuZnIn(Se,S)3 compounds have been identified as promising materials for optoeletronic devices based on their excellent luminescent properties, and working solar cells have already been reported. In this work, we have synthesized CuZnInSe3 by a two-step process: sputtering of metallic precursors and subsequent selenization in Se atmosphere. The crystal structure of the resulting thin films was proven to be cubic and of the zinc-blende type. Solar cells using CuZnInSe3 as absorber were fabricated following standard procedures of back contact and front-end termination as commonly implemented in kesterite and CIGS thin film technologies. In order to investigate the influence of composition, mainly the Cu content, CuZnInSe3 samples were prepared from Cu-poor (Cu/Zn+In =0.3) to slightly Cu-rich (Cu/Zn+In=0.53) compositions. This variation resulted in a change of the band gap from 1.16 to 1.32 eV. A first optimization showed that the performance of CuZnInSe3 solar cells depends slightly on the temperature of the first of the two stages during selenization and - strongly - on composition. Solar cells with high Cu deficiency show very high VOC (up to 560 mV), whereas devices with only slightly Cu-poor composition exhibited higher fill factors and power conversion efficiencies (PCE). The characteristics of the best solar cell device were: VOC=520 mV, JSC=22.6 mA/cm2, FF=64%, and PCE=7.6 %, which, to our knowledge, is the highest efficiency reported for this material so far.

Resume : Copper Zinc Tin Sulphide (CZTS, Cu2ZnSnS4) is a promising earth-abundant thin film photovoltaic material. Current devices have disappointing open circuit voltages considering the band gap of the material. At open circuit all charges are recombining; the open circuit voltage is set by the degree of recombination. From studies of the photoluminescence, there appears to be significant band-gap tailing even in high quality bulk CZTS material. Independent of the source of disorder, the device physics are particularly affected due to the low dielectric constant of this material. In this work we combine electronic structure calculations with different representations of disorder. We write custom Monte Carlo codes to simulate both substitutional disorder; and sample positional disorder with both molecular dynamics and lattice dynamics (phonons). With these models of disorder we simulate the band tailing observed in bulk material, and estimate its effect on the device physics, and predict what experimental observables would agree with the different models.

Resume : Absorbers made from the kesterite material Cu2ZnSn(S,Se)4 have gained interest as low-cost alternative to Cu(In,Ga)Se2 for high-efficiency thin-film solar cells. One of the major ongoing research topics in kesterites is the investigation of possible disorder amongst the Cu+ and Zn2+ cations within the Cu/Zn crystal lattice planes and its influence on material properties. The degree of disorder can be influenced by thermal treatment at different temperatures and the rate of subsequent cooling. It is typically determined in an indirect way by changes in the optical band gap of the material. Following few publications on the impact of disorder on the absorber layer itself, we present experimental results on cyclic changes between the ordered and disordered state and its direct effect on complete solar cells. We compare samples that we firstly ordered or disordered, respectively, and then repeated two additional cycles of alternate ordering or disordering. The processes were done both on pure absorber layers on Mo substrates and complete cell stacks simultaneously. In addition, after each step parts of the pure absorber samples were finalized to complete cells as well. Hence, we could compare the influence of ordering effects in the absorber layer and thermally induced changes in the interface between absorber and CdS buffer layer. The results indicate locally different behavior but a general trend for Voc that follows directly Egap and is normally improved after an ordering step. Nevertheless all cell parameters are degrading with cyclical annealing steps. Further details on layer and cell properties will be discussed.

Resume : Understanding of the interplay between structural, chemical and electronic properties of kesterite type compound semiconductors, applied as absorber in PV devices, can give crucial informations aiding continuous improvement of device efficiencies. These absorber layers exhibit in general an off-stoichiometric composition. The presence of secondary phases and deep defect levels caused by intrinsic point defects are efficiency limiting factors. The kesterite type structure can be described by a stacking sequence of cation layerd Cu/Sn-Cu/Zn - Sn/Cu-Cu/Zn-Cu/Sn perpendicular to the crystallographic c-axis. An off-stoichiometric composition originates from the prospensity of the structure to stabilize defects, charge balancees beeing commonly insured by appropriate cationic substitutions forming anti-sites, vacancies or interstitials. This study reports a detailed crystallographic investigation of cationic point defects in off-stoichiometric Cu2ZnSnS4 and Cu2ZnSnSe4, based on powder sample series synthesized by solid state reaction. Due to the isoelectronic character of Cu+ and Zn2+ it is necessary to apply neutron diffraction to investigate the distribution of the 3 cations on the 4 cationic sites of the kesterite structure. Rietveld refinements and the average neutron scattering length analysis method enables us to determine point defect type and concentrations. We will present the possibility to deduce occuring point defects from the chemical composition of the kesterite phase.

Resume : The interest in Cu2ZnSn(S,Se)4 (CZTS) for photovoltaic applications is motivated by the similarities to the promising material Cu(In,Ga)Se2 (CIGS) while being comprised of non-toxic and earth abundant elements. The structural similarity of a number of competing phases makes identification through standard x-ray diffraction (XRD) challenging. Further, CZTS is thought to contain potentially high levels of point defects, the exact effect of which is unknown. The strong possibility of point defects on the Zn and Cu sublattices may lead to a reduction in solar cell efficiency. Recently there have been reports of a low temperature (200 ± 20 °C, for the selenide) order/disorder transition identified by Raman spectroscopy. This transition is thought to be reordering of the Zn and Cu sublattice but direct evidence of this by Raman is difficult. The tunable energy x-rays available at modern synchrotron sources provide a site and element specific probe to investigate such disorder. We have used resonant XRD techniques to quantitatively determine the level of CuZn, ZnCu, VCu, and VZn point defects present in thin films of polycrystalline CZTSe in order to shed light on this low temperature transition. By annealing just below and quenching from just above the transition temperature we have targeted both ordered and highly disordered films in the Cu/Zn sublattice. We aim to understand and characterize the structural differences and defect levels of films annealed under different conditions.

Resume : Abstract Sb2S3 and Sb2S3:Snx (x=1, 2, 3 at. Sn %) thin films have been deposited by single source vacuum thermal evaporation onto glass substrates. The substrate temperature Ts and the annealing temperature Tr vary between 30 °C and 350 °C. The films have amorphous structure, but after annealing at temperatures above 250 °C for 1 hour, they change to polycrystalline structure. The structure and surface morphology of the films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical absorption coefficient becomes higher for subgap absorption at higher annealing temperatures. The optical investigation showed that, films have direct allowed transitions in the range 1.72-2.2 eV depending on the annealing temperature. The dark electrical resistivity shows a decrease by about an order of magnitude when increasing the isothermal annealing treatment. Doping by tin makes the P-type layers and the electrical resistivity decreases from 2.6 107 to 6 105 Ω cm by increasing the percentage of doping.

Resume : Among the ongoing investigated absorber materials, SnS has attracted interest as a photovoltaic absorber material due to its simpler composition and potential for large-scale, cost-effective and environmentally friendly power generation. It possesses favorable optical properties including an energy band gap of 1.3 eV and a high optical absorption coefficient greater than 104 cm-1. Moreover, it also exhibits tunable carrier concentrations in the range of 1015 to 1018 cm−3 and hole mobilities as high as 100 cm2V-1s-1 or higher. Additionally, a single-junction SnS-based solar cell shows a high spectroscopic limited maximum efficiency (SLME) of 32%. Nevertheless, the best conversion efficiency of SnS solar cell has been recorded 4.36% by ALD deposition technique until now. In addition, it is difficult to obtain pure SnS phase due to the various oxidation state of Sn (0, +2 and +4), which makes other phases of Sn-S (Sn, SnS2 and Sn2S3) along with SnS. In our study, to overcome the weakness, solution and liquid precursors were used to fabricate pure SnS thin films on Mo-coated soda lime glass using direct solution coating method and subsequent annealing at different reaction temperatures. Direct solution coating is easy, cost-effective and rapid process for large scale fabrications, comparing to ALD. Also, it is very easy to control the exact stoichiometry. The compositional, morphological, and structural characterization of the films prepared by both the precursors will be presented.

Resume : Recently, Cu-Sb-S phase has been studied as an absorber in solar cell because of its low toxicity, low cost and an abundant of raw materials. In the Cu-Sb-S phases, there are four major phases (CuSbS2, Cu3SbS3, Cu3SbS4, Cu12Sb4S13) and most of research have reported only about synthesis and characteristics of nanocrystals. However, in order to be used as an absorber, the synthesis of thin films with these materials is important. In this study, thin films of Cu-Sb-S phases were formed using non-vacuum process which is cost-effective. To obtain single phase thin films of each material, different Cu/Sb ratios of hybrid ink was firstly considered and different sulfurization conditions were also required, such as sulfurization temperature, reaction time, and reaction pressure. The fabricating thin films were analyzed and confirmed as target phases. The characteristics of each Cu-Sb-S absorber and photovoltaic performances of fabricated cells will be presented.

Resume : Tin mono-sulfide (SnS) thin film has been investigated as a photovoltaic absorber material due to not only a high optical absorption coefficient but also non-toxicity and earth abundance. However, the highest photovoltaic conversion efficiency is as low as 4.4%. One of the reasons for the low efficiency is considered to be the poor quality of the SnS layer, such as the formation of secondary phases and a high grain boundary density. Thus, many research groups have dedicated their own effort to obtaining a high quality of SnS thin film. Nano-particle based approach can be one of the suitable fabrication methods of SnS thin films because the thin film can be prepared by a simple coating process using the nano-particle based ink, and the subsequent thermal annealing. The thermal annealing can transform the nano-particle precursors into the thin film and promote the grain growth. Moreover, it can affect the micro-crystalline structure of the thin film. Thus, we can control the physical properties of the SnS thin films by the modification of the thermal annealing condition. In this study, we focused on the effect of the thermal annealing condition on the properties of the SnS thin films. We have found that the crystallinity and band gap (Eg) of thin film are highly dependent on the thermal annealing temperature. The partial pressure of hydrogen sulfide (H2S) gas during the thermal annealing affects the valence of Sn atom and the composition of the thin film. Finally, we demonstrate a SnS thin film solar cell and characterize its photovoltaic performance.

Resume : A promising concept to surpass some of the inherent limitations of single junction solar cells are tandem approaches stacking high- and low-bandgap cells, thus reducing thermalization losses and increasing the efficiency potential. The emergence of perovskites as high efficiency, high bandgap solar cells opened up the possibility for tandem devices with a Cu(In,Ga)Se2 (CIGS) bottom cell. In order to reach the current matching conditions for a Perovskite/CIGS monolithic tandem, a low bandgap CIGS or even CIS is necessary. Unfortunately, even the best pure CIS cells published so far suffer from comparatively low VOC and FF, rendering the gain in current ineffective for tandem solar cells. One possible reason for the observed reduced device quality is the misalignment of the conduction band between buffer and absorber layer. In this work, two possible approaches to circumvent this shortcoming are reported: i) downward shifting of the conduction band in the buffer layer ; ii) upward shifting of the conduction band in the absorber layer by applying a Ga front grading at the surface. We will show how the electrical properties improve due to composition gradings and the influence of buffer adaptation in low Gallium CIGS on single cell parameters. The resulting cells are implemented into CIGS/perovskite tandem solar cells.

Resume : In this work, Cu0.5Ag0.5InSe2 (CAIS) thin film structure was deposited by thermal evaporation technique on n-Si wafer substrate in order to characterize p-CAIS/n-Si heterojunction diode. This quaternary compound is an electronic chemical analogue of I-III-VI2 ternary semiconductor compounds, CuInSe2 (CIS) and AgInSe2 (AIS). It has been a subject of interest since quaternary alloys can provide opportunity to a large degree of variation of their properties as a function of change in the elemental composition, and as a result of this, it provides to adjust the properties of these materials. The thin film charactersitics of CAIS thin film deposited on glass substrates were investigated by EDS, XRD, transmission, dark resistivity and Hall Effect measurements. Then, in order to determine the device parameters of In/p-CAIS/n-Si/Ag heterojunction, I-V characteristics of the diode at the temperature range of 220-360 K was measured. This p-n junction showed a good diode behavior with about 2 order rectification factor. The series and shunt resistance values were calculated from the parasitic resistance relation in the forward and reverse bias, respectively. In addition, the ideality factor n and the barrier height values of the diode were determined by performing different I-V plots according to Schottky diode equation. Calculated n values were higher than unity and it indicated that there can be other transport mechanisms different from the pure thermionic emission mechanism. Thus, the main current transport mechanism of this heterojunction was found tunneling-like transport of the dark diode current. As observed in the forward transport behavior, the leakage current characteristics of the junction was found to be approximated by the tunneling mechanism. The temperature dependence at reverse bias indicated that they were in between 10^1 to 10^2 number of tunneling steps and about 10^5 traps act role in the reverse current mechanism across the barrier. In addition, C-V measurements were carried out at room temperature under dark condition. The barrier height of the device was found around 0.70 eV and the number of interface states was estimated as in the order of 4.8x10^10 cm-2V-1. From these results, the energy band diagram of the p-CAIS/n-Si heterojunction was constructed by Anderson’s model.

Resume : In this work, ZnInSe2 (ZIS) polycrystalline thin films were prepared by sequential evaporation of its elemental sources on ultrasonically cleaned soda lime glass substrates. Their compositional, structural and surface characteristics were checked by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. Moreover, post-thermal annealing were applied under nitrogen atmosphere to see the effect of annealing and to provide the thermal energy to the constituent elements to complete thermal kinetics for the nucleation stage in the thin film structure. Under this aim, the crystallinity of the films improved as annealing temperature with increasing up to 500°C by 100°C step. With the insight into the structural information, ZIS films were found in n-type semiconductor behavior with tetragonal crystal structure oriented along (112) direction. The optical properties and constants for deposited ZIS thin films as well as the effect of heat treatments were estimated from their transmission spectra in the wavelength interval of 200-2000 nm. The refractive indices were calculated by using envelope method (EM) and also modeled by Cauchy fitting processes to get a continuity in the whole wavelength range. The absorption coefficients in the visible and near-infrared region (NIR) were computed from the transmission values. Then, the extinction coefficient calculations were done and also by using the results of the refractive index calculations, the real and imaginary part of the dielectric constant were found. Analysis of the absorption coefficient data showed the existence of allowed direct transition and corresponding band gap values were calculated from their Tauc plots. These values were found to decrease with increasing annealing temperature. The refractive index dispersion over the measured wavelength range was investigated with single-oscillator model (SOM) and the related parameters were obtained.

Resume : We report the changes induced by alpha particles irradiation on the physical properties of zinc selenide layers (ZnSe) deposited by rf-magnetron sputtering onto optical glass substrates covered with indium tin oxide (ITO) thin films. Deposition time was varied in the range 2 – 10 minutes and the other working parameters, power, pressure and substrate temperature, were maintained constant during the procedure. ZnSe/ITO interface was irradiated using Cockcroft-Walton Tandreton accelerator with 1.5 MeV energies and 1012 particles/cm2 fluencies. Induced changes by irradiation on the optical properties were analyzed using a UV-VIS Lambda 35 spectrometer and by spectroscopic ellipsometry. Bandgap energy of ZnSe thin films slightly decreased for both as grown and irradiated samples with decrease of deposition time. Optical transmittance was higher than 70% and no important changes were observed after irradiation procedure.Structural and morphological features were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), for as grown and irradiated fabricated thin films. To completely understand the changes induced by alpha particles irradiation Monte-Carlo numerical simulations were also discussed. Keywords: ZnSe, alpha particles, irradiation Acknowledgements: This work was supported by Romanian Executive Unit for Financing Higher Education, Research and Innovation (UEFISCDI) under PN-II-PCCA program, grant no. 288/2014.

Resume : SnSb4S7 powder is one of the most promising compounds used as evaporation source material for the next generation of optoelectronic and thin films photovoltaic devices. SnSb4S7 powder was synthesized by horizontal Bridgman method using high purity tin (Sn), antimony (Sb) and sulfur (S) elements. Structural, chemical and morphological properties of the obtained powder were investigated by X-ray diffraction (XRD), Raman spectroscopy, energy dispersive X-ray (EDX) and transmission electron microscopy (TEM), respectively. The XRD results demonstrated that the powder was polycrystalline in nature and crystallizes in monoclinic structure with a preferred orientation along ( 1 ) plane. This analyses also reveal that the lattice parameters of the powder were a = 11.51 Å, b = 4.17 Å, c = 13.96 Å and β = 105.41°. The formation of SnSb4S7 powder is also confirmed by Raman spectroscopy. This analysis shows that the Raman modes for the SnSb4S7 powder were observed at 117, 140, 155, 187, 250, 303, 371 and 450 cm-1. TEM/EDX was also used to determine the chemical composition of the SnSb4S7 powder. The EDX analysis of various grains of the powder shows that the compositions vary from one grain to another. Transmission Electron Microscopy (TEM) and high-resolution transmission electron microscopy (HREM) were employed to establish the crystalline nature of the SnSb4S7 powder. The TEM observations demonstrated that the SnSb4S7 powder polycrystalline in nature with rod shaped structure.

Resume : In this work, we investigated the physico-chemical properties of Indium-free Sn-Sb-S materials as promising candidates for thin films solar cells. SnSb4S7, SnSb2S4, Sn4Sb6S13, Sn2Sb2S5 and Sn3Sb2S6 thin films were deposited by single source vacuum thermal evaporation onto unheated glass substrates. The as-deposited samples were annealed at 300 °C under nitrogen during 2 hours. The structural properties of annealed films were studied using X-ray diffraction (XRD), Raman and FTIR spectroscopies. The surface morphology was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The optical properties were carried out using UV-Vis spectroscopy and the electrical measurements were focalised on the impedance spectroscopy. XRD results revealed that the nitrogen annealing improves considerably the crystallinity of these films, which is confirmed by the Raman measurement. The surface images show that the roughness increases when the Sn content increases. The optical properties of the films were obtained from the analysis of the transmittance and reflectance data. All films exhibit a high absorption coefficient (104-105 cm-1) in the visible region which is a good advantage for solar cells applications. The band gap energy decreases from 1.71 to 1.39 eV by increasing Sn content. We exploited the models of Wemple-DiDomenico and Spitzer-Fan for the analysis of the dispersion of the refractive index and the determination of the optical constants of the films.

Resume : Orthorhombic SnS thin films doped by silver with various percentages were deposited onto glass substrate by Chemical Spray Pyrolysis technique. Structural, morphological, composition, optical, and electrical properties were studied using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Energy Dispersive Analysis of X-Ray (EDAX) attached to SEM, spectrophotometry, and four points probe technique, respectively. XRD shows that the crystallinity of films enhanced by the increase of silver dopant with (111) as preferential peak for all films. EDAX revealed the presence of sulphur, tin and silver in all films with good stoichiometry and an increase of silver proportional to silver doping. All films had almost same surface morphology. Atomic force microscopy confirms the results of scanning electron microscopy in the sense that Ag-doping had no effect on the topography of films. The resistivity decreases with the percentage of silver doping reaching the lowest value (108 Ω·cm) for films sprayed with solutions containing 10% of Ag. Band gap energy measurement of 1.45 eV was obtained by film doped with silver at 5%.

Resume : In this study, p-n junction and the thin films of SnSe and ZnO used were investigated. The properties of the deposited films and the p-n junction were studied by means of X-ray diffraction (XRD), UV-VIS spectrophotometry, Profilometry, Hall Effect and I-V Solar Simulator. SnSe material obtained by ultrasonic spray pyrolysis, has a forbidden gap of 1.1 eV and p-type conductivity, which is adequate to be used as an absorber in a p-n junction. ZnO obtained by sputtering, has n-type conductivity and an energy gap of 3.3 eV who made the material a candidate to use it as a window layer. Stainless Steel 430 (Steel) was used as substrate and back contact, obtaining the following configuration Steel/SnSe/ZnO/ZnO:Al, where ZnO:Al was deposited by sputtering and has been used as transparent conductor oxide (TCO). A theoretical and a physical p-n junction were form, the theoretical junction obtained by Solar Cell Capacitance Simulator (SCAPS) fed by the measured parameters shows a Voc= 0.9 V and a Isc= 22 mA/cm2 regardless serial and shunt resistance. The theoretical results show the ability to improve the physical p-n junction (working on improving results) so as the principal parameters of the solar cell. This solar cell do not contain toxic materials and all the constituents are earth abundant elements which is now, one of the principal reasons to study, improve and fabricate thin film solar cells.

Resume : In this paper, Schottky diodes (Al/p- Cu3SbS3/Mo) were fabricated by simple deposition of pure Aluminum on the front side of the Cu3SbS3 thin films. The current-voltage (I–V) characteristics show that the Aluminum makes Schottky contact with p- Cu3SbS3. Ideality factors and serial resistances were extracted from the experimental data. The capacitance-voltage (C-V) characteristics reveal the values of the barrier height of the heterostructures. Impedance spectroscopy technique was used to evaluate the conduction processes of the samples as a function of temperature. We found a decrease with temperature of the entire serial resistances Rs, the parallel resistances Rp and the capacitances Cp. In effort to modeling these results equivalent electrical circuits were used. From the Arrhenius diagrams, we estimated activation energies which represent the energy difference between the trap level and the valence band.

Resume : We fabricate inorganic planar solar cells of the pin type with a Sb2S3 absorber using thermal evaporation, and compare the results to solution-processed cells. The efficiency of the evaporated Sb2S3 solar cell is more than double the efficiency of the solution deposited Sb2S3 solar cell. The evaporated Sb2S3 film is sulphur-rich, which results in perfect flattening of the film surface during the annealing step. Having a flat Sb2S3 surface allows deposition of a thin and uniform hole transport layer. In contrast, the solution deposited Sb2S3 is sulphur deficient and does not flatten during the annealing step. The internal quantum efficiency is also higher for the evaporated Sb2S3 solar cells, suggesting longer diffusion lengths. We attribute this to differences in the preferred crystal plane orientation for the evaporated and solution deposited Sb2S3 films and the 30% larger grain sizes in the evaporated films. The difference in the preferred crystal plane orientations is consistent with studies of Sb2Se3 solar cells that also showed a correlation between crystal plane orientation and efficiency. For Sb2Se3 the difference in efficiency with different preferred crystal plane orientations is due to more efficient carrier transport and lower recombination along particular crystal planes. Given that Sb2S3 and Sb2Se3 are closely-related materials, it is very likely that our results for Sb2S3 solar cells can be explained by the same mechanism, although confirmation of this will require a detailed theoretical analysis of atomic bonding in Sb2S3. These insights are useful for the progress of solar cells of Sb2S3 and related materials.

Resume : SnS is an attractive absorber material for solar cells due to its high absorption coefficient, non-toxicity and availability of tin and sulfur. Chemical spray pyrolysis is a simple, fast and inexpensive method to deposit thin film. Acidic aqueous solutions containing tin chloride (SnCl2) and thiourea (SC(NH2)2) at molar ratios of 1:1 and 1:8 were pulverized onto glass substrates at 200 °C in air to obtain SnS films. In this study we investigated the influence of post-deposition annealing in nitrogen and vacuum at 450 °C for 60 min on properties of sprayed SnS films. Films were characterized by X-ray diffraction (XRD), Raman and UV-Vis spectroscopies, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). As-deposited films are composed of cubic SnS as the main crystalline phase independent of precursors’ molar ratio in the spray solution, the mean crystallite size is 35 and 17 nm for 1:8 and 1:1 films, respectively. Thermal treatment of 1:8 films in nitrogen results in Sn2S3 as the main phase, SnS is present as the minor phase. Annealing of 1:8 films in vacuum results in orthorhombic SnS phase, films show optical band gap of 1.4 eV. Thermal annealing of 1:1 films in vacuum leads to metallic Sn, whilst annealing in nitrogen results in films composed of a mixture of SnS and SnO2 phases. Formation of SnO2 in an inert atmosphere indicates presence of oxygen containing phases already in the as-grown film. Chemical reactions taking place during thermal treatments as well as optimal deposition and post-deposition treatment conditions for fabrication of SnS single phase films will be discussed.

Resume : The limited amount in nature of Te and In and the toxicity of Cd and Se in CdTe and CuInGaSe2 solar devices could be overcome with alternative thin film absorbers like Cu2ZnSnS4 or SnS. Recently we presented SnS solar cells with 1.5% efficiency, now we report a detailed study of the SnS growth by congruent vacuum evaporation, producing solar cells in superstrate configuration. The SnS layers have been deposited under different substrate temperatures from 150 to 400 °C. Morphology, grain size, and crystalline structure of SnS thin films are strongly influenced by the substrate temperature. Samples grown at low temperature exhibit small flake-like grains and a poor adhesion, whereas those grown at high temperature show a very compact structure with rectangular grains. Moreover, we found that while in the low and high temperature regimes there are two different preferred orientation of crystallites, in the 250 – 270 °C mid region samples exhibit a lower degree of preferential orientation of the grains. We also investigated the effects of different temperatures of the SnS evaporation source on the deposited absorber layer. The evaporation rate has been kept fixed to 0.5 nm/s, despite the source temperatures were varied (600 – 810 °C) by adding source caps to the crucible. It seems that the congruent evaporation of SnS occurs in a narrow range of temperatures. These evidences and their correlation with the electrical performance of SnS solar cells will be presented and discussed.

Resume : SnS is considered a promising candidate for cost-efficient solar cell devices using earth abundant elements. Being formed by only two elements, this material can simplify the processing involved in the preparation of photovoltaic devices. Nevertheless, the record efficiencies achieved for SnS are still low, requiring further efforts to solve main fundamental and technological problems such as the coexistence of multiple SnxSy phases (including SnS2 and Sn2S3). In this work, we synthesize SnS thin films using a sequential process based in the sputtering of Sn layers onto Mo-coated soda lime glass substrates, followed by a reactive annealing under S+Sn atmosphere. We focus on the variation of the annealing conditions (total pressure between 0.1-10 mBar and temperature 300-550 ºC). The layers were characterized using XRF, SEM, XRD and Raman spectroscopy, and solar cells were fabricated and tested using a CdS/i-ZnO/ITO front-end design. Combining XRD and Raman spectroscopy we determine the p-T zone for the preparation of SnS single phase materials. The morphology and composition of the layer is strongly influenced by the reactive annealing conditions. Synthesizing the SnS layer at 10 mBar and 500 ºC, we report devices with rectifying behaviour, but still poor efficiency (<1%). Further challenges of the material system and the involved technology will be discussed.

Resume : Colloidal semiconductor nanocrystals are providing for new opportunities for the design and fabrication of competitively-performing optoelectronic devices but are largely based on materials containing Cd and Pb. Here, we investigate the use of copper indium sulfide (CuInS2) as an alternative nanocrystal material system, having no toxic component and having already demonstrated high conversion efficiencies in the bulk. We synthesized various sizes of emitting copper-poor zinc-treated CuInS2 nanocrystals with long luminescence lifetimes, broad and tunable emission spectra and high exciton binding energies. We found that these nanocrystals couple to each other as the distance between them is reduced. With a ligand exchange procedure, these films became conductive, with modest mobilities, low carrier concentrations and a slow response to illumination. These properties were implied to cause the observed limitation in the thickness of the CuInS2 absorber in heterojunctions with titania, showing a cap in photocurrent. This limitation was overcome by increasing the CuInS2-titania interface through nanostructuring and by reducing the energetic barrier between nanocrystals. This has led to an increase of more than 10 times in photocurrent and power conversion efficiencies of >1%, which is a record for non-annealed solid-state CuInS2 nanocrystal devices.

Resume : During the last decade, PbS colloidal quantum dot (CQD) are attracting tremendous attention due to its excellent optoelectronic properties for photovoltaic applications. Up to now, a maximum power conversion efficiency (PCE) of 10.6% has been reported by surface modification of PbS CQD, resulting in reduction of the deep trap states. Herein, we synthesize PbS CQDs with hot injection method and modify the dots surface by using solid-state ligand exhchange. We utilize short ligands of methylammonium iodide (MAI) for ligand exchange. The results show that the MAI-passivated dots have higher life time and lower charge-transfer resistance compared with the passivated dots by mercaptoperopionic acid (MPA). The photovoltaic measurements of the devices demonstrate that MIA-passivated PbS CQDs solar cell has a slightly higher performance than its counterpart. Besides, we fabricate three-dimensional plastic nanostructure using inverted-nanocone structure of Anodized-Aluminum oxide. This layer has attached to the FTO glass in order to increase the absorption. In fact, the nanostructure thin film on PbS CQDs solar cell works as not only anti-reflection but also self-cleaning layer. The results of UV-visible and external quantum efficiency tests confirm the effect of this nanostructured layer on device performance. Finally, we achieve a PCE of 6.35% and 7.1% for PbS CQDs solar cell without and with anti-reflection layer, respectively.

Resume : This research focuses on the fabrication of stoichiometric CuInSe2 nanostructures with controllable height, spacing and size of the nanocrystals, suitable for hybrid (organic/inorganic) photovoltaics. The CuInSe2 nanostructures were prepared by the chemical close-spaced vapor transport (CCSVT) method [1] onto Mo/barrier/glass substrates by using In2Se3 source material (volatilized by HCl at 550C) and three different types of Cu precursors: (i) Cu thin films (6-250 nm thick), (ii) Cu nanoparticles deposited by spray pyrolysis (at substrate temperatures of 220-500C), and (iii) Cu nanoparticles formed by applying shadow nanosphere lithography (using a monolayer of 450 nm nanospheres). The CCSVT process parameters were varied to reveal the influence of the precursors on the formation of the CuInSe2 nanostructures. A summary of structural characterization by x-ray diffraction in both grazing incidence and theta-2theta configuration confirmed the presence of CuInSe2 chalcopyrite phase independent on the applied precursor approach. The elemental composition of the as-prepared CuInSe2 nanostructures was analyzed by laser ablation-inductively coupled plasma mass-spectrometry. An excess of Se compared to stoichiometric composition was detected and was attributed to the formation of molybdenum selenide and indium selenide phases. However, the formation of the latter secondary phases was suppressed by tuning of the deposition parameters. 1. M. Rusu et al. Thin Solid Films 451–452 (2004) 556

Resume : Presently, the earth-abundant and non-toxic SnS2 and SnS compounds could be considered as the promising optoelectronic material. This is due the fact that SnS2 has n-type conductivity, high carrier mobility and wide band gap of 2.2 eV. These properties make it possible to use SnS2 as a window layer in solar cells. Whereas SnS has p-type conductivity, high absorption coefficient and band gap of about 1.3 eV is suitable material for absorber layer in thin film solar cells. Also opposite conductivity of SnS and SnS2 gives an opportunity to create n - SnS2/p - SnS heterojunction. SnS2 films were obtained by the close-spaced vacuum sublimation method. SEM images of the surface and cross-section of the obtained samples shows that films consist of plate-like crystallites with average grain size of 2 µm. Thickness of the films was 4 µm. The annealing of the samples was carried out at 300, 400 and 500 0C for 30, 60 and 90 min for each of temperatures in vacuum. In order to study phase composition of the SnxSy films XRD and Raman spectroscopy were used. It was determined that annealing in vacuum of SnS2 films at 500 0C for 90 min leads to the formation of single phase SnS. While, the smaller time and temperature of annealing leads to the mixed phase composition of SnS, Sn2S3 and SnS2. Annealing at 500 C for 90 min lead to the porous structure of the material. EDS analysis shows that the non-annealed films has typical for SnS2 value of Sn:S ratio of 0.49 . Annealing at higher temperature of 500 0C for 90 min shows that films have Sn:S ratio of 0.96. That is close to stoichiometric composition of SnS compound. These results confirm the XRD and Raman data indicating that annealing at 500 0C for 90 min lead to phase transition from hexagonal SnS2 to orthorhombic SnS.

Resume : This paper will discuss our research approaches for printable CZTS/CIGS thin film solar cell, which will include CZTS/CIGS nanoparticle syntheses using thermal injection method; forming high quality CZTS/CIGS layer by solution based method and annealing; Using MoNa electrode or Na ion solution process to further improve the crystalline of the CZTS/CIGS layer ; forming printable Mo back contact using Mo ink; and using solution method to form AZO nanorod sandwich layer to improve light absorption. We have observed significant improvement on the efficiency of CZTS/CIGS solar cell by optimizing MoNa back contact, using Na ion solution, by special ambient annealing process and using AZO nanorod sandwich layer as the window layer. Detail will be addressed at the presentation.

Resume : The use of a High Resistant Transparent (HRT) layer has been shown to increase the efficiency of thin film CdTe heterostructure solar cells. However, the precise mechanisms involved are not fully understood. This study aims to examine the physical properties of ZnO films with their effectiveness as a HRT layer for CdTe devices. The microstructure of ZnO was manipulated by variation of the deposition parameters. The ZnO layers were deposited by RF magnetron sputtering using different deposition working gas pressures and substrate temperatures. The pressure during deposition was varied between 1mTorr (1.33 x 10^-1 Pa) and 9mTorr (1.2Pa). The substrate temperature was varied from 20°C to 400°C. UV-VIS spectrophotometry, Hall Effect measurements and X-Ray Diffraction (XRD) spectroscopy were used to characterize the ZnO films. The deposition pressure influenced the transparency of ZnO films while temperature did not. The resistivity of ZnO films increased steadily with increasing pressure. No trend was observed by increasing temperature. Resistivity was minimised at 100°C and was maximised at 300°C. All films displayed a strong (002) XRD peak indicating a strong c-axis orientation. CdS/CdTe solar cells were fabricated on ZnO coated TEC 10 glass. Transmission Electron Microscopy (TEM) was used to investigate the cross-sectional microstructure of completed devices and J-V measurements were obtained to relate ZnO HRT microstructure to cell performance.

Resume : The polycrystalline CdTe/CdS thin-film solar cell is one of the most important photovoltaic devices for cost-effective generation of solar electricity on terrestrial applications. The typical superstrate structure of a CdTe/CdS solar cell is composed of four layers: one of them, a transparent and conducting oxide (FTO) which acts as front contact, the window and absorbent layers CdS and CdTe films were used respectively, and finally Te/Cu:Au was deposited as back contact. CdS and CdTe thin films have been prepared by chemical bath deposition and by close space vapour transport (CSVT) technique respectively. An efficiency of 12% has been reached on laboratory scale. Higher short-circuit current densities (Jsc) can be achieved by reducing the CdS thickness (60nm) and using an appropriated thermal annealing process. Elemental composition was analyzed by using SIMS (Secondary Ion Mass Spectrometry) system along the solar cell volume focusing on Cd, Te, S, Cu, Au elements distribution and their relation with the final photovoltaic efficiency.

Resume : CdS/CdTe heterojunctions are used on photovoltaic devices. Substantial evidence exists that interdiffusion between CdTe and CdS layers occurs during CdCl2 treatment and post annealing processes. A sulphur diffusion into the CdTe layer leads to the formation of a ternary CdTe1-xSx phase within the CdTe layer. In this work was studied some electrical properties by using a simple model known as matching method considering a ternary compound in the interface. Comparing theoretical results with I-V experimental measurements on CdS/CdTe solar cells good results were obtained, due to using a x= 0.75 concentration were obtained a match with the theoretical and experimental values.

Resume : CdTe solar cells have recently reached the highest conversion efficiencies, comparable to CuInGaSe2 and Si devices. Moreover they still are a step ahead in terms of industrial scalability and reproducibility. One of the key factors for this success is the so-called “activation” treatment, which typically consists in depositing a CdCl2 film on the CdTe absorber layer and in a subsequent annealing in air or nitrogen atmosphere. Nevertheless CdCl2 is a carcinogenic and water-soluble compound; for this reason recent studies have found MgCl2 as a good alternative. In this work we have studied devices (exceeding 13% efficiency) prepared by low-substrate temperature CdTe deposition and activated with MgCl2 treatment (applied by wet deposition). The devices were characterized by of capacitance-voltage, drive level capacitance profiling and admittance spectroscopy techniques. Carrier concentration and also concentration and distribution of shallow and deep defects are presented and compared with data taken from analogous devices but made with the standard CdCl2 activation treatment. Despite similar structural properties of the CdTe layers treated with the two different processes (analyzed by X-ray diffraction and atomic force microscopy), the distribution of deep and shallow defects resulted to be different in the two cases and admittance spectroscopy technique reveals the presence of different defects.

Resume : CdTe solar cells have demonstrated very high efficiency with latest results from First Solar that have shown a record efficiency of 21.5%. One of the crucial points for high efficiency is the insertion of copper that dopes CdTe and delivers a good ohmic contact, however it contributes to cell degradation. For this reason other degradation effects are generally neglected. However in a recent work we have shown that the performance degradation of the solar cells (measured at different time steps, having stored the devices under one sun at 80°C) is also depending on the TCO. In this work CdTe solar cells were prepared with different transparent conductive oxides (TCO), namely indium tin oxide (ITO), fluorine doped tin oxide (FTO) and a bi-layer ITO/ZnO. The finished devices perform quite similar conversion efficiencies and exhibit similar morphology (analyzed by atomic force microscopy) and structural properties (studied by x-ray diffraction). We confirm that aged cells show that stability is connected with the different TCOs: ITO based devices show a limited stability compared to the other ones. Analysis of capacitance-voltage and drive level capacitance profiling shows a different defects distribution for solar cells with less stable front contact. Moreover here we reveal by admittance spectroscopy (performed at different temperatures) the presence of impurities coming from the glass and from the front contact degrading the cells conversion efficiency.

Resume : Traditional CdCl2 thermal treatment of CdTe/CdS heterostructure results in grain growth, sintering and incorporation of Cl into CdTe lattice, substantially improving performance of the cells. The process is complicated by high vapor pressure of CdCl2 and presence of oxygen. Considering volatility of CdCl2 and presence of residual oxidation products we provide systematic comparative study of CdCl2 treatment in air, in evacuated quartz ampoules and in a process tube under nitrogen normal pressure for CSS CdTe films (thickness 2-3 µm) deposited at 200 and 500 oC onto roughned glass. The films treated in air at 420 oC have high resistivity (106 Ω×cm) , high dark to light resistance ratio (25≤KF≥13 and 50≤KF≥35 for 200 and 500oC respectively) and intense photoluminescence band at 1.5 eV. Treatment in ampoules and in the process tube at 600 oC decreases the resistivity up to 4 orders of magnitude and dark to light resistance ratio (1.3≤KF≥1.2) of the films. P-type resistivity around 102 Ω×cm and dark to light resistance ratio KF ~1.02 of CdTe films was achived by subsequent vacuum annealing at 470 oC. Hall effect measurements indicate a p-type conductivity with carrier density of 1015 cm-3 and hole mobility about 1 cm2 V−1 s−1. The low value of mobility suggests the need for optimization of CdCl2 concentration for stronger sintering which implyies dcreased density of grain boundaries. By controlling the thermal annealing conditions, substantial improvement of CdTe opto-electronic properties has been achieved.

Resume : In this study, the p-i-n structure of homojunction CdTe solar cell was fabricated by electrodeposition. The kinetic model based on the Butler-Volmer equation was developed and used to obtain the potential of perfect stoichiometric (PPS) under a variety of deposited conditions. Kinetically model and numerical simulation of CdTe thin film deposition process were confirmed experimentally. Owing to the critical quality of the CdTe layer of i-region is required, which leads to high efficiency photovoltaic cell. Therefore , the electrodeposition process of i-layer of CdTe cell is improved to act as the best the dominant strategies to achieve the ideal efficiency of CdTe solar cell. By precise potential-control during electrodeposition process, the intrinsic CdTe thin film with near-stoichiometric, n-type CdTe film with Cd-rich and p-type CdTe film with Te-rich non-stoichiometric films were obtained, respectively. The defect engineering process was used to improve the quality CdTe thin film for photovoltaic cell. Theoretical approach was used to improve the performance of solar cell by varying the electrodeposition parameters, Hence, the high efficiency p-i-n CdTe photovoltaic cell can then be achieved. CdTe with special property is very well suited for use as active material in thin-film solar cells s [1]. [1] R.D. Engelken and T.P. Van Doren, J. Electro -chem. Soc., I32 (1985) 2910.

Resume : Close spaced sublimation (CSS) is one of the leading technologies for the deposition of absorber layer for CdTe solar cells. For the large scale production CSS means a dynamic process where the substrates are moved above the crucible at a close distance. CdTe layers of 3-4 µm can be grown in less than one minute. In this study oxygen as a process gas is injected during dynamic CSS deposition. Solar cells are prepared from these layers by chlorine activation, copper doping and back contact deposition. The samples are investigated using current voltage measurement, quantum efficiency (EQE), scanning electron microscopy (SEM) of cross sections, deep level transient spectroscopy (DLTS) and photo reflectance (PR). The results show that the precise introduction of oxygen can improve the solar cell efficiency. The reasons of this effect are discussed in this study.

Resume : We report on the electrical properties of CdTe-based photocells grown by MBE method on the semi-insulating (100) GaAs substrate. The diodes with two different absorbers: CdTe and CdMgTe were analyzed. The dark current–voltage (I–V) characteristics measured in the temperature range of 77–320K reveal the dominant carrier transport mechanisms through the junctions. The studied diodes exhibit strong rectifying properties, however the obtained ideality factor value is greater than unity suggesting that the diodes are not ideal ones. The double logarithmic forward bias dark I–V plots indicate that the charge transport mechanism in a wide voltage range is governed by the space charge limited current, moreover, in the case of the diodes with CdMgTe absorber - with the participation of deep traps which are exponentially distributed in the band gap. From the temperature dependence of saturation current the activation energies of the traps present in the studied junctions were calculated and their possible origin has been ascribed. The light I–V characteristics were measured under 1-sun illumination. The diodes exhibit a significant photosensitivity for different light intensities proving that they can be regarded as good candidates for photodiode applications. From the analysis of double logarithmic plot of short-circuit current vs. applied illumination it was found that the trapping centers govern the current flow through the junction what is in agreement with dark I-V analysis.

Resume : Our CZTSSe technology, based upon the instantaneous precipitation of CZTS nanoparticles, the spray coating of a water-ethanol colloid and the subsequent annealing of the films in a chemically controlled environment, allowed us recently to reach an efficiency of 10.8%. Among the various possible levers we have tuned, it appeared that controlling the Sn content was essential to reduce the number of deep defects, and thereby achieve this level of performances. Further light-management optimizations yielded an increase in the photocurrent that translated in a maximum of 11.5% power conversion efficiency under AM1.5G. In spite of this encouraging value close to the best ones reached with vacuum processes or liquid processes based upon less environment friendly solvents, the Voc deficit at work in these devices still appears prohibitive. We have performed detailed investigations in order to identify the potential sources of this major loss. Our study reveals that i) the Cu/Zn order/disorder is not the main culprit of the Voc deficit, ii) in spite of the latter observation, high ordered CZTSSe is preferred because of much lower non-radiative recombination, iii) Quasi-Fermi-Level-Splitting measurement suggest that the majority of the Voc losses occur in the bulk of the photoactive materials and not at an interface. All these new results obtained on efficient devices will be discussed together with the strength, impact, and potential origin of Urbach tails.

Resume : Cu2ZnSnS4 (CZTS) compound with a bandgap of 1.5 eV is a promising candidate for low-cost, high-efficiency thin film as well as tandem solar cells. These earth abundant elements make this compound a feasible option for photovoltaic (PV) thin film technologies scalable to TW/year. The key limitation to high efficiency PV conversion for this compound remains optimization of the growth conditions. In this paper we present the possibility to increase PV efficiency in Mo/CZTS/CdS/ZnO solar cells fabricated by two step annealing process starting from co-sputtering precursor performing (i) sequential etching of CZTS using KCN, KMnO4/H2SO4 + Na2S, HCl solutions [1-2] allowing to adjust cationic ratio at the absorber surface, (ii) sodium incorporation using NaCl and NaF powders during annealing allowing to increase absorber grain size, (iii) post-annealing treatment of completed device ranging from 200 to 250 °C which improved CZTS/CdS interface. A comparative study of Cu2ZnSnS4 and Cu2ZnSnSe4 absorbers was later performed. Overall efficiency of more than 8% has been obtained for Cu2ZnSnSe4 based solar cells. Also an open circuit voltage of 510 mV was extracted after post-annealing treatment due to an increase of carrier concentration. [1] A. Fairbrother et al., J. Am. Chem. Soc. 2012, 134, 8018 [2] S. López-Marino et al., Chem. Eur. J. 2013, 19, 14814

Resume : While substantial research on the Cu2ZnSn(SxSe1-x)4 [CZTSSe] materials system is focused on either the pure selenide (x=0) or sulfide (x=1), the highest efficiencies are obtained for alloys containing both sulfur and selenium. One of the potential advantages of such alloys is the possibility to obtain a band gap grading. This work presents initial experiments aiming for this goal. A sulfur containing Cu-Zn-Sn-S precursor was deposited by reactive sputtering from binary targets. This precursor was then annealed in a graphite box containing selenium. Various process parameters were investigated to optimize the process including background pressure, temperature and annealing time. It is demonstrated how the precursor gradually selenizes as a function of annealing time, but no S/Se gradient is observed in initial experiments. As part of the optimization air annealing of the bare absorber was investigated. A significant improvement of the device efficiency was observed when increasing the air annealing temperature from 100 C to 300 C. The effect of air annealing on the absorber surface was investigated by XPS. The results indicate that SnO, SeOx and Na compounds form on the surface during air annealing. These are removed in the subsequent etching in either ammonia or KCN. The highest efficiency obtained for an air annealed CZTSSe solar cell in this study was 9.7%. Starting from devices with efficiencies in this range, we will aim to introduce an S/Se grading.

Resume : Recent work has aimed to determine the origin of performance limitations in kesterite solar cells, namely through current-voltage (JV) and quantum efficiency (QE) analysis [1,2]. In this work, detailed voltage-, temperature-, and intensity-dependent QE analysis is applied and further developed to understand recombination and diffusion limitations for kesterites; devices with a variety of processing conditions, compositions, and efficiencies are considered. Analysis on related materials, namely CIGSe with >17% efficiency, is also performed for comparison of the analysis method and results in understanding the performance gap relative to kesterites. First, voltage-dependent QE is used to distinguish between collection/voltage losses due to diffusion and that of photocarrier recombination; this analysis affords relevant diffusion, absorption, and recombination parameters. Next, light-bias QE and intensity-dependent JV is used to characterize non-idealities of the injection current/collection efficiency and their impact on data analysis. Lastly, temperature-dependent QE is considered to understand the origin of photocarrier recombination as well as to investigate photoconductivity and the temperature-dependence of diffusion and absorption parameters. While CZTSSe and CIGSe can exhibit similar diffusion limitations, CZTSSe is characterized with detrimental photocarrier recombination in forward bias.[1] Hages et al., JAP, 115, 23504 (2014); [2] Hages et al., JAP, 119, 014505 (2016)

Resume : It is known that charge carrier depletion width at the hetero-junction interface is one of the key parameters that affect the performance of thin film solar cells. The relative dielectric constant of kesterite Cu2ZnSn(Sx,Se1-x)4 varies from 6.8 to 8.6 as x changes from 1 to 0 according to theoretical calculation and experimental results. Based on this result, the carrier density of CZTSSe absorber is generally in the range of 5×1015 ~ 1×1016 cm−3, and the depletion region width (Wd) is typically in the range of 150~220 nm. Generally speaking, a narrow depletion region is not only adverse to the photo-generated charge carrier collection in thin film solar cell, especially for the charge generated at the long wavelength region close to band edge, but also may lead to increased interface recombination and tunnel recombination, which result in significant open circuit voltage (VOC) loss. Hence, increase of the depletion region width of CZTSSe solar cells is crucial to push the performance of CZTSSe solar cells to a higher level. In this study, high performance Cu2ZnSn(S,Se)4 (CZTSSe) solar cells are fabricated by selenization of the precursor films of Mo/Sn/Cu/ZnS/Sn/ZnS/Cu deposited by magnetron sputtering. The investigation of the solar cells with different Zn/Sn ratio in CZTSSe film discloses that the charge carrier concentration and depletion region width of the device is very sensitive to Zn/Sn ratio of CZTSSe layer. The CZTSSe film with Zn/Sn=1.05 has lower carrier density (5.0 × 1015 cm−3), which is half of the cell with Zn/Sn=1.12, whereas the depletion region at the CdS/CZTSSe hetero-junction interface of the former (200-250 nm) is 100 nm longer than the latter. As a result, better collection of photo-generated charge carrier is found with the cell with longer Wd in the longer wavelength region above 800 nm. Therefore, the average power conversion efficiency is increased from 6.53% to 9.16% with enlarged depletion region width, and the best performance with 10.2% efficiency is achieved.

Resume : Efficient Kesterite devices require absorber layers free from detrimental secondary phases. The highest efficiency devices are grown in the two phase Kesterite + Zn(S,Se) region. To reduce its current blocking effects, Zn(S,Se) should preferably form on the absorber surface and grow in nanometer sized islands or be removed. Zn(S,Se) formation and distribution is not well understood for absorber layers synthesized from metallic precursors. We show that the size and distribution of the ZnSe phase depends on the size and distribution of the metal alloys that form during the heating of the precursor, before selenization. This conclusion was enabled by a method that we developed to perform all analyses (SEM, EDX, micro-Raman, micro-PL) on a sample in one identical location, therefore tracking all changes occurring during each step of the process. Absorbers were prepared by electrodeposition of Cu/Sn/Zn layers followed by pre-alloying at 350°C and annealing in Se/SnSe atmosphere, allowing us to achieve a best efficiency of 7%, comparable to the best reported device of 8.2 % with a similar process. As-deposited metals form smooth continuous layers, whilst after pre-alloying 5-20 micron CuZn alloy islands are formed surrounded by a Sn rich matrix. After selenization, analysis reveals that Kesterite forms everywhere but ZnSe mostly formed on the Sn rich matrix! In order to reduce the size of the ZnSe areas on the absorber, smaller CuZn islands or longer selenization times are required.

Resume : Post deposition treatment (PDT) of CIGS solar cells with Potassium is known to be beneficial to both open circuit voltage and fill factor and is therefore crucial for obtaining the highest efficiencies. Here we investigate the impact of Potassium incorporation at different stages of the solution processing of kesterite CZTSSe absorbers and its effect on the opto-electronic properties of solar cells. The different incorporation strategies include the addition of potassium to the precursor solution, pre-deposition of a KF layer onto the precursor before the selenization step, and a PDT with KF of the selenized absorber in a separate annealing step. The experiments were conducted with and without additional Sodium doping in order to understand the intrinsic and combined effects of two alkali dopants. The respective solar cells were examined utilizing advanced characterization methods as admittance spectroscopy (AS), time-resolved photoluminescence (TRPL) and external quantum efficiency (EQE) measurements. Potassium incorporation before the selenization step yields an improved grain growth similar to the effects of Sodium, resulting in a higher minority carrier lifetime, improved EQE in the long-wavelength region, and improved short circuit current. Incorporation of Potassium prior to selenization changes the doping concentration as revealed by AS. Interesting results are observed for the PDT with KF, which leads to a blocking of the device resulting in a collapse of the fill factor, the short circuit current and hence the efficiency, whereas the open circuit voltage and the minority carrier lifetime improve on the other hand.

Resume : We investigate exchange of the standard CdS buffer layer in Cu2ZnSnS4 (CZTS) solar cells with Zn1-xSnxOy (ZTO). For sulfide CZTS, the band alignment between CZTS and CdS is not ideal (“cliff-like”), which enhances interface recombination. In this work, we show how a ZTO buffer layer results in improved open circuit voltage values (Voc) for CZTS devices as compared to using a CdS buffer layer. The ZTO is deposited by atomic layer deposition (ALD), with a process previously developed for Cu(In,Ga)Se2 solar cells. By varying the ALD process temperature, the position of the conduction band minimum of the ZTO is varied in relation to that of CZTS. A ZTO process at 95°C is found to give higher Voc and efficiency values as compared to the CdS reference devices. For a ZTO process at 120 °C, the Voc and efficiency values are similar to those of the CdS reference. This is compatible with an expected conduction band alignment similar to that of CdS. The position of the conduction band minimum can be further increased as compared to CZTS by lowering the ZTO deposition temperature to 80 °C. These devices show blocking of forward current and reduced fill factor, which is consistent with barrier formation at the junction. Temperature-dependent current voltage analysis measures the activation energy for recombination to be 1.36 eV for the best ZTO device as compared to 0.98 eV for CdS. Therefore, we argue that the Voc of the best ZTO devices is limited by bulk recombination, which is in agreement with a room temperature photoluminescence peak at around 1.3 eV for both devices, while the CdS devices are limited by interface recombination.

Resume : Kesterite absorbers often undergo a post deposition chemical and/or heat treatment in order to improve the power conversion efficiency of the resulting solar cell devices. Passivation of grain boundaries and changes in the chemical composition in the near surface region and at the grain boundaries are discussed in literature. In order to investigate the origins of the efficiency improvements amplitude modulated Kelvin Probe Force Microscopy (KPFM) is carried out on selenium based absorbers with efficiencies in the range of 6-7%. We measure large variations in the workfunction on the as-grown absorbers (FWHM>100meV). Upward and downward band bending at grain boundaries are observed. The workfunction variations can be divided in two categories: Grain to grain changes, which can be linked to the different crystallographic orientations and small intra-grain variations on the length scale of roughly 100nm. All measurements are carried out on as-grown absorbers, after rinsing with H2O and NH4OH and after several different heat treatments in order to study the evolution of the workfunction after the different processing steps. In order to quantify the impact of the treatments on device performance the KPFM measurements are supplemented with calibrated photoluminescence measurements where the quasi-fermi level splitting and consequently the open-circuit voltage can be extracted.

Resume : Cu2ZnSnS4 (CZTS) nano-particles ink was successfully synthesized by a simple solvothermal method. A low cost non-vacuum method was used to deposit the ink on Mo-coated glass substrates by doctor blade process followed by selenization in a tube furnace to form CZTSSe layer. Different selenization conditions and particle concentrations were considered in order to improve the crystallinity and surface morphology, the annealing temperature was varied between 400 oC to 550 oC and the annealing time was varied between 5 to 20 min in selenium containing nitrogen atmosphere. The influence of selenization conditions variation on the structural and morphological characteristics of the films was investigated to reach the optimum conditions. To figure out the structure of the deposited layers and the presence of secondary phases, X-ray diffraction analysis and Raman spectroscopy were used. The morphology and the homogeneity of the layer were investigated using SEM. The best layers and homogeneous films were achieved using CZTS nano-particles concentration of 150 g/l, temperature range of 475 - 525 oC and time of 10 min.

Resume : Kesterite solar cells (Cu2ZnSn(S,Se)4) are considered to be a sustainable alternative to highly efficient chalcopyrite (CIG(S,Se)2) solar cells albeit the highest efficiency is currently limited to 12.6%. Preparation of kesterite absorber by sputtering of metal precursor with subsequent annealing is a well-established technique to obtain controlled and reproducible compositions with a possibility to introduce metal gradients with respect to bandgap engineering at the interfaces. In contrast to a stacked layer arrangement, where intermixing of the individual elements is highly dependent on the annealing conditions, we employ confocal co-sputtering of elemental metal targets to obtain homogenous precursor. An evaporated Se top layer guarantees uniform absorber formation during rapid thermal annealing. Based on a reproducible baseline process yielding >7% solar cells on 5x5cm2 substrates, a systematic study is performed on how precursor composition influence the absorber morphology and device performance. By reducing the Zn content, the formation of ZnSe is decreased as revealed by EDX, XRF and SEM measurements. An enhanced grain growth is observed using higher temperatures during the annealing step. KPFM measurements are performed on complete devices using a novel contacting approach to highlight the contribution of grain boundaries and compositional inhomogeneities to the Voc.

Resume : Kesterite absorber layers are promising in terms of cost and material abundance, but still lag relatively far behind Cu(In,Ga)Se2 (CIGS) in terms of device efficiency. This raises questions about their potential for large scale production. This work aims at assessing the challenges of scaling up the existing PVD processes of Cu2ZnSnSe4, Cu2ZnSnS4 or Cu2ZnSn(S,Se)4 for industrial fabrication. Through literature review and exchanges within the European project Kestcells, the main issues that can affect kesterite and their causes are listed. These issues are then prioritized in terms of criticality for the process, using the methodology of Design Failure Mode and Effects Analysis (D-FMEA). This analysis indicates, in particular, that bandgap and electrostatic potential fluctuations are the most critical risks to date for the fabrication process, not so much because of their impact on efficiency than because of the inability to detect them or prevent them in the current state of our understanding. Applying the acquired knowledge to the known fabrication routes for kesterite, it is shown that some of the most critical shortcomings can originate from unsuitable metallic ratios in the precursor and the absorber. In this respect, the volatility of Sn in kesterite fabrication processes is an important challenge. In the current state of the technology, this volatility forces the sequential route (i.e. deposition/selenisation) even on co-evaporation processes that would otherwise have benefited from the maturity of the higher throughput 1-step industrial processes currently used for CIGS by companies such as Solibro, Manz or Miasolé for example.

Resume : Copper Zinc Tin Selenide (Cu2ZnSnSe4, or CZTSe) is emerging as an potential absorber material to the present thin film solar cell technologies, alternative to the industrially produced thin film solar cell structures, Cu(In,Ga)Se2 (CIGS) and CdTe. Although CIGS thin film based solar cell applications can reach to the world record efficiency, in the availability on In for highly demand on these type cells, have been directed the market to the cost-effective materials. Therefore, by identifying the earth’s abundant solar energy materials with suitable material characteristics to reduce the cost of solar cell have been attracted to technologically material changes from In/Ga to Zn/Sn [1]. In fact, compared to the traditional thin film based solar cell absorber layers, all the elements in CZTSe are abundant, environmental-friendly, and inexpensive [1, 2]. In addition, CZTSe has suitable band gap energy of 1.4–1.5 eV which is the ideal range for converting the maximum amount of energy from the solar spectrum into electricity. CZTSe has a good light absorption characteristics with high absorption coefficients (>104cm^-1 in the visible region) and only a few micron thick layer of CZTSe can absorb all the photons with energies above its band gap. In addition, solar cells based on these novel structures have already reached power conversion efficiency >10% [3, 4]. However, there are still limited works on CZTSe, and in this study, CuZnSnSe2 quaternary structure has been studied under different post-thermal heating processes to determine the stoichiometric, single phase and poly crystalline structure. Then, these different kinds of CZTS thin film samples have been characterized to provide structural, optical and electrical properties of the samples under the insight of possible solar cell application. To obtain this film structure, elemental Cu, Zn and Sn and Se sources were thermally evaporated on ultrasonically cleaned soda-lime glass substrates in a stacked layer form. During the deposition process, the substrate temperature was kept at about room temperature and the vacuum was controlled at about 10^-6 Torr. Then, different annealing procedures were applied both during and the after the deposition. Firstly, after the deposition process, the films were selenized with Se evaporation at the substrate temperature of 200 °C under the pressure of 10^-6 Torr. Under this ambiance, thin films annealed for one hour (CZTSe-1). After annealing, three sample sets were prepared; one of them were kept in as-grown form, another set were annealed at 300 °C under vacuum condition (CZTSe-2) and the other samples were annealed at 400 °C under Se environment (CZTSe-3). The structural and the morphological evolution were analyzed by means of X-ray Diffraction (XRD), Raman spectroscopy, energy dispersion X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Moreover, the optical characteristics of these samples for selenized just after the deposition, and annealed under vacuum and Se evaporation conditions were determined in detail and it was observed that there has been a remarkable influence of annealing on these quantities. It was determined that these changes affected the band gap values relating with the structural changes with annealing. To complete the material characterization processes, the electrical and photo electrical properties of the films have been investigated by carrying out temperature dependent conductivity, photo conductivity under different illumination intensities in the temperature range 100–400 K, and Hall effect and photo response measurements between 300 and 1200 nm at room temperature. [1] S. R. Kodigala, Thin Film Solar Cells from Earth Abundant Materials, Elseiver, USA, 2014 [2] X. He, H. Shen, W. Wang, B, Zhang, Y. Dai and Y. Lu, J. Mate.r Sci: Mater. Electron. 24 (2013) 572–575 [3] I. V. Bodnar, V. F. Gremenok, W. Schmitz, K. Bente ve T. Doering, Crys. Res. Technol. 39 (2004) 301-307 [4] T.K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, and D.B. Mitzi, Adv. Energy Mater. 134 (2012) 15644–7

Resume : Cu2ZnSnS4 (CZTS) thin films were deposited onto Mo/glass substrates by thermal evaporation and subsequent thermal treatment in Ar+S atmosphere. CZTS powder was synthesized previously by solid state reaction of the elements to be used as precursor material. 1.5 µm-CZTS films were evaporated at a nominal substrate temperature of 250º C for only 12 minutes. Different thermal treatments at 550º C by using different heating rates were performed. CZTS thin films were investigated by grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy, X-ray fluorescence and Glow Discharge Optical Emission Spectroscopy (GDOES) measurements. SnS secondary phase was identified next to the Mo layer by GIXRD and corroborated by GDOES depth profiles. Solar cells were fabricated by deposition of a CdS buffer and ZnO/ITO or i-ZnO/AZO layers. Different chemical etchings were performed, KCN or (NH4)2S just before the CdS deposition. Moreover, Cd-free solar cells were fabricated by using Zn(O,S) buffer layer. I-V and external/internal quantum efficiency measurements were used to characterize the photovoltaic devices. The presence of SnS limited the efficiency up to 3.2 % (active area) and 1.8 % (total area) for Cd- and Cd-free devices respectively. In this presentation, the effect of the different thermal treatments on the CZTS thin film properties and the device performance will be discussed. These results show the potential of this simple and fast absorber thin film growing method.

Resume : Cu2SnS3 (CTS) is an abundant, less expensive, and non-toxic alternative to other chalcogenide compounds such as CdTe and Cu(In,Ga)(S,Se)2 (CIGS) used as absorber layers in thin-film solar cells. Its low bandgap makes it a good candidate for heterojunction structures with Si. The low diffusion length of the carriers due to the presence of spurious phases, holes, defects and small grain size hampers its use. To improve the crystalline quality we have grown CTS by co-sputtering of target of Cu/SnS on soda lime glass /Mo substrates and later annealed with a two-step process at 250°C for 15 min and 550 °C for 60 min in the presence of sulfur. The intermediate annealing temperature decreases holes, defects and spurious phases in the layer. The quality of our samples has been improved by studying the influence of the deposition and sulphurization parameters on its structural characteristics. Once optimized the film of CTS an heterojunction with Si will be fabricated and characterized.

Resume : Solution processing of thin film semiconductors is a key area of research in thin film solar cells. These methods avoid the high costs of vacuum based deposition techniques and have a high throughput, ideal for large scale manufacturing. Currently the record efficiency for CZTS was obtained using solutions of binary chalcogenides in hydrazine; however, more recently a solvent combination of dithiol and diamine has been shown as a viable safer alternative. In this study, a novel approach has been developed to deposit CZTSe absorber layers. Using a new safer thiol source, elemental Cu, Zn, Sn and S/Se can be fully dissolved in solution at ambient conditions, with the composition easily controlled. The preparation and the spray deposition of these solutions are performed in air, allowing a quick, easy and inexpensive process. Upon selenization, large crystals can be observed which are identified as CZTSe using XRD and Raman, the latter showing no distinctive signs of any binary or ternary phases. Using this method, high quality CZTS absorber layers have been prepared for use in thin-film solar cells. The sprayed absorbers have been successfully implemented in a solar cell with promising initial results. Further device optimisation and characterization will result in improved device performance. This novel approach is a simple, safe and inexpensive method which is scalable, and can therefore serve as a safer alternative to the hydrazine based process.

Resume : We have investigated Cu2ZnSnSe4 (CZTSe) two thin films by using spectroscopic ellipsometry (SE), optical transmission, Raman scattering and photoluminescence (PL). Samples were prepared by high temperature co-evaporation. The front surface and the back interface region of the films were analyzed after absorber lift-off from the substrate. We show detailed modeling of SE data which allows extraction of optical parameters from the SE experiments, such as refractive index and extinction coefficient. These were then used to extrapolate the band gap of the absorbers which match the results from optical transmission/reflection measurements and amount to 0.92 and 0.97 eV. The detailed modeling of the SE results further allows for detection of the secondary phases MoSe2 and ZnSe which is confirmed by Raman and PL measurements. PL measurements were done at different excitation wavelengths with the aim of differentiating between the optical transitions of the absorber and of present secondary phases. Surprisingly the position of the main PL peak appears to be at around 0.83 eV for both samples, independently of the extracted band gaps. Furthermore, we discuss a PL peak at 1.2eV which has been assigned as a ZnSe defect transition in literature. We show that the detection of secondary phase ZnSe by observation of this PL peak depends strongly on the excitation wavelength. The results support and complement the findings from SE and Raman for detection of the secondary phase ZnSe.

Resume : The non-toxic, abundant material Cu2ZnSnS4 (CZTS) is promising for thin film solar cells, where solution processing allows fast and inexpensive large-scale fabrication. However, crystal growth during selenium-free annealing is far more pronounced in films prepared from precursor inks compared to those made from nanoparticle (NP) inks. It is not well understood why annealing in sulfur atmosphere is challenging, but it is clear that sintering of CZTS NPs is strongly impeded by remnants, e.g. ligands or other contaminants, on the NPs’ surface. Here, we attempt to design a suitable annealing procedure for NPs with and without organic ligands. We study the effect of the annealing atmosphere on grain growth behavior of CZTS NP-based thin films by thermal analysis. We use thermogravimetric analysis (TGA) and differential thermal analysis (DTA) coupled with mass spectrometry (MS) to simultaneously monitor the mass and heat flow changes of both NP powders and inks. The investigation focuses on annealing in air, nitrogen and forming gas (5% H2 in Ar), i.e. oxidizing, inert, and reducing atmosphere. We observe that most of the grain growth occurs for both ligand-coated and ligand-free samples in reducing atmosphere. We find that the organic ligands thermally decompose in smaller fragments, with the highest decomposition rate in reducing atmosphere. The results from the TGA-DTA-MS measurements are compared with the structural changes of the samples after annealing in a sulfurization oven.

Resume : Several routes to produce the solar cell material Cu2ZnSnS4 exist. Most successful are still the two stage methods, starting with precursor deposition and thereafter annealing to crystallize into Cu2ZnSnS4. We have investigated reactively sputtered, homogeneous, precursors with a composition close to stoichiometric Cu2ZnSnS4, and how a limited variation of sulfur content in the precursor affects the annealed films in terms of crystal quality. A reason to investigate this is that compound sputtering, an alternative sputtering route which has the advantage of possibly omitting the corrosive gas H2S in the process, may yield sulfur poor precursors. In this work, the sulfur content was lowered by reducing the H2S/Ar-ratio in a 3-target reactive sputtering process, while target power was adjusted to compensate for the change in deposition rate that occurs as the targets switch from compound to metallic mode. Comparison is also made to precursors from compound sputtering, using CuS, ZnS and SnS targets. Annealing was done in a graphite box with added elemental sulfur. Characterization was made by scanning electron microscopy, x-ray diffraction and Raman scattering. The annealing time and temperature have a large influence on the film quality. However, we find no strong correlation between precursor sulfur content and grain size after annealing, within the compositional range investigated. This is positive since it allows larger flexibility in precursor deposition by sputtering.

Resume : CZTS nanoparticles were synthesized by solvothermal method. The ink of CZTS nanoparticles dispersed in chloroform was deposited on Mo-coated glass substrates by Doctor blade technique, which is cheap, simple and suitable technique for large scale production. In the present work, Effect of annealing conditions (temperature, duration and the annealing atmosphere) on the structural, morphological and optical properties of CZTS thin films was investigated. X-ray diffraction analysis and Raman scattering measurements were used to confirm the formation of the crystalline CZTS. The morphology and the chemical composition of the CZTS thin films were studied by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. It was found that CZTS films show the diffraction peaks of (112), (220) and (312) with slight appearance of diffraction peaks of (112) and (332) attributed to kesterite CZTS (JCPDS: 26-0575). At elevated annealing temperatures (≈ 550 °C), the CZTS phase decomposes to other secondary phases. Annealing with elemental sulfur enhances the intensity of the diffracted peaks of CZTS but it also may results in the appearance of Cu2S or CuS as a secondary phases. Keywords: Cu2ZnSnS4, Kesterite, Doctor blade, CZTS nanoparticles.

Resume : Over the past years solution-processed absorbers for photovoltaics got highly competitive in comparison to vacuum-processed films. For kesterite and chalcopyrite materials nanoparticle ink-based film deposition is one of the main directions. Nevertheless, the synthesis of monodispersed phase-pure CZTS colloidal nanocrystals with a desired composition without secondary phases is challenging. In this work the influence of the injection temperature and growth time on the formation process of CZTS nanoparticles and their final properties have been systematically investigated applying TEM, XRD, XRF, ICP-MS, Raman spectroscopy and thermogravimetry coupled with mass spectrometry. The analysis of aliquots has been used to investigate the growth process of nanoparticles in its intermediate stages. CZTS nanocrystals have been obtained by hot-injection of a sulfur/oleylamine solution into a metal salts mixture with a high-boiling temperature solvent under N2 atmosphere, using a Schlenk line. Within this approach a preferable formation of small sized nanoparticles (in the range from 3 to 10 nm) has been achieved. Based on XRF and ICP-MS analysis we show that an increase of the injection temperature results in significant changes in the Cu/Sn- and Zn/Sn-ratios of the nanoparticles. By Raman analysis of aliquots we find that the crystalline quality of the nanoparticles depends on the injection temperature and growth time, while for all processes Cu2-xS is detected as the initial phase.

Resume : Cu2ZnSnSe4 (CZTSe) compound is one of promising light absorber candidates for thin-film solar cells. Nonetheless, its band gap needs to be increased to the optimal band gap for high light absorption efficiency. Herein, the front band gap tuning of Cu2Zn(Sn,Ge)Se4 (CZTGSe) thin films is investigated, depending on the Ge ratio. CZTSe thin films were deposited by electron beam co-evaporation on Mo-coated soda lime glass. After that, for front band gap grading, Ge layers of various thicknesses are added on the absorber layers with diverse annealing time up to 50 min. Furthermore, we examined its structural, electrical, and optical properties of CZTGSe thin films with different composition ratio of Ge/(Ge+Sn). As the annealing time is long, Ge can diffused into the CZTSe layers well, resulting in the increase in the grain size and the thickness of precursor layers. The phase shift of the CZTGSe thin films occurs as Ge ratio is high because Ge will substitute for Sn. Moreover, with large Ge composition, the increase in the band gap is observable. CZTGSe thin films annealed less than 30 min shows only single work function peak, whereas several work function peaks appear in the samples annealed more than 30 min. From this, we can comprehend the band alignment between an absorber layer and a buffer layer in thin-film solar cells. Accordingly, the front band gap grading of CZTGSe thin films will present high conversion efficiency.

Resume : A novel low temperature Cu2ZnSnS4 (CZTS) formation path using co-sputtered SnS2-ZnS-Cu precursors has been employed for CZTS solar cell fabrication*. This method involves a low-temperature annealing stage for CZTS phase formation with a followed short high-temperature annealing stage for grain growth and secondary phase removal. The two stage process wisely separates the phase formation and crystal coalescence, thus makes the fabrication of CZTS film more controllable. Furthermore, given the controllability of the two stage process, the effect by the atmosphere at different stages of the process has been studied separately and the process has thus been further optimized. The demonstration of the low temperature formation path provides new opportunities to fabricate high efficiency, cost-effective and environmental friendly CZTS solar cells, in the hope of rivalling CIGS solar cells, its predecessor. * Y. Feng, T.-K. Lau, G. Cheng, L. Yin, Z. Li, h. luo, Z. Liu, X. Lu, C. Yang and X. Xiao, CrystEngComm, 2016, DOI: 10.1039/C5CE02279G.

Resume : Copper zinc tin sulfide (Cu2ZnSnS4) has gained attention as one promising quaternary absorber material for photovoltaic applications. Solution based methods could represent a low processing temperature and low cost alternative for the synthesis of CZTS. In this work, Cu2ZnSnS4 thin films have been deposited by the Successive Ionic Layer Adsorption and Reaction (SILAR) method on top CdS films deposited on glass substrates. Some parameters such as the concentration of the metal precursors and the complexing agent were optimized for the deposition of homogeneous CZTS thin films. Thin films were prepared using metal chlorides as the cationic precursor, sodium citrate as complexing agent and sodium sulfide as the anionic precursor, applying 70 cycles and post-deposition thermal annealings at 450 and 500 °C under N2 flow. The films were characterized using X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy and Scanning Electron Microscopy (SEM). Due to the similarity between the position of diffraction peaks for the two structural phases corresponding to the ternary Cu2SnS3 and quaternary Cu2ZnSnS4, XRD analysis is not conclusive to determine which one other two phases dominates. A Raman peak at 332 cm-1 corresponding to the A vibrational mode of the quaternary Cu2ZnSnS4 was observed. XPS depth profiling analysis using Ar+ ions revealed that the annealed films are poor in Zn and rich in Cu, also a Cd diffusion caused by temperature was observed.

Resume : An attractive possibility for the synthesis of In and Ga free chalcogenides which may allow terawatt range photovoltaic (PV) applications relies on I2-II-IV-VI4 species, such as copper zinc tin sulfide, copper zinc tin selenide and their sulfur-selenium alloy. They share similar structure with the first developed chalcopyrite matrix (i.e. CuInS2): half of the In atoms is replaced with Zn and another half with Sn, resulting in the kesterite phase. A further alternative belonging to this class of materials is Cu2MnSnS4 (CMTS), which consists of abundant and non-toxic elements and shows high absorption coefficient and direct band gap suitable for PV applications. In this work, a two-step process (evaporation in vacuum of the metal precursors followed by annealing in Sulfur vapors) has been implemented to grow CMTS thin films (TFs) on Mo-coated soda lime glasses. The effects of the most critical growth parameters on TFs quality (homogeneity and stoichiometry) have been investigated, paying particular attention to the order of the metal precursors in the evaporated stack and to the annealing temperature. Several techniques, including Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Raman, X-ray Diffraction and Photoluminescence have been used to test the CMTS TFs and to identify possible secondary phases. Good layer compactness, high absorption coefficient (5x104 cm-1) and direct band (1.26 eV) suitable for PV applications have been obtained.

Resume : One of the main challenges that the kesterite community faces is the reduction of the Voc deficit of the Cu2ZnSnS(e)4 solar cells. We present the synthesis of Cu2ZnSnSe4 (CZTSe) absorber layers by the selenisation of Cu-Zn-Sn sputtered metallic precursors with different compositions. It observed that the tetragonal distortion parameter c/2a has, in all samples, a value below 1. The value of c/2a decreases monotonically as the composition of the samples becomes Cu poorer and Zn richer. Solar cells were fabricated from this series of absorbers. It is observed that the efficiency of the solar cells increases from 6.9% to over 8% as the Cu/(Zn Sn) ratio of the film decreases and the Zn/Sn ratio increases. This increase in performance is mainly driven by a monotonic increase of Voc, which in the best case reaches a value of 450 mV, being comparable with and even larger than the current record CZTSe device and appearing to be linked to c/2a. These results are compared with 29 CZTSe kesterite powders with different composition which show a minimum c/2a value located in the Cu-poor and Zn-rich region, which is the range of composition where all the best kesterite based solar cells are produced and consistent with the trends of this work. We propose that the determination of c/2a (which partly determines the crystal field splitting) could be used as a predictor of the quality of CZTSe based solar cells, and, to some extent, as an indicator of phase inhomogeneity in the absorbers. Furthermore, we suggest synthesis routes to induce a decrease of c/2a that could help to solve the problem of Voc deficit that kesterite solar cells are facing.

Resume : It is well established that the addition of sodium (Na) to chalcopyrite (CIGSSe) or kesterite (CZTSSe) solar cells markedly increases solar cell performance. An enhancement of the grain size and increase of the mobility of carriers is typically shown but the fundamental reasons for these improvements are still a matter of debate. In this work, we explore the impact of other alkali elements – like potassium, rubidium, caesium and lithium (K, Rb, Cs and Li) – on pure selenide CZTSe solar cells. Therefore, alkali salts are first dissolved in water, then deposited at low temperature, and finally introduced into the CZTSe absorber layer at high temperature. Deposition of the solution is by spin or dip coating using moderate conditions, very easily scalable techniques. Two different sets of substrates are applied: Type (i) soda lime glass (SLG) / silicon nitride (barrier layer) / Mo (rear contact) / Cu, Zn, Sn (metal precursors), and Type (ii) SLG / Mo / Cu, Zn, Sn. Hence, using the samples with barrier layer in between the SLG and Mo, the alkali elements are introduced in a well-controlled manner. We have successfully deposited Na on type (i) substrates using spin coating achieving an efficiency of 8 % (without anti reflection coating). Preliminary results of K-deposition are equally promising, and work on Rb, Cs and Li is ongoing. Characterization techniques such as Scanning Electron Microscopy, X-ray Diffraction, Time Resolved Photoluminescence Spectroscopy, Current-Voltage and Quantum efficiency measurements are employed to study the physical and optoelectronic properties of solar cells. With this study, we explore the effect of alkali metals on CZTSe solar cell performance and enhance its understanding.

Resume : Cu2ZnSn(S,Se)4 (CZTSSe) material is one of the most promising solution for low cost production of thin film based solar cells mainly due to its use of abundant material. Standard manufacturing processes usually require high control of material composition and long deposition and post-treatment processes. CZTS powders synthesized by ball milling followed by doctor blade deposition in thin films and post reactive annealed in Se atmosphere, have already been reported, leading to the fabrication of CSZTSe solar cells with efficiency of 6.7%. Synthesis by ball milling allows an accurate control of composition, and the avoidance of secondary phases which are critical parameters for high efficiency CZTSSe solar cells. Moreover this technique is compatible with mass production requirements as it is a fast and cheap process which has a high production yield. In this work, we propose a fast dry process based on the direct synthesis of nanometric CZTSSe powder by ball milling, followed by its thermal evaporation on Mo substrates (evaporation rate of 30 Å/s) and finally recrystallized. Crystallinity, morphology and composition of the final absorber layers have been characterized by XRD, XRF, SEM and Raman techniques Additionally, because of its high control over composition and thickness of synthetized layers, we discuss about the application of this process for the growth of layers with a controlled gradient composition leading to band gap engineering of CZTSSe devices.

Resume : In this research work, CZTS, CZTSe and CZTSSe powders were prepared by mechanochemical ball-milling process using Tungsten carbide balls. Using the source materials of Cu, Zn, Sn, S, Cu, Zn, Sn, Se and Cu, Zn, Sn, S and Se for the CZTS, CZTSe and CZTSSe, in the ratio 1.6:1.1:0.9:4, which correspond to [Cu]/([Zn] [Sn]) and [Zn]/[Sn] values of 0.8 and 1.22, respectively. Furthermore, was chosen [Se]/[Se] [S]=0.4 for the synthesis of CZTSSe powders. These relationships are considered as the optimum for high efficiency solar cells. The powders are investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) for structural and morphological studies, EDS and XRF for compositional studies and Raman Spectroscopy technique for the existence of possible secondary phase formation, defects and crystalline quality of the powders. Our results are discussed in terms of using these powders in the processing of low-cost and high efficiency solar cells, as alternative technique to traditional methods used so far.

Resume : Cu2ZnGeS4 (CZGS) and mixed Cu2Zn(Ge,Sn)S4 (CZGTS) semiconductors have a strong interest for the development of cost efficient photovoltaic devices with suitable band gap engineering. Development of these materials requires for a deeper knowledge of their fundamental properties, which have been very little explored for these Ge based compounds. In this frame, this work reports a first theoretical and experimental detailed study of the vibrational properties of these compounds, This includes the theoretical DFT analysis of the lattice dynamics of CZGS wurtzstanite and kesterite structures, that has allowed deriving pictures of the atomic displacements in the unit cell for all the optical vibrations. These theoretical results are compared with the experimental spectra measured in the kesterite structure at low temperature in non-resonant and resonant excitation conditions. For most vibrations, an excellent agreement (≤ 2-5 cm-1) has been achieved. Higher differences (15-20 cm-1) observed in fully symmetrical modes are related to the existence of an anharmonic Fermi resonance mechanism. For the mixed CZGTS, in spite of the small frequency differences between the dominant A bands from CZGS and CZTS, the spectra measured in crystals with different composition show the existence of a two mode behaviour that has been experimentally confirmed from resonant Raman 2nd and 3rd order peaks. This is related to the small dispersion of the phonon branches in the Brillouin zone.

Resume : Chemical solution deposition techniques have been widely used for the production of kesterite thin films [1,2]. The processing conditions such as selenization parameters (e.g. temperature, pressure etc.) play a critical role and have significant impact on the properties of the resulting CZTSSe thin films and hence the device performance [3]. In this study we have deposited CZTS thin films by spin coating technique followed by an annealing step in selenium atmosphere to obtain CZTSSe thin films. Two different configurations for the selenization process were used, one in a tube furnace and the other one in a rapid thermal processor (RTP). The effect of the different selenization processes on the structural and morphological properties of the prepared thin films were investigated using SEM, XRD and Raman spectroscopy. Depth profiling Raman analysis was employed with different excitation wavelengths in order to study the formation of CZTSSe and secondary phases during selenization. CZTSSe thin film solar cells were fabricated and the resulting efficiencies are discussed in context of the varied selenization parameters. [1] X. Zeng et al., Sol. Energy Mater. Sol. Cells 124 (2014) 55–60. [2] H. Xin, J.K. Katahara, I.L. Braly and H.W. Hillhouse, Adv. Energy Mater. (2014) 1301823. [3] Y.E. Romanyuk et al., Sol. Energy Mater. Sol. Cells 119 (2013)181–189.

Resume : Kesterite-type Cu2ZnSnSe4 (CZTSe) based PV devices are limited by a low open-circuit voltage [1]. A possible reason is an effective band gap reduction due to structural inhomogeneities [2] like disorder among Cu and Zn in lattice planes perpendicular to the crystallographic c-axis at z=¼ and ¾ [3]. Decreasing Cu/Zn disorder was reported at T<533 K (CZTS) [4] and T<473 K (CZTSe) [2]. The implications of such a low critical temperature TC would be significant, because such absorber layers are grown far above TC. The exact extent of disorder by the end will depend only on the part of cooling that occurs below TC, a temperature region in which cooling history often is neither controlled nor reported. Indeed, low T postdeposition annealing has been shown to increase efficiency significantly [5]. However, low temperature ordering effects were only deduced indirectly, e. g. by Raman spectroscopy [4]. We used anomalous X-ray diffraction on the Cu- and Zn-K absorption edges to determine the Cu-Zn distribution over the crystallographic sites in a B-type CZTSe kesterite powder (Cu1.949Zn1.059Sn0.983Se4), thus determine the Cu/Zn disorder directly. Rietveld refinement allowed a quantitative determination of occupancies for all relevant sites. From this, the temperature dependency of a structure-based, quantitative order parameter could be determined. The critical temperature of the phase transition was confirmed at 460±10 K. The mechanism is in agreement with an order-disorder transition.

Resume : Doping with alkaline metals is one of the approaches for achieving high efficiency thin film chalcogenide solar cells. The latest record of 21.7% for Cu(In,Ga)Se2 (CIGS) solar cell which is achieved by alkali post-deposition treatment shows the importance of introducing impurities to absorber material. Similar type of doping can be also applied for Cu2ZnSnS4 (CZTS) solar cells. In this work, potassium iodide (KI) treatment for CZTS monograins is studied to improve the performance of CZTS monograin layer solar cells. CZTS monograins are submitted to high temperature heat-treatment in the presence of KI. Photovoltaic parameters of the solar cells were compared as a function of KI amount used in treatment. The highest efficiency of 5.8% is achieved with 10% KI treatment (percentage of the KI/CZTS mass ratio). I-V measurement shows an increase of Jsc by 10%, compared to corresponding untreated CZTS. The gain in Jsc could be a result of band gap decrease of CZTS and improved current collection. Bias-dependent EQE results confirm the improvement of carrier collection. In contrast to the improvement in Jsc, decreasing tendencies are observed in Voc and FF with increasing amount of KI. In addition, time-resolved photoluminescence (TRPL) is used to investigate PL decay time constants of KI treated monograins at low temperatures to understand the influence of KI treatment on charge carrier dynamics.

Resume : A comparative study of Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) thin films absorbers annealed on ZnO:Al (AZO), SnO2:F (FTO) and Mo substrates was investigated. Cu-Zn-Sn-S sulfide and Cu-Zn-Sn metallic precursors with a near stoichiometry composition were deposited by cosputtering followed by an annealing in an atmosphere containing Se or Se/S vapors. Detailed comparisons of structural, morphological and optical properties were performed using X-ray diffraction, Raman spectroscopy, scanning electron microscopy and glow discharge optical emission spectroscopy. The optoelectronic properties of the solar cells have been characterized by the means of PL, C(V), I(V) measurements. The results show different trends depending on the precursor and back contact used. While efficiencies more than 8% are reached whatever the precursor used on Mo back contact, preliminary studies on TCO substrates show that: the precursor morphology is strongly dependent on the substrate, AZO substrate reacts with the chalcogens while FTO is very chemically stable, however Se diffuse more easily in CZTSSe using AZO back contact compared to FTO. Moreover pure Se annealing compared to Se/S annealing exhibits a stronger gradient of S/(S+Se) in the CZTSSe absorber. In this work the impact of the substrate on the kinetic rate of selenization and sulfurization, the reaction occurring at the interface CZTSSe/substrate and their impact on the optoelectronic properties of solar cells will be discussed.

Resume : A disordering in kesterites due to [ZnCu+CuZn] antisite defect pairs has attracted significant interest in recent years. Cu–Zn disorder and different defect clusters are the primary cause of the absorber material band gap fluctuations. Degree of Cu–Zn disordering can be changed by using different cooling regimes after high temperature treatments. Currently, there are no experimental results about order-disorder affect to the Cu2ZnSnS4 (CZTS) solar cell parameters. In this study we investigated the influence of different cooling regimes after annealing of CZTS powder in S and/or SnS2 vapor to the performance of monograin layer solar cells. In the first experimental series the powders were cooled to room-temperature by using different cooling rates and in the second series, the powders were annealed additionally at low-temperature for different time periods. EDX analysis showed that the bulk composition of CZTS monograin powders did not change significantly by increasing cooling time after post-treatment. PL studies showed that slow cooling increased the level of ordering in crystals by shifting the PL band maxima from 1.27 to 1.34 eV. Current-voltage measurements revealed that increased amount of ordered kesterite makes significant impact on the CZTS monograin layer solar cell characteristics, mostly, the values of Voc were increased. EQE measurements confirmed that additional low-temperature annealing increases bandgap energy about 60 meV.

Resume : The emerging of kesterite Cu2ZnSn(S,Se)4 photovoltaic is foreseen to open a new application of white bronze CuSnZn alloy films. This is due to white bronze compositional range which can be tuned to meet the kesterite metallic composition. In order to obtain device-quality kesterite film, a white bronze precursor with smooth surface morphology is demanded. Typical rough and irregular corrugated surface has been found in white bronze precursors prepared by wide film deposition methods such as evaporation, electrodeposition and sputtering. The rough precursor surface is caused by the formation of Volmer-Weber Sn islands as a direct consequence of Sn high surface tension. In this contribution, a systematic study to improve the white bronze film surface morphology by preventing the Volmer-Weber Sn island formation is presented. The white bronze alloy films were electrochemically deposited by successive deposition of Cu, Sn and Zn on Mo-coated glass substrates. The effects of additional 0.025 g/l to 0.4 g/l sodium dodecyl sulfate (SDS) in citrate-based Sn electrolyte on the Sn surface morphology were investigated and discussed. The Sn island formation on Cu layer was significantly inhibited as SDS concentration increases toward an optimum SDS concentration of 0.1 g/l, where a smooth and highly reflecting Sn layer was produced. A new approach to evaluate the white bronze film morphological improvement by using reflectance measurement is also proposed. The application of white bronze was demonstrated by sulfurizing the CuSnZn films at 560 °C for 30 minutes which yields single phase and large grain of Cu2ZnSnS4 films.

Resume : In recent years, the Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) compound semiconductors are potential alternative materials to CuInGaSe2 (CIGS) for thin film photovoltaic absorber layers. This is because all of the elements in CZTS and CZTSe are abundant in the earth, and the bandgap energy is controlled by the ratio of S and Se. Therefore, environmental concerns are eliminated with these materials, paving the way for gigawatt scale mass production of solar cells. Despite their brief history in solar cell technology, CZTS, CZTSe, and Cu2ZnSn(S,Se)4 (CZTSSe) solar cell devices are rapidly advancing in the solar cell markets. For example, CZTSSe solar cells fabricated at IBM have achieved an efficiency of 11.1% using a hydrazine solution process [1]. Shin et al. have reported CZTS thin-film solar cell with an efficiency of 8.4% using a vacuum based thermal evaporation process [2]. Repins et al. have reported CZTSe solar cells with an efficiency of 9.2% using co-evaporation process [3]. In this work, CZTS thin film on glass substrate is grown by dipping-coat from Cu-, Zn- and Sn-xanthate solution as precursor materials. The samples are annealed under nitrogen atmosphere. X-ray diffraction (XRD), electron probe microanalysis (EPMA), scanning electron microanalysis (SEM), thermoprobe analysis and the four-point probe method are carried out. The XRD spectra indicate that a peak of CZTS (112) starts to observe at 150 °C. This temperature is lowest in non-vacuum process of CZTS film. The all samples indicate chalcopyrite structure and polycrystalline as evidenced by the XRD spectra. A value of full width at half maximum of (112) peak increases with increasing temperature. This indicates that an grain size increases with increasing annealing temperature. The samples are non-uniform composition such as Cu-poor and Zn-rich at low temperature and become S-poor with increasing temperature. Sulfur evaporates because vapor pressure of sulfur is high. The all samples are p-type conductivity by thermo prove analysis because Cu vacancy (VCu) defects is dominant in the samples from EPMA results. The resistivity increases with the increasing annealing temperature. It is assumed that this reason is due to decreasing carrier concentration. Donor type defects such as (Vs) increases with increasing the annealing temperature. [1] T. K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, and D. B. Mitzi, Adv. Energy Mater. 3, 34 (2013). [2] B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey, and S. Guha, Prog. Photovol: Res. Appl. 21, 72 (2013). [3] I. Repins, C. Beall, N. Vora, C. DeHart, D. Kuciauskas, P. Dippo, B. To, J. Mann, W. C. Hsu, A. Goodrich, and R. Noufi, Sol. Energy Mater. Sol. Cells 101, 154 (2012).

Resume : Despite progress and developments of quaternary Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTS). today solar cells with initial efficiency of about 12.6%, further understanding of CZTS as well as ways for its improvement is necessary. The control of formation/stability of point defects is one of promising approaches for increase of efficiency and stability of CZTS solar cells, because defects are one of major factors, related to phase formation, electrical and optical properties. Because of this, in this work, using hybrid functional calculations, we perform investigation of not only CZTS but also its alloys with Ge or Si. In particular, we analyze trends in electronic properties of CZTS materials with Ge or Si doping at different dopant concentrations. We further analyze optical properties of the materials in term of dielectric function and absorption coefficient. Based on the predicted results, we expand understanding of Cu-based solar cell materials and ways for controlling CZTS properties by doping. References: C. Persson, R. Chen, H. Zhao, M. Kumar and D. Huang, in Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells, John Wiley & Sons Ltd, 2014, DOI: 10.1002/9781118437865.ch4, pp. 75-105. W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D. B. Mitzi, Adv. Energy Mater., 2014, 4, 1301465

Resume : The best efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is still far below Cu(In,Ga)Se2 solar cells, mainly due to a large Voc deficit (about 600 mV), which is linked to band tails and interface recombination caused by non-optimized CZTSSe/CdS interface and CZTSSe grain boundaries. In this work, we managed to passivate the kesterite absorbers surface with group Ⅲ-S (Al-S, Ga-S) compounds by a wet chemical treatment, which was confirmed by XPS measurements. After the passivation process, the Voc and FF of the devices have improved 30-40 mV and 5-15%, respectively, while Jsc is almost unaffected, leading to 1-2.5% increased efficiency in absolute. A best CZTSSe device with 8% efficiency (Jsc=25.7 mA/cm2, Voc=459 mV, FF=68.1%) was obtained after passivation of the 5.6% reference device (Jsc=24.6 mA/cm2, Voc=428 mV, FF=53.4%). This passivation effect can be explained by: 1) Voc increase and less Voc deficit: better p-n junction quality, less interface recombination and increased hole concentration linked to a decrease of diode factor and saturation current, higher electronic band gap and possible removal of donors, which is studied by I-V, IV-T and C-V measurements; 2) FF increase: lower series resistance and higher shunt resistance could be related to better CZTSSe/CdS band alignment and decrease of shunt paths; 3) Jsc unaffected: better EQE in the p-n junction region while lower EQE in the back, which is ascribed to better p-n junction quality and shorter depletion width.

Resume : The quaternary compound Cu2ZnSnS4 (CZTS) is a promising non-toxic absorber material for solar cells made from earth abundant elements. It is known that CZTS is subject to disorder in the Cu and Zn containing lattice planes of the kesterite structure by Cu-Zn exchanges. The impact of Cu/Zn disorder on properties of CZTS is an intensely discussed field and there are still unresolved problems. In this work we performed time resolved photoluminescence (TRPL) studies of small CZTS single crystals grown in molten KI salt. The ordered/disordered structure of CZTS was varied by the post thermal treatment temperature. PL spectra measured at T=5K showed an asymmetric band with a peak position of 1.33 eV and 1.27 eV for ordered and disordered structures, respectively. TRLP was measured for both crystals and their decay curves were fitted with a stretched exponential function. The differences in the TRPL decays of ordered and disordered crystals and the mechanisms causing such kind of behaviors are discussed in the paper.

Resume : Our group has developed a relatively benign binary solvent system consisting of ethylenediamine and a short chain thiol that is capable of dissolving a myriad of bulk inorganic materials under ambient conditions. Upon thermal annealing at low temperatures, phase pure, crystalline chalcogenide materials can be recovered. This method is a simple and safe alternative to using hydrazine chemistry to solution process similar materials. Recently this binary solvent system has been used to solution process thin films of ternary materials such as CuIn(S,Se)2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTS) for photovoltaic (PV) applications. In the work presented here, the thiolamine solvent mixture is used to dissolve bulk inorganic oxides and chalcogenides to generate precursor inks that can be spin coated onto a desired substrate, and upon mild annealing, yield phase-­‐pure chalcogenide thin films. For example, thin films of Sb2S3 and Sb2Se3 can be recovered from inks made with bulk Sb2O3, or Sb2O3 in the presence of Se, respectively, and show high photoelectrochemical current response. More interestingly, by varying the nominal composition of Se in the precursor ink, a series of Sb2Se3-xSx alloys can be recovered and their direct band gaps can be tuned from 1.2­-1.6 eV, thus covering the optimal range for a PV absorber material in a single junction cell. Thin films of CuSbS2 (Eg = 1.5 eV) are also readily prepared using this ink formulation method with Cu2S and Sb2S3 as precursors.

Resume : The net dopant concentration of the absorber layer is a crucial parameter for understanding and optimizing the performance of a solar cell. Reported dopant concentrations for comparable specimen however often vary by more than one order of magnitude. Most data on the dopant concentration of polycrystalline thin film absorbers is obtained by either Hall or capacitance-voltage (C-V) measurements. Although both in principle permit a fast and convenient characterization, the simple models almost exclusively employed in the analysis neglect any influence of grain boundaries on the measurements. In our contribution, we obtain the dopant concentration for a set of Cu-poor Cu(In,Ga)Se2 thin film absorbers from Hall measurements, carefully taking into account the effect of grain boundaries on the analysis. We obtain the same net dopant concentrations for films grown on glass and for films mechanically ripped from a conductive molybdenum back contact, which permits to compare Hall and C-V measurements on the same sample. Furthermore, the low mobility values well below 1 cm2/Vs agree with theoretical predictions of a depleted grain boundary and allow us to identify the dopant concentration obtained from Hall measurements with a lower limit of the true in-grain dopant concentration. However, C-V measurements on the same absorbers yield significantly lower dopant concentrations. This might be linked to the potential distribution in the absorber, which can no longer be described adequately by a one-dimensional model in the presence of grain boundaries.

Resume : Photoinduced current transient spectroscopy (PICTS) was proven to be a powerful tool for investigation of the electronic parameters of deep bulk levels in the CIGS absorbers. In our previous publications we were able to distinguish deep levels appearing in both epitaxial and polycrystalline layers of various Ga content. Some of them, and these are of particular interest, have been found in all CIGS materials irrespective of their morphology and Ga/In ratio, and also observed also by capacitance methods in CIGS solar cells. In this report we would like to present a new approach for studying these levels: Excitation PICTS. First we measure the PICTS spectrum using LED diode and box-car averaging of the transient photocurrent. Then the excitation spectra for levels differentiated by PICTS are measured using chopped illumination in the range 400-1800 nm and phase-sensitive detection. The frequencies and temperatures of measurements corresponding to the PICTS maxima are chosen in order to determine excitation energy for a given defect level. We will present the results both for the epitaxial and polycrystalline thin films. Special attention will be paid to the cases of the disagreement between the sum of the thermal activation and excitation energies with the bandgap. The failure to observe such agreement and its possible sources such as negative correlation energy or barrier for carriers capture will be discussed.

Resume : Recent efficiency improvements in Cu(In,Ga)Se2 (CIGS) thin-film solar cells were achieved by the application of a potassium post-deposition treatment (K-PDT), applied after the deposition of the CIGS absorber layer by a co-evaporation process. We use Kelvin probe force microscopy (KPFM) to study the effect of the K-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We investigated the CIGS sample surface, and also the initial growth of the CdS buffer layer, after deposition of a thin CdS layer for 1 minute by chemical bath deposition. We observe an increase in the overall contact potential difference (CPD) for samples with K-PDT as compared with samples without K-PDT, while the variations of the overall CPD are comparable for all samples. The thin CdS layer reduces the CPD by 80 - 130 mV. A clear difference is observed in electronic properties of grain boundaries (GBs) observed by KPFM: the K-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of CPD values at the GBs. This effect of the K-PDT on GBs electronic properties could additionally contribute to the improved efficiency values observed for CIGS thin-film solar cells with K-PDT. This work was supported by cooperation program Portugal-France FCT-Pessoa 2015/2016.

Resume : The highest efficiency of Cu(In,Ga)Se2 (CIGS) solar cells has currently been demonstrated using CIGS thin films with a band-gap energy (Eg) of about 1.1 eV, corresponding to the [Ga]/([Ga]+[In]) (GGI) composition ratio of about 0.3, though the ideal Eg value is speculated to be about 1.4 eV (GGI ~ 0.6) for the case of single junction cells. To enhance CIGS cell efficiencies, therefore, material and device properties of highly Ga-containing CIGS or ternary CuGaSe2 (CGS) have long been studied and many efforts have been made to improve wide-gap CIGS solar cell performance thus far. In our recent work, we have for the first time demonstrated double digit efficiency (>10%) for elementally In-free ternary CGS thin film solar cells. The Se to metal flux ratio used during co-evaporation growth of CGS films was found to be an important parameter and affected alkaline Na and K diffusions in CGS films and led to the formation of a Cu-deficient layer (so-called ordered vacancy defect: OVC) on the CGS film surface, resulting in the formation of n-OVC/p-CGS junction. We have also found that the interface properties of ternary CGS solar cells are different from quaternary CIGS cells and thus post-growth processes for CdS buffer and ZnO window layer depositions on CGS films require specialized conditions different from the case for CIGS to demonstrate good device performance. These understandings are expected to contribute to explore highly efficient wide-gap CIGS solar cells as well.

Resume : Recently developed potassium fluoride post-deposition treatment (KF-PDT) of Cu(In,Ga)Se2 absorber has allowed the achievement of 22.3% devices. Such a treatment improves both the open circuit voltage (Voc) and the fill factor of the solar cells. Moreover, it allows the reduction of the CdS buffer layer deposition duration by chemical bath deposition (CBD), leading to thinner CdS layers thus increased short circuit current. In the present work, we have investigated the heterojunction formation in 20 % energy conversion efficiency device treated with KF. We have analyzed by X-ray photoelectron spectroscopy and scanning electron microscopy the surface of KF-treated absorbers right after the KF-PDT and after different dipping durations in the CBD. KF-PDT treatment yields the formation of a thin KInSe2 phase at the CIGS surface. We show that (i) the KInSe2 induces a much more homogeneous growth of the buffer layer than onto the CIGS and (ii) the KInSe2 is turned into CdIn2(S,Se)4 in the CBD by substitution of K by Cd and Se by S. Transmission electron microscopy has revealed that a 5-nm thick CdIn2(S,Se)4 interface layer is present in complete device between the CIGS and the CdS. This material has been reported in literature to have n-type conductivity. Finally, KF-PDT and (CBD)CdS lead to an asymmetric junction between CIGS and CdIn2(S,Se)4, that could explain the increase of the Voc and why it is possible to thin the CdS layer.

Resume : Flexible and lightweight Cu(In,Ga)Se2 solar modules have a drastically increased potential for building-integration and allow for an industrial roll-to-roll process with increased throughput. Polyimide foils are a promising candidate to replace the standard glass substrate. The incorporation of alkaline dopants into the CIGS is a major challenge for alkaline-free substrates. Post-deposition treatments (PDT) with NaF and KF led to cell efficiencies up to 20.4 % but the influence of the PDT conditions (temperature, duration) on the alkaline concentration-profile and the morphology of the absorber layer and solar cell is not clear yet. This work aims to close this gap. Using a 3-stage co-evaporation process yielding a cell efficiency of 9.7 % (alkaline-free), KF and NaF PDT under various conditions were first studied separately and then combined. A pure KF PDT increased the efficiency to 11.8 % and a NaF PDT to 14.1 %. Bare and H2O rinsed absorber layers as well as complete solar cells were investigated by GD-OES and SEM. The rinsing washed off nanometric agglomerates from the CIGS surface and the influence of the PDT conditions on the Na profile (duration increased Na concentration, temperature increased penetration) was found to disappear. In certain cases a slight nano-structuration of the surface was observed. Finally as the alkaline treatments influence the p-n junction quality, solar cells with CdS and alternative Zn(O,S) buffer layers will be compared.

Resume : Highly efficient co-evaporated Cu(In,Ga)Se2 (CIGS) absorbers are typically grown with a [Ga]/([Ga]+[In]) (GGI) gradient across the thickness and a Cu-poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the near infra-red (NIR) response. In this contribution [Cu]/([In]+[Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometry point. In order to obtain comparable Ga contents at the interface for proper band alignment, the GGI gradings were modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. Adequate tuning of front GGI allows maintaining Voc and FF values, yielding an overall increase of efficiency as compared to a reference baseline. Further reductions in the final stage duration do not lead to additional improvements. The net majority carrier concentrations, measured by capacitance-voltage profiling, are found to be similar up to a relative CGI increase of 15%. We further characterize the modified absorber properties by reflection-transmission spectroscopy and time-resolved photoluminescence. We suggest that optical absorption and carrier collection can be improved by composition and grading modifications for state-of-the-art devices.

Resume : In CIGS(e) solar cell research, a novel area of attention is the Mo/CIGS(e) rear interface, where recombination of charge carriers typically is rather high. A rear surface passivation layer with nano-sized contacts has been suggested and developed as innovative approach to reduce recombination of charge carriers at the rear CIGS(e) semiconductor interface. This design stems from the Si PV industry, but employing the concept in a CIGS(e) solar cell requires a scale down of all dimensions. It is needed to shrink the Si rear passivation design about 100 times and, instead of a contact hole size of several micrometres, aim for a hole size of 20-200 nm with a pitch of a few micron. Another area of attention where recombination of charge carriers is too high is the CIGS(e)/CdS front interface. Very recently, several techniques that could generate a front surface passivation layer with nano-sized point openings have been proposed, e.g. the spray-ILGAR® (ion layer gas reaction) and the SALT (self-assembled alkali-templates) approaches. Initial calculations indicate that in this case the design is even more demanding: A hole size of 10-100 nm with a maximal pitch of a few hundreds of nm is aimed for. This presentation will discuss recent developments in electrical passivation of CIGS(e) surfaces: From its fundamentals to solar cell applications, and their perspective.

Resume : Chalcogenide thin films (i.e., CdTe, Cu(In,Ga)Se2, Cu2ZnSnSe4) have attracted intensive research over the past two decades to produce low cost and high efficiency thin film solar cells. As recently demonstrated [1], one of the major challenges to improve the performance of such devices is the control of the surface properties of the absorber. In the present contribution, we report on alternative chemical approaches that can be applied at room temperature to modify the surface of Se-based chalcogenide thin films, using ammonium sulfide (AS) as a treating agent. A first chemical treatment process has been developed based on the immersion of Cu(In,Ga)Se2 (CIGSe) absorber into an AS solution [2]. This treatment results in the removal of the undesirable CuxSe secondary phase from the absorber surface and in the incorporation of sulfur into the surface region of the material. Both of these effects lead to the improvement of the open circuit voltage and the minority carrier lifetime of the treated devices. A second chemical treatment has been developed based on the use of AS vapor as a sulfur source. When applied to CuInSe2 (CISe) absorbers, the exposure to AS vapor leads to the progressive formation of CuIn(S,Se)2 at the surface of CISe thin films. The open circuit voltage of the CISe/CdS/ZnO devices is found to be reproducibly improved by about 30 mV after 10 min of treatment compared to a reference device. The impact of the AS treatments duration on the physical and electrical properties of chalcogenide solar cells was also evaluated for Cu2ZnSnSe4 photovoltaic devices. [1] Chirila A. et al., Nature Materials 2013, 12, 1107–1111. [2] Buffière M. et al., Adv. Energy Mater. 2015, 5: doi: 10.1002/aenm.201401689

Resume : It has been a long-standing discussion in a variety of semiconductor communities [in particular for CuInGaSe2 (CIGS) thin film solar cells] to determine the band gap of a specific material. Variations between bulk and surface, but also between optically and electronically determined band gaps have been controversially debated. For CIGS, pioneering photoelectron spectroscopy (PES) measurements [Schmid et al., JAP 73, 2902-2909 (1993)] pointed towards a band gap widening at the surface of the chalcopyrite absorber, which was subsequently directly confirmed and quantified by a combination of PES and inverse PES (IPES) [Morkel et al., APL 79, 4482-4484 (2001)]. In this contribution, we use reflective electron energy loss spectroscopy (REELS) as a complementary method to study surfaces and interfaces in such systems. By analyzing the characteristic inelastic scattering cross-section λK(E), we have determined the surface dielectric function and thus the optical properties of a Cu(In,Ga)(S,Se)2 (CIGSSe) absorber, an as-grown indium sulfide (InxSy) buffer, and an annealed InxSy buffer. A comparison of the optical values at the surface of the InxSy film with bulk ellipsometry measurements indicates a good agreement between bulk- and surface-related optical properties. In contrast, the CIGSSe absorber shows a larger electronic surface band gap than for bulk-sensitive measurements, providing a complementary and independent confirmation of the earlier results.

Resume : The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin film solar cells. Especially impurity driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect level density and unusual optoelectronic grain boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provide evidence that sodium diffusion from the glass substrate does not affect the surface defect passivation or grain boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6eV) due to sodium. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n-interface in standard chalcopyrite thin film solar cells is dominated by band bending within the CIGSe absorber, rather than the result of Na impurities or a n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as origin for the Cu-depletion and band gap widening at the surface of chalcopyrite thin films.

Resume : The efficiency of Cu(In,Ga)Se2 (CIGS) solar cells is strongly affected by structural and compositional inhomogeneities in the absorber layer, which locally influence its electronic properties and also the charge carrier transport. However, a deeper understanding on how such nanoscale inhomogeneities influence electronic loss mechanisms in CIGS is still needed to further improve the solar cell performance. Therefore, the local characterization of electronic properties such as the band gap energy (Eg) is of high interest. Valence electron energy loss spectroscopy (VEELS) provides information about the Eg in the nanometer scale. However, the high energy resolution, which is required to access the small Eg of CIGS, is pushing towards the achievable limits of state-of-the-art electron sources. In this work we use a 2nd generation monochromator that provides a very high energy resolution to study the electronic properties at selected sites in CIGS by VEELS. First results of local Eg measurements within the bulk, at grain boundaries (GBs) and across the surface of CIGS are presented. E.g. we observe a variation in electronic properties that is correlated with the nanopatterned surface structure of CIGS. Further, the Eg varies as a function of compositional inhomogeneities in the bulk, but no distinct Eg fluctuations are observed at GBs. The possible effects of the observed nanoscale fluctuations in electronic properties are finally discussed in view of the solar cell performance.

Resume : The benefit of reducing the thickness of the Cu(In,Ga)Se2 absorber (CIGSe) is beyond the reduced material costs and production time. It also lowers the minimum quality requirements for the CIGSe layer, since shorter pathways for electron extraction allow shorter electron diffusion lengths. However, the design of ultrathin solar cells raises specific issues in terms of absorber quality, light management and electronic transport and in terms of the requirements on the interface quality due to the increased charge carrier density at the contacts. The aim of this presentation is to give an overview of the state of the art of ultrathin CIGS solar cells and to present some results and perspectives for improvement of the opto-electronic properties of ultra-thin solar cells. The impact of the deposition techniques for CIGS such as coevaporation, reactive sputtering and electrodeposition on the properties of ultrathin absorbers will be discussed.

Resume : Low temperature annealing of completed cells is reported by several authors as beneficial to CZTX/CdS based solar cells. [Neuschitzer, 2015 / Hironiwa, 2015] Cations diffusion at homo and hetero interfaces (grain boundaries and CZTX/CdS) plays a major role in this feature: Cu migration into the CdS, Cd into the CZTX, copper/zinc ratio changes at grain boundaries were observed by different authors. Similarly, In-supply to CdS and CZTS can boost the efficiencies, providing a good control of In-diffusion. [Kim, 2014] The present study consists in comparing the impact of annealing of CZTSe based solar cells with different buffer layers, in order to better understand the role of the different elements. CBD deposed CdS and Zn(O,S) and evaporated In2S3 and CdIn2S4 have been used. IV and EQE measurements are used to determine trends on the solar cell parameters evolution with annealing at different temperature/time, and check what features are reversible with time. Samples leading to the most remarkable parameters evolution are selected for advanced electrical measurements. The outputs of this study are: _A better understanding of the role of the different elements migration during annealing on CZTX-based solar cells, leading to a boost of cells efficiencies thanks to fine tuning of the annealing conditions (from 2 to 8% with CdS buffer without ARC) _Developpement of a new buffer layer material (CdIn2S4) for CZTSe based solar cells (efficiencies of 6.5% without ARC)

Resume : Chemical-bath-deposited (CBD) ZnS buffer layer is being steadily studied in Cu(In,Ga)Se2 (CIGS) thin-film solar cell because of their beneficial properties, for examples, good transparency with large direct bandgap, less toxicity and cost effectiveness. CBD-ZnS buffer layer, however, is easily affected during subsequent sputtering process for TCO layer and the conversion efficiency decreases with sputtering power. The efficiency decrease of the CBD-ZnS/CIGS thin-film solar cells is known to be related to plasma damage originated from high energy negative oxygen ions or neutral particles at the interface. It is important to develop a process for a high-quality interface between CBD-ZnS and CIGS. In this study, we developed the CBD-ZnS film by changing the process condition and applied the dual-facing targets sputtering process to TCO layer for reducing the plasma damage. The effects of plasma damage were investigated and the properties of TCO layer deposited by low damage process were characterized. Finally, the effects of sputtering process on the conversion efficiency of CBS-ZnS/CIGS thin-film solar cell were analyzed.

Resume : Several works studying the effect of the intrinsic ZnO (i-ZnO) layer on the performance of CIGSe solar cells have been reported nevertheless, many questions concerning its beneficial role still remains under debate. This oxide layer is of particular importance not only for CIGS but for most of thin film based technologies including CZTSe. In those technologies the same window strategy is used: i-ZnO followed by a transparent conducting oxide (TCO) as front contact. We report on the impact of incorporating 0%, 5%, 10% and 15% oxygen during i-ZnO sputtering correlated with the optoelectronic properties of CZTSe solar cells. The optical properties of the window layers (i-ZnO/TCO) were first investigated using normal incident transmittance by UV-VIS-NIR spectrometer, before and after performing hot plate air annealing at 200ºC (commonly used after cell fabrication to improve heterojunction quality). The annealed films showed improved transmittance over all visible electromagnetic spectrum and enhanced crystallinity as revealed by Raman scattering. The CZTSe solar cells fabricated with these window layers have improved optoelectronic properties following the oxygen incorporation by increasing the open circuit voltage, fill factor and hence efficiency, from 6.5% (0% oxygen) to 7.7% (15% oxygen). The results on the J-V characteristics under different light conditions indicate that the oxygen addition during the i-ZnO deposition has a strong effect on the commonly observed red-kink and cross-over that increases with oxygen.

Resume : Zn(O,S) used as an active layer of thin film transistors has been considered as an attracted material because of its wide energy band gap and high transparency for various optoelectronic devices. In addition, it is a prominent candidate for a Cd-free buffer layer in thin film solar cells due to its no environmental issues. There are lots of researches on Zn(O,S) are performed by using the solution-based processes because of their low cost, high throughput, inexpensive, and continuous roll-to-roll advantages. In this study, Zn(O,S) thin films was synthesized by a new solution-based method designed by utilizing printing technique. In the preparation of precursor inks, ZnSO4 and thiourea were used with continuous flow reactor and ratio of O/S was controlled by the concentration of thiourea. Then, Zn(O,S) thin films was deposited with the precursor inks by the printing process. The prepared Zn(O,S) films were characterized with X-ray diffraction spectrometer (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), UV-visible spectrophotometer, and X-ray photoelectron spectroscope (XPS). Based on the physicochemical analyses, it was found that the good quality of Zn(O,S) thin films were successfully deposited by the printing process and a very short period time was taken to deposit an enough thickness for the TFT devices. We believe that this is the first experimental research work on the deposition of Zn(O,S) thin films using the printing process.

Resume : Zinc sulfide (ZnS) thin film is a promising material for its use in various application devices due its high refractive index and high transmittance in the visible wavelength range. ZnS thin films were prepared on the glass substrates by CBD method using aqueous solutions of zinc acetate and thiourea at pH 10 and temperature was maintained at 950C. The non-toxic complexing agent tri-sodium citrate (Na3-citrate) was used to control the unwanted release of Zn2+ ions. The solutions were prepared in deionized water and films were deposited on İTO glass substrates. The reaction bath solution was prepared using 40 mL of a 0.2M zinc acetate dihydrate (Zn(CH3COO)2.2H2O) and 40 mL of a 0.4M thiourea (SC(NH2)2). The concentration of the complexing agent Na3-citrate was 0.2 M and the pH was adjusted to 10 by adding 25% ammonia (NH4OH) solution. Finally, a sufficient amount of deionizedwater was added so as to make the total volume of the deposition bath 100 mL. Before deposition, the substrates were ultrasonically cleaned with acetone followed by rinsing in methanol, isopropyl alcohol, and deionized water for 10 min, respectively. The prepared solution was stirred vigorously before dipping the glass substrates. The substrates were mounted on the substrate holder and kept vertically in the reaction bath. The temperature of the reaction bath was then maintained at 950 C. The films were sequentially removed in an interval of 15 min. It is well known that the atomic force microscopy (AFM) is one of the effective ways for the surface analysis due to its high resolution and powerful analysis software The ZnS and ZnS:Ag thin films were morphologically characterized using Atomic Force Microscopy (AFM) technique. Also the surface structure of a coating gains more and more importance. In the extreme case of vthin films the surface roughness may be in the order of the film thickness and can influence all film properties such as mechanical, electrical, magnetical or optical properties. Also film morphology, inner structure, texture and crystallinity are strongly connected to roughness evolution. Generally, the reason for the development of roughness during the deposition process is the finite extension of the film forming particles and their random, temporally and spatially uncorrelated impingement at he growth front. The "building blocks" of the film do not necessarily have to be single atoms as it is the case for PVD coatings. They can also be complex molecules (e. g. for organic coatings) multi-particle aggregates as e. g. for cluster deposition or macroscopic aggregates like the ceramic or metallic droplets in the case of thermal spraying. It can be seen that films are uniform, densely packed and pinhole free, and it shows that the morphology of these films has larger number of grain size and are homogeneously distributed, which indicates the crystalline nature of the film. Initial visual investigations of the deposited film have shown that they are compact and have good adherence to the substrate. No evidence of cracking observed The grains are made of different sizes varying from 74.0-76.0 nm. Based on AFM image, the grain density reduced indicating the smaller grains agglomerate together to form larger grains of ZnS and ZnS:Ag. On the other hand, the surface roughness and Root Mean Square (RMS) of the films were measured using AFM technique. The surface roughness defined as the standard deviation of the surface height profile from the average height is the most commonly reported measurement of surface roughness. The surface roughness is unavoidable since the grains are grown with different sizes.

Resume : Zinc oxysulfide is emerging as one of the most promising materials to replace toxic CdS in the buffer layer of chalcopyrite based thin-film solar cells. Zn(S, O) thin films were grown by three step process: (1) The deposition of Zn layer on glass substrates by vacuum thermal evaporation, (2) The oxidation of the as-deposited Zn layer in air atmosphere and (3) The sulfurization of the oxidised samples in N2 atmosphere. The samples were characterized for their structural, morphological, optical and electrical properties by using X-ray diffraction (XRD), atomic force microscopy (AFM), UV-Vis spectroscopy and hot probe method, respectively. XRD analysis revealed that the films were polycrystalline in nature. The optical constants (band gap energy, absorption coefficient) of the films were determined, in the spectral range 300 - 1800 nm, from the analysis of the transmission and reflection data. The surface morphological studies revealed that the films had an average roughness between 2 and 5 nm. All films exhibited n-type conductivity.

Resume : Understanding how the solar cell device performance is influenced by changing material properties of different layers in the window layer structure of Cu(In,Ga)Se2 (CIGS) solar cells is important when developing new processes or materials. In this work, we study the interaction of intrinsic ZnO layers grown by chemical vapor deposition (CVD) and by magnetron sputtering (PVD), together with two different transparent conducting oxide (TCO) materials, ZnO:Al grown by PVD and atomic layer deposition (ALD) of In2O3. The solar cell device structure in the study is SLG/Mo/CIGS/CdS/i-ZnO/TCO/metal grid. It is found that the thickness of the intrinsic CVD ZnO layer affects the open circuit voltage (Voc) significantly when using the ALD In2O3 TCO. The average Voc drops by 28 mV when the intrinsic ZnO thickness is increased from 20 to 160 nm. This detrimental effect on Voc could not be seen when using a ZnO:Al TCO. In addition, the Voc drop for the CVD i-ZnO/ALD In2O3 structure could not be reproduced by using a PVD grown intrinsic ZnO layer. It has yet to be shown how and when the intrinsic ZnO thickness affects the Voc. However, due to the observed thickness trend, the Voc decrease cannot simply be explained by an i-ZnO/In2O3 interface property. A bulk property of ZnO is therefore likely one part of the explanation.

Resume : The development of a novel chemistry for the chemical bath deposition (CBD) of Zn(O,S) buffer layers for Cu(In,Ga)Se2 (CIGS) solar cells aims at a higher growth rate, hence reduced deposition time, while reducing simultaneously the required concentration of reactants. State-of-the-art recipes are based on thiourea as “S2-“ precursor requiring a high molarity of reactants and relatively long deposition times due to the slow decomposition rate of thiourea. In this contribution new sulphide precursors were in-vestigated for their decomposition and growth behaviour. A co-solvent approach in an aqueous am-monia medium was evaluated omitting the need for additional complexants. Homogeneous dense Zn(O,S) layers of around 30 nm were grown with a greatly decreased deposition time of 8 minutes compared to 25 minutes for thiourea. Likewise the concentration of the sulphide precursor was 40-fold reduced. X-ray photoelectron spectroscopy characterization was carried out to investigate the [S]/([S]+[O]) composition showing a more sulphur rich surface as compared to the thiourea process. The photovoltaic performance was characterized by EQE and IV measurements and showed compara-ble efficiencies to the thiourea based process of 15% cells.

Resume : This paper reports on the synthesis of Cd1-xZnxS thin film buffer layers prepared by chemical bath deposition (CBD) onto indium thin oxide coated glass substrates using aqueous solutions for solar cell photovoltaic application due. Cadmium sulfide, zinc sulfide and thiourea were used as sources of Cd+2, Zn+2, and of S-2, respectively. Triethenolamine was used as complexing agent to control the Cd+2 and Zn+2 ions concentrations and ammonia to adjust the pH 9.8±5 of the solution. The temperature of the bath was kept at 70 °C. The as deposited films are well adherent, homogeneous and free from pinholes. The structure of the films as observed by X-ray diffraction was polycrystalline with hexagonal structure and (100), (101), (102) and (110) as main peaks. Energy dispersive spectroscopy (EDS) shows non-stoichiometric films due to a deficit of sulfur by increasing Zn content. The strong absorption edge shifts towards the lower wavelength region and hence the band gap of the films increases as the Zn content increases. The values of the absorption edge are found to shift towards the shorter wavelengths with Zn content and hence the direct bandgap energy varies from 2.42 eV for the CdS film and 3.50 eV for the ZnS film. Cd1-xZnxS thin films can be useful as buffer and window layers in Cu(In,Ga)Se2 thin films solar cells due to ability to tune the band gap through the Zn/Cd ratio present in the chemical bath.

Resume : CdS is used as a buffer layer in thin film solar cells and it is subjected to thermal treatments during deposition of ZnO window or during CdTe vapor deposition and CdCl2 activation, influencing the performance of the devices. An unambiguous understanding of physico-chemical processes taking place during annealing and resulting in changes of CdS properties is still missing. This paper aims to study systematically the changes in the properties of CBD CdS films during annealing and to describe the physico-chemical mechanism of these changes. Based on IR spectroscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Hall, Van der Pauw and optical band gap studies, the analysis is given in terms of defect chemistry involving OH groups substituting the sulfur in CdS lattice. Incorporation of cadmium hydroxide in the deposition process causes formation of cadmium hydroxysulfide alloy as a compensated semiconductor with a stressed crystalline lattice. Thermal treatments of CBD CdS films from 200 °C to 450 °C generate destruction of incorporated hydroxide group, creation and out diffusion of excess Cd, and modification of the film electron concentration in the range of 10E16-10E19 cm-3. We have found that during the sputtering of ZnO layers the sheet resistance of underlying CBD CdS film was decreased by three orders corresponding to the electron density of 10E19 cm-3, indicating to the importance of these results for further development of substrate type thin film solar cells.

Resume : CdS thin films are used as a buffer layer in the hetero-junction structured CIGS thin film solar cells. However, because Cd is a toxic material, the needs for its alternative materials have been discussed to avoid the environmental issues. ZnS is considered as a prominent substitute material to replace Cd. It was reported that some amount of ZnO and Zn(OH)2 as the impurities, which are denoted as Zn(O,S), are required to be present in ZnS for fabricating the efficient CIGS solar cells. In this study, Zn(O,S) films were synthesized by a solution-based deposition method designed by a combination of a continuous flow reactor (CFR) with another method. Their structural and physicochemical properties were characterized by XRD, SEM and UV-vis spectroscopy. It was confirmed that the CFR process was very effective method to deposit Zn(O,S) thin films. Based on our result, Zn(O,S) thin films were successfully deposited in a very short period of time using the combined CFR process. Cd-free B:ZnO/Zn(O,S)/CIGS solar cells were also fabricated on Mo coated glass substrates in order to investigate the influences of the experimental parameters such as deposition time and deposition method on the performance of Zn(O,S) buffer layer in the devices. Photovoltaic performance was measured under AM. 1.5G illumination. The fabricated CIGS solar cell yielded an efficiency of 9.64%. It is expected that a higher efficiency can be obtained when our experimental conditions are optimized.

Resume : Thin film solar cells based on Cu(In,Ga)Se2 (CIGS), where just the buffer layer is changed, were fabricated and studied. The effects of two buffer layers are compared, CdS and ZnxSn1-xOy (ZnSnO), using several characterization techniques. The general trend from the literature where CdS-based solar cells show higher values of open circuit voltage (Voc) and fill factor (FF) compared with Cd-free buffer layers is confirmed in this work. Kelvin probe force microscopy (KPFM) results indicate that CdS provides junctions with slightly higher surface photovoltage (SPV) than ZnSnO. The higher SPV value of CdS is reflected in the higher Voc values of resulting devices when compared with ZnSnO. Contrasting the higher SPV and Voc values of the CIGS/CdS devices, the CIGS/CdS interface exhibits more defects as concluded by photoluminescence (PL) results and in accordance with C-V measurements. Our results indicate that the CIGS/CdS interface still has a vast potential to be improved and that future studies of ZnSnO buffer layers should focus on understanding its limitations in SPV and Voc.

Resume : The development of non-toxic and sustainable materials and devices is of paramount importance for the future deployment of solar energy. In this frame, kesterites are at the forefront of sustainable photovoltaic technologies due to the abundance of its constituent elements and the relatively low environmental impact of the associated processes. Nevertheless, kesterite devices (and almost all the thin film photovoltaic technologies), rely in the use of CdS as “universal” buffer layer, requiring for a medium-long-term Cd-free substitute in order to ensure the sustainability of the technology. In this work we present a comparative study of chemical bath deposited ZnS and In2S3 as buffer layers for Cu2ZnSnSe4 (CZTSe) based solar cells. A complete analysis of the impact of bath composition (mainly [M]/Tiourea, being [M] the concentration of Zn or In), bath temperature (60-80 ºC) and growth time will be presented. The Cd-free buffer layers were characterized regarding their optical (transmittance and photoluminescence), structural (Raman spectroscopy and XRD) and morphological (SEM and AFM) properties, as well as implemented in CZTSe based devices. We will show the high potential of these buffer layers reporting a CZTSe/In2S3 device with 4.5% efficiency (VOC = 460 mV, JSC = 23.9 mA/cm2), FF = 41%) and CZTSe/ZnS with 5.2% efficiency (VOC = 366 mV, JSC = 27 mA/cm2), FF = 53%), and we will discuss the challenges of these buffer layers to become a real alternative to CdS.

Resume : Replacing the toxic CdS buffer material in thin-film solar cells based on Cu(In,Ga)(Se,S)2 (CIGSSe) has been a crucial issue for a long time. A promising alternative buffer material is InxSy which stands out due to its tuneable characteristics, e.g., the band-gap or the electron-affinity by adding third elements. The specific influence of additional elements has been subject of many publications, however, with inconsistent results e.g. concerning the nature of the band-gap or the conductivity. The experimental access to the optical and electrical parameters of InxSy thin-films is a necessity to fully understand the formation of the solar-cells hetero-junction. For this work, CIGSSe solar-cells with a varied sodium-concentration in the InxSy:Na-buffer-layer and comparable single InxSy:Na-layers on sodium-free glass substrates were investigated. The solar-cells were characterized by means of current-voltage-measurements (IV), whereas spectroscopic-ellipsometry (SE) and conductivity-measurements were performed on bare InxSy:Na- layers. The IV-measurements indicate a formation of a transport-barrier with increasing sodium content which is in agreement with a decrease in electron affinity reported in the literature. The SE analysis shows band-gap values of Eg=2.1 eV for sodium-free thin-film InxSy and Eg=2.33 eV for the layers with the highest sodium content. The absorption coefficient shows a clear indirect nature of the band-gap transition. The conductivity under illumination is in the range of σ=10^(-6)(1/Ωm) and shows a strong decrease in the dark. The conductivity shows no significant trend with increasing sodium-concentration.

Resume : We designed a precursor ink based process for the fabrication of indium sulfide (In2S3) thin films, as a buffer layer in CIGSe thin film solar cells. Inkjet printing has been successfully utilized to deposit the ink. A working In2S3 buffer was obtained after drying the film in air and annealing it in flowing Ar or Ar/H2S. Solar cells with the buffer produced by our process demonstrated performance comparable to that of standard CdS-buffered cells. As a drop-on-demand technique, inkjet printing has advantages in terms of material usage and generation of waste. The whole process is also vacuum-free and the combination of these specific properties promises low cost in industrial production. In addition, inkjet printing enables highly localized deposition of materials, which is attractive for implementing advanced device concepts, such as micro-concentrator solar cells. X-ray photoelectron spectroscopy (XPS) has been used for a first assessment of substrate coverage, interdiffusion, contaminations, and reaction mechanisms. The spectra indicate the presence of In, S, Se, Cu, O, N, and C. The transformation of the film during the annealing step is deduced from a strong decline in the O and N concentrations. The origin of the remaining N contamination (In salt or S source) has been studied by comparing films prepared from inks with different thiourea concentrations. Cu and Se signals indicate incomplete coverage and/or interdiffusion between absorber and buffer. Both are crucial for optimum performance of In2S3 buffer layers and could be assessed separately by taking into account the Cu/Se ratio.

Resume : Low cost processes are very important problem for solar cell devices. The spray method is one of non-vacuum processes and is respected as low cost method. In our previous works, high quality transparency ZnO films were successfully grown at 100 °C by a conventional atmospheric spray pyrolysis using diethylzinc (DEZ) based solution [1]. The DEZ was diluted with diisopropyl ether to control its reactivity to air and water (supplied by Tosoh Finechem Corporation, JAPAN). Moreover, the growth of Ga-doped ZnO (GZO) /glass films was carried out by spray pyrolysis at 150 °C [2]. The samples had an average optical transmittance of more than 80% and were strongly a-axis orientated. The sheet resistivity of 30 /sq. could be obtained. In this work, the GZO films were successfully grown on ZnO/CdS/CIGS/Mo/glass by spray pyrolysis using DEZ based solution. After covering clean SLG substrates with back electrodes of Mo films by sputtering, CIGS films were deposited using physical vapor deposition. Buffer layers of CdS films were prepared by chemical vapor deposition. Buffer layers of ZnO films (≈ 20 nm) were also prepared by RF sputtering method [3]. The efficiency of the obtained device was 10.3%. The short-circuit current density (Jsc) of 34.3 mA/cm2, open circuit voltage (Voc) of 0.50 V and fill factor (FF) of 0.60 are obtained. [1] K. Yoshino, Y. Takemoto, M. Oshima, K. Toyota, K. Inaba, K. Haga, and K. Tokudome, Jpn. J. Appl. Phys. 50, 040207 (2011). [2] Y. Takemoto, M. Oshima, K. Yoshino, K. Toyota, K. Inaba, K. Haga, and K. Tokudome, 50, 08801 (2011). [3] T. Minemoto, Y. Hashimoto, W. S.-Kolahi, T. Satoh, T. Negami, H. Takakura, Y. Hamakawa Solar Energy and Solar Cell Materials, 75, 121 (2003).

Resume : The main characteristic of Cu2ZnSnS4 (CZTS) base solar cells is the presence of secondary phases which may introduce a local compositional and structural inhomogeneity. Among of the various secondary phases generally observed in CZTS material, one of the most limiting is the ZnS phase. In this work, in order to probe ZnS phase and quantify inhomogeneity in CZTS thin film material, we use, on one hand, spatially resolved Raman and PL spectroscopy measurements at different wavelength, and on other hand, three kinds of samples: CZTS crystal with controlled phase purity, CZTS crystal with controlled phase purity and intentional localized ZnS phases inclusions, and thin film CZTS fabricated by sputtering method. By comparing the spatially resolved micro-Raman spectra of thin film CZTS to that of controlled phase purity CZTS crystal, we are able to identify the main Raman pics of thin film CZTS and to quantify its distribution within an area of 50x50 um. Additionally, micro-PL cartography at multiple wavelength gives information on layer quality and secondary phases localization. Hence, the effects of ZnS phases in the structure of CZTS thin film are also successfully identify by comparing different excitations wavelengths (325, 532, 640, 784nm) Raman spectra of CZTS crystals with localized ZnS inclusions to that of thin film CZTS. These results, will be compared to those determined by others groups, as a lot of work has been done on CZTS secondary phases identification, and we will discuss the possible correlation with photovoltaic cells response parameters.

Resume : To further characterize the composition distribution, a promising nondestructive analysis of field-emission electron probe microanalysis (FE-EPMA) was proposed to probe the compositional distribution within Cu(InxGa1-x)Se2 absorbers. FE-EPMA obtained the high S/N ratio and collaborated with high detector resolution (10 eV) resulting in 10 ppm in detection limit; additionally, advanced field emission gun achieved much tinier probe size about 10 nm, and by well controlling the accelerating voltage of electron beam, the escape depth could be lower to sub-micron meter level. FE-EPMA shows the excellent ability for detecting the fine distinction of Se, trace elements, and provides the direct evidence for uniformity issue. Using field-emission electron probe microanalysis (FE-EPMA), electrometry (IV, EQE, C-AFM), and photometry (PL) we investigate the relationship between the Se concentration environment and the corresponding spatial distribution of films. We show that the deficient Se environment gives rise to its non-uniform spatial distribution, resulting in poorer grain boundary quality and enhancing deep defects recombination. By improving the uniformity of Se, the efficiency is leading to the increase about 5%. Our results suggest that not only the Se quantity but also the spatial distribution limits the CIGS cell performance.

Resume : Luminescent downshifting layer (LDL) has a real interest to improve the performances of a solar cell. LDL allows to convert photons from UV to visible range, increasing the photon fraction absorbed by the solar cell. The LDL is constituted of a polymeric matrix doped with fluorophores. We propose a review of different categories of fluorophores (organic dyes, inorganic dyes, quantum dots, etc…). Figures of merit such as Absorption range, emission range, photoluminescence quantum yield (PLQY), the emission overlap, and the emission spectral matching integral were calculated for each fluorophores. The final aim is to find the good combination between fluorophores, matrix and solar cell. Starting from figures of merit we identify, for example, the following materials: Lumogens (BASF) with the Lumogen red305 as reference, quantum dots CdSe/ZnS with different absorption/emission ranges and homemade rare-earth complex. The Lumogen family has a high PLQY (>90%), more precisely our reference Lumogen red 305 has a PLQY of 96%. For the quantum dots family, we retained CdSe/ZnS for two reasons: the large absorption (max at 376nm) and emission (max at 623nm) range and the large Stokes shift. Optical properties of the matrix (transmission, reflexion) w/o fluorescent materials (absorption and emission range, photoluminescent quantum yield) were studied. Experimental work of CIGS cell and LDL, to quantify the downshifting efficiency will be presented.

Resume : Today, the world solar photovoltaic modules market is dominated by PV cells based on silicon and thin films, including CdTe and CIGS cells. Yet, their conversion performances are limited by the so-called spectral mismatch between solar cells and the solar spectrum. For instance, the spectral response of CIGS cells has maximum external quantum efficiency (EQE) in the region 550-900 nm and its efficiency is dramatically decreased between 500 and 300 nm, where the total sun illumination power is around 19 %. One solution to this issue is to use downshifting materials based on Eu(III) complexes to convert UV light into red light. Their major advantages are very limited reabsorption and large Stokes? shifts. The goal of this work is to increase the photon conversion efficiency of CIGS cells in the region 280-500 nm by implementing encapsulating polymers containing highly luminescent europium complexes. Herein, we will present a complete study on the influence of the [Eu(TTA)3(TTPO)2] complex concentration and thickness of the ethylene-vinyl acetate (EVA) copolymer to increase the conversion efficiency of CIGS cells. We have demonstrated an improvement of the EQE at 350 nm by 30 % absolute. We pointed out that the highest concentration of [Eu(TTA)3(TTPO)2] in EVA obtained from spectral response does not automatically result in the best conversion efficiency. Finally, the best cells exhibit an increase of the conversion efficiency of CIGS cells by 0.8% absolute.

Resume : CuGaSe2 and CuGaS2 polycrystalline thin films absorbers have been prepared by one-step electrodeposition from an aqueous electrolyte containing CuCl2, GaCl3, and H2SeO3. The pH of the solution was adjusted to 2.3 by adding HCl and LiCl. Annealing improves crystallinity of CuGaSe2 and further annealing in sulphur atmosphere is required to obtain CuGaS2 layers. The morphology, topography, chemical composition and crystal structure of the deposited thin films were analysed by Scanning Electron Microscopy, Atomic Force Microscopy, Energy Dispersive X-Ray spectroscopy and X-Ray Diffraction, respectively. The X-Ray diffraction shows that the as-deposited CuGaSe2 film exhibits poor crystallinity, but it improves dramatically when the layers are annealed in forming gas atmosphere for 40 min. Subsequent sulfurization of CuGaSe2 films was performed at 400 °C for 10 min in presence of molecular sulphur and under forming gas atmosphere. The effect of sulfurization is the conversion of CuGaSe2 into CuGaS2. The formation of CuGaS2 thin films is evidenced by the shift observed in X-Ray diffraction pattern and by the blue shift of the optical band gap. The bandgap of CuGaSe2 was found to be 1.66 eV while for CuGaS2 it raises up to 2.2 eV. A broad intermediate absorption band associated to Cr and centred at 1.63 eV was observed in Cr-doped CuGaS2 films.

Resume : By now, the progress in CIGSSe based thin film solar cells led to conversion efficiencies of more than 22% [1]. By establishing an intermediate band within the energy gap of a wide gap semiconductor, theoretical predictions state the possibility to increase the utilization of solar energy [2]. With such an intermediate band gap it is proposed, that the efficiency can be raised up to 63.3% [3]. In the presented study CuGaS2, which has the widest band gap in the solid solution series of CIGSSe, is used as basis to establish an intermediate band by the incorporation of transition metals. Martì et al. [4] proposed different transition elements, such as Ti4+/3+ and Fe3+/2+. In this study we focused on incorporating Cr instead of Ti into the crystal structure of CuGaS2, because of better solubility proposed from thermodynamic calculations as well as Mn as analogue [5]. Aiming in a systematic study to determine the solubility limits of these cations in CuGaS2, powder samples have been prepared by solid state reaction of the elements (900°C) in evacuated silica tubes. The synthesized materials were analyzed in terms of chemical composition and phase content using WDX spectroscopy as well as in terms of crystal structure by X-ray and neutron diffraction. The presentation will summarize structural trends of lattice parameters, tetragonal distortion and tetragonal deformation in dependence on the Mn/Cr content. Moreover the solubility limits of Mn/Cr in CuGaS2 are discussed.

Resume : Na incorporation in Cu(In,Ga)Se2 thin film solar cells improves photovoltaic properties. However, it remains an open question why –especially at low growth temperatures– the beneficial effect is stronger when Na is added after the layer growth. To examine a possible influence of Na, CuInSe2 thin-films deposited at low substrate temperatures (below 400°C) by multi-stage coevaporation were analyzed in this study. A 12 nm NaF precursor layer on the molybdenum back contact acted as alkali metal source during the growth of the samples. The effect of Na on the structural defects in the absorber layer is characterized via high-resolution scanning transmission electron microscopy (HR-STEM) in combination with electron energy-loss spectroscopy (EELS) for two samples from processes interrupted during Cu-Se deposition at Cu-poor and Cu-rich compositions. The defect concentration in the samples was analyzed via STEM low-angle annular dark-field imaging. Although most of the defects were expected to have been annihilated for the sample with Cu-rich composition, both samples showed high concentrations of planar defects. We observed stoichiometric elemental distribution at twin boundaries and at stacking faults, however, elemental redistribution was found at lower-symmetry grain boundaries (GB). We also found evidence of CuPt ordered CIS phase within the grains and around GBs.

Resume : Polycrystalline CuIn1-xGaxSe2 thin films solar cells present a high conversion efficiency (h≈22 %) for x*≈0.4 while theoretical predictions indicate better performance for larger x. Recent results obtained by Atom Probe Tomography [1] on CIGSe layers on soda-lime substrate strongly suggest that below x*, Grain Boundaries (GBs) are In-enriched compared to the bulk while Cu-enriched GBs are more frequent for larger x. The chemical difference of GB-segregating species below and above x* may explain the unexpected limited performance described hereabove. Hence to understand this two-regime segregation a part of the puzzle is to carefully analyze the coupling between the structure and the chemistry of the GBs. On one hand, our study aims at assessing the role of Na onto the GB structure by alternative techniques (EBSD, XRD). On the other hand, we perform atomic-scale computations to refine the state-of-the-art knowledge of CIGSe bulk phase diagram [2]. This may give insights on the segregation driving forces at GBs in CIGSe. [1] M. Raghuwanshi, E. Cadel, P. Pareige, S. Duguay, F. Couzinie-Devy, L. Arzel, N. Barreau, Applied Physics Letters 105, 013902 (2014) [2] H. T. Xue, W. J. Lu, F. L. Tang, X. K. Li, Y. Zhang, Y. D. Feng, Journal of Applied Physics 116, 053512 (2014)

Resume : The fabrication of semitransparent thin film solar cell based on Cu(In,Ga)Se2 needs a substitution of molybdenum as back contact. The structure is the stack Glass/ZnO:Al/CIGSe/CdS/ZnO/ZnO:Al. The work involves optimizing the optical and electrical properties of the back contact made of ZnO:Al, deposited by RF-sputtering. Moreover, the thin film of CIGSe needs to have a high gallium rate at the CIGSe/ZnO:Al interface, so an unusual deposition process is used, referenced to as CURO process. This structure permits to obtain cells with a 12% power conversion efficiency with a 700 nm thin absorber and a 50% transmission beyond the semiconductor band gap. First results are promising and show the suitability of the choice of ZnO:Al as an alternative to molybdenum as back contact. Then, some alternative back contact will be investigated and tandem solar cell with CIGSe and crystalline silicon will be realized. Key word: semitransparent solar cell, tandem solar cell, thin film, Cu(In,Ga)Se2, ZnO:Al, CURO process, alternative back contact.

Resume : Thickness reduction of CIGSe films is a subject of economic importance. The problems so far are non-sufficient light absorption by single or double light pass, pin holes, and carrier recombination. From silicon technology, the concept of point contact solar cells is known which, in combination with a back contact reflector, allows collecting long wavelength photons. Using a model system of 190 nm thick CIGSe films, we follow this approach and prepare a SiO2 back side film with periodic openings to the molybdenum layer being the electrical contact. The openings are plasma etched through a mask prepared by laser interference lithography. We find a maximum increase of the short circuit current density of 25% for a SiO2 thickness of 60 nm and structure length of 1.1 µm. This gain is mainly due to (1) a roughness increase of the CIGSe layer due to the back contact height profile which enhances the quantum efficiency at all wavelengths and (2) an anticipated wave guide effect which boosts the quantum efficiency particularly at long wavelength. Interestingly, due to the absorber growth process not all structure lengths lead to a roughness increase: The CIGSe film tends to smooth out the back contact height profile for too small structure length. The best efficient solar cells with 190 nm thick CIGSe has a conversion efficiency of 9% (FF=70%) without anti-reflection coating. These experimental results can largely be represented by simulation. The success of ongoing optimisation will be reported in the paper.

Resume : Thin film solar cells based on chalcopyrite Cu(In,Ga)Se2 (CIGS) exhibit efficiencies up to 22 % with an absorber layer thickness of 2-3 µm. In order to overcome the bottleneck of the limited primary resources such as indium and gallium, one of the solution is to synthesize the CIGS absorber with a thickness lower than 500 nm. In this work, we have developed a hybrid one-step co-sputtering/evaporation ultra-thin CIGS deposition process, where Cu, In and Ga are sputtered simultaneously with the thermal evaporation of selenium, thus avoiding H2Se use. To meet this goal, studies have been conducted to ensure control of the multiple deposition parameters. Ultra-thin CIGS absorbers down to 500 nm were synthesized with a stabilized hybrid process. These absorbers were deposited in one and three stages to analyze the influence of composition gradients on their morphological, structural and optoelectronic properties. The film composition and thickness of the samples were measured by XRF, EDX and GD-OES. The different phases formed during the process were identified by Raman spectroscopy and XRD. The impact of the deposition conditions on nucleation step and morphology of the absorbers were studied by SEM. The optoelectronic properties analysis of each sample was also performed by I(V) measurement under illumination and dark conditions and by spectral response. CIGS solar cells with conversion efficiencies up to 5 % were fabricated with an absorber layer thickness of 300 nm.

Resume : A conversion efficiency of 15.5% on a pure-sulfide Cu(In,Ga)S2 solar cell with a Cd-free buffer layer was achieved. This is a big progress of pure-sulfide Cu(In,Ga)S2 cells whose conversion efficiency have not been improved since 2009. Our pure-sulfide Cu(In,Ga)S2 cells were fabricated by combing Cu-poor absorbers and Zn-based buffer layers. Current breakthrough to exceed the limit of 15% efficiency was brought by optimizing the Zn-based buffer layer. In this paper, we will review our recent progresses of our pure-sulfide Cu(In,Ga)S2 cells, especially in the impact of buffer layers for the pure-sulfide Cu(In,Ga)S2 cells. We keep exploring the potential of pure-sulfide Cu(In,Ga)S2 cells, and the latest results will be shown at the conference.

Resume : The recent efficiency breakthrough of Cu(In,Ga)Se2/CdS heterojunction-based solar cells results from KF post-deposition treatment (KF-PDT) of the completed absorber layer. KF-PDT implies the formation of a very thin CdIn2S4-like layer at the surface of the CIGSe during the chemical bath deposition (CBD) of CdS, yielding CIGSe/CdIn2S4/CdS junction. With the aim of better understanding the electro-optical characteristics of such a junction, thin films of CdIn2S4 have been co-evaporated on different types of substrates, namely soda-lime glass (SLG), SLG/Mo and regular SLG/Mo/CIGSe structures for cells preparation. In the present contribution, physico-chemical, optical and electrical properties of the layers will be presented as well as performances of cells based on p-CIGSe/n-CdIn2S4 junctions. The efficiencies reached by the latter devices appear strongly dependent on the amount of sulfur supplied during CdIn2S4 deposition. The first results of theoretical calculations, supporting the hypothesis that S-vacancies are at the origin of the n-type conductivity of CdIn2S4, do provide fundamental bricks to interpret cells performance.

Resume : In this study thin films of In2O3 were fabricated by physical vapor deposition (PVD), namely magnetron sputtering, with and without a post-annealing step, and by atomic layer deposition (ALD). The electro-optical properties on glass as well as the performance as a transparent conductive oxide (TCO) layer in CuIn1-xGaxSe2 (CIGSe)-based solar cells are compared and related to a ZnO:Al (AZO) baseline TCO. Corresponding TCO film thicknesses were adjusted to a resulting sheet resistance of about Rsh = 20 Ohm/sq for all samples. Structural investigations were conducted by X-ray diffraction and scanning electron microscopy, while Hall and optical absorption measurements were performed to analyze the electrical and optical quality of the window layers. It is shown that the fully crystallized In2O3 layers processed by ALD and PVD plus post-annealing show similar microstructural and electro-optical properties, which are superior to the AZO baseline. The finalized solar cells were characterized by current-voltage and reflectance-corrected quantum efficiency measurements. While there is no significant gain in short circuit current density for as-deposited PVD In2O3 layers, the application of crystalline In2O3 TCOs leads to an improvement of more than 1 mA/cm2 due to an increase in ”optical” band gap energy and less free charge carrier absorption. The open circuit voltage (Voc) of the best cells is 10-15 mV higher compared to the AZO reference, independent on the crystallinity of the In2O3 films. Since very comparable absorber and buffer layers were used (stemming from the same run) the differences in Voc are reliable and the detected slight gain is most likely caused by material properties (lower electron affinity or reduced interface defect density by a more coherent interface to the buffer layer) rather than by differences in the deposition processes.

Resume : We study the effect of post-growth annealing on the Cu(In,Ga)Se2 (CIGS) thin film and its interface with CdS, as well as the effect of the delay between the annealing and the photoluminescence (PL) measurements. A CIGS/CdS sample was annealed in air at 185oC for 30 mins and PL was measured before (15 min) and after annealing (1, 16 and 64 h). Before the annealing, a broad and asymmetric band is observed, suggesting the influence of fluctuating potentials in the radiative transitions. After the annealing, a redshift of the PL (~23 meV) and a decrease of the asymmetry of the band are observed. For a time interval of 16 and 64 h, a blueshift and an increase of the asymmetry is observed. We used the dependence on the excitation power of the peak energy to investigate the influence of the annealing on the fluctuating potentials at the CIGS layer. With the increase of the excitation power, the observed peak energy shifts were 7.0(0.5), 5.8(0.4), 6.2(0.8) and 6(2) meV for before annealing and 1, 16 and 64 h after the post-growth annealing, respectively. The results suggest that a post-growth annealing step is beneficial for the reduction of fluctuating potentials. Nevertheless, a partial “recovery” of the absorber layer and/or the absorber/buffer interface compared to their properties prior to the annealing is observed.

Resume : CIGS modules with an aperture area of 667cm2 and efficiencies up to 15.6% were prepared using a sputtered Zn(O,S)-buffer layer. The AVANCIS Cd-free buffer layer is based on InxSy, deposited with a soft but more complex thermal evaporation process. Here we demonstrate that a simple and low-cost industrial sputter process can be used to substitute the conventional buffer and i-layer with a single film of Zn(O,S). We evaluate the influence of an additional i-ZnO film between the Zn(O,S)-buffer and the ZnO:Al front electrode layer. In comparison with our state-of-the-art InxSy:Na reference, the CIGS modules with a Zn(O,S)-buffer show a superior short-circuit current, which is slightly overcompensated by a loss in the open-circuit voltage. Quantum efficiency measurements reveal that the gain in the short-circuit current can be attributed to a higher transparency in the UV-region due to a larger band gap of the Zn(O,S). Electroluminescence images demonstrate a low and uniform recombination across the active module area. The quality of the Zn(O,S)/CIGS interface, as judged from saturation current and ideality factor, is close to that of the InxSy:Na reference. Hence, by further optimizing the deposition parameters and interface properties, the sputtered Zn(O,S)-buffer is a promising candidate in order to increase the efficiency and reduce the costs of CIGS modules.

Resume : Selective removal of functional layers without affecting the functionality is still challenging for laser-assisted methods. In the present study laser pattering of copper indium gallium selenide (CIGS) thin films which are deposited on flexible metal substrates are presented. The pattering experiments were performed with a laser source having a wavelength of 1550 nm and pulse duration of 6 ns. The 1550 nm wavelength was selected due to the high transparency of the CIGS material in this wavelength range. It was found that due to the high transparency of the CIGS material two different material removal processes can be activate. By using a fixed laser repetition rate with a high pulse overlap a material ablation process starting front the sample was observed. By decreasing the pulse overlap the material removal process induced by laser ablation changes to a stress assisted delamination process. The material modification of the processed sample material in dependence on the laser parameters used was analyzed in detail for both processes. Therefore the composition of the sample material after the laser treatment was analyzed by energy dispersive X-ray spectroscopy (EDX) and micro Raman spectroscopy.

Resume : The range of applications of kesterite photovoltaic devices can be widely broadened by the use of alternative substrates to soda-lime glass. This work explores the use of commercial ceramic tiles as ecological substrates for CZTSe thin film solar cells in view of an application in building integrated photovoltaics (BIPV). As a novel approach to control the Na-incorporation into the absorber, the substrates are coated with enamels containing different concentrations of Na2O. This way, the enamel acts not only as barrier, but also a source of Na during the absorber growth. Cu2ZnSnSe4 (CZTSE) absorbers were produced on Mo coated substrates by sequential process including sputtering deposition of the metallic precursors followed by a reactive annealing. The analysis of the samples processed with different Na2O contents shows an optimum region before degradation of the back contact morphology. Resonant Raman scattering measurements have revealed the formation of SnSe2 as main secondary phase in the layers, being the content of this phase strongly related to the thermal annealing conditions. Correlation of these measurements with the optoelectronic properties of test cells fabricated with these layers shows that this phase deteriorates the device performance mainly by limiting the short circuit current. Optimisation of these processes has allowed the synthesis of SnSe2 free layers with good crystallinity that lead to a promising device efficiency of 7.5%.

Resume : High efficiency Cu(In,Ga)Se2 solar cells are typically Cu-poor but have passed through a Cu-rich intermediate stage during the three-stage co-evaporation process. This strongly increases the solar cell performance, however, the mechanisms behind this improvement are not yet fully understood [1]. Similarly, the presence of alkali elements such as Na and K is known to increase the conversion efficiency, but again, the underlying mechanisms are not yet clear. We have therefore used X-ray absorption spectroscopy (XAS) to study the local atomic structure of Cu(In,Ga)Se2 absorber layers prepared by the three-stage co-evaporation process. The films are characterized by different final Cu contents and have or have not passed through the Cu-rich intermediate stage. Additionally, samples were prepared with and without the presence of alkali elements. The XAS analysis yields local structural parameters such as element-specific bond lengths and bond length variations [2]. Comparing these parameters for the different Cu(In,Ga)Se2 samples provides unique insight into the correlation between preparation conditions and subnanoscale structure and thus contributes to a better understanding of the improved device performance of absorber layers that have passed through a Cu-rich intermediate stage and were subjected to alkali treatment. [1] Caballero et al., Prog. Photovolt. 21, 30 (2013). [2] Schnohr et al., Thin Solid Films 582, 356 (2015).

Resume : A Cu-poor/Cu-rich compositional transition is essential in multi-stage co-evaporation of high-efficiency Cu(In,Ga)Se2 thin film solar cell absorbers. For manufacturing purposes a precise understanding of the influence of Cu is necessary to simplify the complex deposition process. The recrystallisation occurring around the point of stoichiometry has been attributed to an increased mobility of grain boundaries. Less attention has been paid to the effect of Cu on the microstructure at Cu-poor compositions. In samples from interrupted processes the typically constant Cu evaporation rate makes it difficult to differentiate between the influence of exposure to high substrate temperature and Cu concentration increase. We overcome this obstacle by using in-situ energy-dispersive X-ray diffraction to study the microstructural evolution of CuInSe2 thin films during Cu-Se deposition with repeatedly interrupted Cu evaporation at low substrate temperatures (420°C). We show that at Cu-poor compositions domain size growth, stacking fault annihilation and stress relaxation are driven by Cu deposition. We deduce an additional driving force for grain growth induced by Cu diffusion and propose diffusion induced grain boundary migration (DIGM) as grain growth mechanism in Cu-poor CuInSe2. DIGM has hitherto not been considered in CuInSe2 thin film growth and differs from the explanations based on grain boundary mobility for grain growth during the Cu-poor/Cu-rich transition. Our results complement the picture of the role of Cu in the microstructural evolution during multi-stage co-evaporation of CuInSe2.

Resume : CIGS (Cu(In,Ga)Se2) thin film absorbers are performing for high yield photovoltaic cells. It’s well known that photovoltaic cells’ performances mainly rely on absorber properties. In this context, the fine optimization of absorber requires an accurate characterization of the surface, the volume of each layer and their interfaces. Especially for CIGS absorbers, determination of gallium gradient (through GGI=Ga/[Ga+In] ratio) is a key parameter. We have developed a strategy of cross physico-chemical characterization methods. First, the chemical composition (major elements, impurities) is investigated combining EDS, ICP-OES, GD-OES and XPS. Then, the surface morphology is studied by SEM and AFM imaging (roughness). All analyzes were carried out on a batch of ungraded CIGS samples prepared by coevaporation with GGI ratio ranging from 0.15 to 0.61. Both thickness and depth composition were set constant to evaluate the sensitivity of each technique. Cross-checking of results will be discussed in terms of calibration and accuracy of composition depth profile linearity. An original XPS approach, to directly access the GGI ratio, based on Ga3dIn4d region study, will be presented as a better way to determine the actual surface composition before buffer layer deposition. Complementary work will be also shown and dedicated to the determination of local scale composition using nanoAuger technique (spot size 12nm). This method allows the investigation of grain boundaries particularities.