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A post-mortem analysis is conducted after potential-induced degradation (PID) of a commercial copper-indium-gallium-selenide (CIGS) photovoltaic module. After PID, the conversion efficiency of the total module decreased by 62%. Electroluminescence images of the module show that the edges of the modules were much more affected by the PID than the middle part of the module. Coring samples were prepared of the different areas and chemical compositional information of the various areas was combined with electrical characterisation, cell modelling and luminescence data to obtain an overall perspective on the root cause of degradation in these modules during high voltage stress. Consistent with earlier studies on cell level, the module analysis shows the occurrence of alkali migration. From current–voltage modelling, it was concluded that the degradation of the most affected areas is due to an increase in bulk and CdS/CIGS interface defects, likely induced by ion migration. Further degradation on the same samples occurred when they are taken out of the argon-filled glovebox and stored under ambient conditions. Remarkably, the PID-degraded areas show stronger degradation when left in ambient atmosphere, as well as a stronger Na redistribution. These new results show that ion migration not only causes the immediate degradation but also strongly affects the longer-term stability of the cells in ambient atmosphere. This indicates that PID degradation makes CIGS devices more vulnerable to hermeticity problems, which are most prominent at the module edges.

Cu(In,Ga)Se-2 (CIGSe) solar cells are among the most efficient thin-film solar cells on lab scale. However, this thin-film technology has relatively large upscaling losses for commercial technology. To tackle this, paradigm shifts are proposed that allow for simpler, cost-effective, and efficient CIGSe solar cells. Front passivation using dielectric layers is one of the options being investigated as this is widely used in Si technology. Research on front passivation for CIGSe is in an early stage and no improvements are made yet. A close comparison with silicon technology is made to understand why it seems to be more difficult for CIGSe solar cells. In general, chemical passivation is less effective, resulting in higher interface defect densities than seen for Si. Also, field-effect passivation requires positive charges, which have not been implemented yet on the CIGSe front surface. Finally, for Si passivation, often a high-temperature annealing step is applied, which is not possible for CIGSe. It is proposed to apply a dielectric tunneling layer with positive fixed charges in combination with an electron transport layer to move forward. A list of potential dielectric layers that could be suitable for CIGSe is provided. Dr. J.D.W., Dr. G.B., and Professor B.V. received funding from the European Union’s H2020 research and innovation programme under grant agreement no. 715027 for this work. G.B acknowledges FWO postdoctoral fellowship 1219423N and R.S FWO Ph.D. fellowship fundamental research 1178022N.

Many economic and environmental studies on novel perovskite solar cells (PSCs), published ex post the development stage to investigate the market competitiveness, have focused on laboratory-scale PSC architectures that are not amenable for upscaling. In this paper, we evaluate the market potential and environmental sustainability of a scalable carbon-electrode-based PSC by benchmarking it to the market dominating c-Si photovoltaics and CIGS thin film photovoltaics. The analysis covers the PSCs full lifecycle, at the module and system levels (residential and utility scale), and is based on realistic annual energy output data derived from energy yield calculations. We find that this PSC can produce electricity at low cost (3–6 €cents/kWh), with lowest energy payback (0.6–0.8years) and greenhouse gas emissions (15–25g CO2eq./kWh) compared with grid-connected PV market alternatives, assuming 25 years of lifetime, expected PV system cost reductions, and PSC module recycling and refurbishment.

Pure sulfide CIGS solar cells are interesting candidates for standalone solar cells or top cells in a tandem configuration. To understand the limits and improve the power conversion efficiency of these devices, a comprehensive approach aimed at composition, interface, and process engineering should be employed. Here, the latter was explored. Using a two-step fabrication technique and one-variable-at-a-time methodology, we found the four processing factors affecting the absorber the most. While two were already backed by the previous literature, we found new and statistical evidence for two other important factors as well. The impact of alkali barrier diffusion was also established with statistical significance and under various processing conditions. Furthermore, the absorber roughness for samples without a barrier indicated a significant negative linear correlation with the devices' efficiency. This contribution could aid engineers in more efficient process designs. European Union's Horizon 2020 Research and Innovation Program [640868]

Ultrathin Cu(In,Ga)Se-2 (CIGS) absorber layers of 550 nm were grown on Ag/AlOx stacks. The addition of the stack resulted in solar cells with improved fill factor, open circuit voltage and short circuit current density. The efficiency was increased from 7% to almost 12%. Photoluminescence (PL) and time resolved PL were improved, which was attributed to the passivating properties of AlOx. A current increase of almost 2 mA/cm(2) was measured, due to increased light scattering and surface roughness. With time of flight-secondary ion mass spectroscopy, the elemental profiles were measured. It was found that the Ag is incorporated through the whole CIGS layer. Secondary electron microscopic images of the Mo back revealed residuals of the Ag/AlOx stack, which was confirmed by energy dispersive X-ray spectroscopy measurements. It is assumed to induce the increased surface roughness and scattering properties. At the front, large stains are visible for the cells with the Ag/AlOx back contact. An ammonia sulfide etching step was therefore applied on the bare absorber improving the efficiency further to 11.7%. It shows the potential of utilizing an Ag/AlOx stack at the back to improve both electrical and optical properties of ultrathin CIGS solar cells. This research was funded by European Union H2020 research and innovation program grant number 715027.

Cu2ZnGeSe4 (CZGSe) is a promising earth-abundant and non-toxic semiconductor material for large-scale thin-film solar cell applications. Herein, we have employed a joint computational and experimental approach to characterize and assess the structural, optoelectronic, and heterojunction band offset and alignment properties of a CZGSe solar absorber. The CZGSe films were successfully prepared using DC-sputtering and e-beam evaporation systems and confirmed by XRD and Raman spectroscopy analyses. The CZGSe films exhibit a bandgap of 1.35 eV, as estimated from electrochemical cyclic voltammetry (CV) measurements and validated by first-principles density functional theory (DFT) calculations, which predicts a bandgap of 1.38 eV. A fabricated device based on the CZGSe as a light absorber and CdS as a buffer layer yields power conversion efficiency (PCE) of 4.4% with V-OC of 0.69 V, FF of 37.15, and J(sc) of 17.12 mA cm(-2). Therefore, we suggest that interface and band offset engineering represent promising approaches to improve the performance of CZGSe devices by predicting a type-II staggered band alignment with a small conduction band offset of 0.18 eV at the CZGSe/CdS interface. SRR and NYD acknowledge the UK Engineering and Physical Sciences Research Council (EPSRC) for funding (Grant No. EP/S001395/1). This work has also used the Advanced Research Computing computational facilities at Cardiff (ARCCA) Division, Cardiff University, and HPC Wales. This work also made use of ARCHER facilities (http://www.archer.ac.uk), the UK's national supercomputing service via the membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202). YAJ is thankful to Savitribai Phule Pune University Post Doctorate Fellowship (SPPU-PDF) program (Grant No. SPPU PDF/ST/CH/2019/0004) and School of Energy Studies for financial support and laboratory facilities. DGB, GB, and BV acknowledge the European Union's Horizon 2020 Research and Innovation Program (Grant agreement No. 640868). SRR and BV acknowledge UHasselt-BOF for the mobility grant project R-8751 (https://www.uhasselt.be/UH/Research-groups/en-projecten_DOC/en-project_details.html?pid=16044&t=en). Rondiya, SR; Dzade, NY (corresponding author), Cardiff Univ, Sch Chem, Main Bldg,Pk Pl, Cardiff CF10 3AT, S Glam, Wales. Vermang, B (corresponding author), Hasselt Univ Partner Solliance, Inst Mat Res IMO, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; Imec Div IMOMEC Partner Solliance, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; EneryVille, Thorpk,Poort Genk 8310 & 8320, B-3600 Genk, Belgium. rondiyas@cardiff.ac.uk; dzadeny@cardiff.ac.uk; bart.vermang@uhasselt.be

Wide band gap thin-film kesterite solar cell based on non-toxic and earth-abundant materials might be a suitable candidate as a top cell for tandem configuration in combination with crystalline silicon as a bottom solar cell. For this purpose and based on parameters we have extracted from electrical and optical characterization techniques of Cu2ZnGeSe4 absorbers and solar cells, a model has been developed to describe the kesterite top cell efficiency limitations and to investigate the different possible configurations with transparent back contact for fourterminal tandem solar cell application. Furthermore, we have studied the tandem solar cell performance in view of the band gap and the transparency of the kesterite top cell and back contact engineering. Our detailed analysis shows that a kesterite top cell with efficiency > 14%, a band gap in the range of 1.5-1.7 eV and transparency above 80% at the sub-band gaps photons energies are required to achieve a tandem cell with higher efficiency than with a single silicon solar cell. The authors would like to acknowledge the SWInG project financed by the European Union's Horizon 2020 research and innovation programme under grant agreement No 640868 and the Research Foundation Flanders-Hercules Foundation (FWO-Vlaanderen, project No AUGE/13/16:FT-IMAGER). Khelifi, S (corresponding author), Univ Ghent, Dept Solid State Sci, Krijgslaan S1, B-9000 Ghent, Belgium. samira.khelifi@ugent.be

Several optoelectronic issues, such as poor optical absorption and recombination, limit the power conversion efficiency of ultrathin Cu(In,Ga)Se-2 (CIGS) solar cells. To mitigate recombination losses, two combined strategies were implemented: a potassium fluoride (KF) post-deposition treatment (PDT) and a rear interface passivation strategy based on an aluminum oxide (Al2O3) point contact structure. The simultaneous implementation of both strategies is reported for the first time on ultrathin CIGS devices. Electrical measurements and 1D simulations demonstrate that in specific conditions, devices with only KF-PDT may outperform rear interface passivation based devices. By combining KF-PDT and rear interface passivation, an enhancement in an open-circuit voltage of 178 mV is reached over devices that have a rear passivation only, and of 85 mV over devices with only a KF-PDT process. Time-Resolved Photoluminescence measurements showed the beneficial effects of combining KF-PDT and the rear interface passivation at decreasing recombination losses in the studied devices, enhancing charge carrier lifetime. X-ray photoelectron spectroscopy measurements indicate the presence of an In and Se-rich layer that we linked to be a KInSe2 layer. Our results suggest that when bulk and front interface recombination values are very high, they dominate, and individual passivation strategies work poorly. Hence, this work shows that for ultrathin devices, passivation mitigation strategies need to be implemented in tandem. Funding Fundaca̧ o para a Cie ̃ ncia e Tecnologia through grants IF/ ̂00133/2015, PD/BD/142780/2018, and SFRH/BD/146776/2019 and projects PDTC/CTM-CTM/28075/2017, PTDC/FISMAC/29696/2017, and UIDB/04730/2020 is acknowledged. European Union’s Horizon 2020 through grant nos. 720887 and 715027 and the Special Research Fund (BOF) of Hasselt University are also acknowledged. ACKNOWLEDGMENTS Fundaca̧ o para a Cie ̃ ncia e a Tecnologia (FCT) is acknowledged through the project IF/00133/2015, PD/BD/142780/2018, and SFRH/BD/146776/2019. The authors want to acknowledge the European Union’s Horizon 2020 research and innovation program under the agreement nos. 720887 (ARCIGS-M project) and 715027. The Special Research Fund (BOF) of Hasselt University, the FCT through the project NovaCell (PDTC/CTM-CTM/28075/2017), and InovSolarCells (PTDC/FISMAC/29696/2017) co-funded by FCT and the ERD through COMPETE2020 is also acknowledged. P. A. Fernandes would like to acknowledge FCT for the support of the project FCT UIDB/04730/2020.

In this work, first principle calculations of Cu$_2$ZnSnS$_4$ (CZTS), Cu$_2$ZnGeS$_4$ (CZGS) and Cu$_2$ZnSiS$_4$ (CZSS) are performed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for CZTS, CZGS and CZSS. In addition, absorption coefficient values of the order of $10^4$ cm$^{-1}$ are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results are used as input data to model the electrical power conversion efficiency of kesterite-based solar cell. In that perspective, we used an improved version of the Shockley-Queisser theoretical model including non-radiative recombination via an external parameter defined as the internal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.88, 19.94 and 3.11% are reported respectively for CZTS, CZGS and CZSS for vanishing non-radiative recombination rate. Using an internal quantum efficiency providing $V_{OC}$ values comparable to experimental measurements, solar cell efficiencies of 15.88, 14.98 and 2.66% are reported respectively for CZTS, CZGS and CZSS (for an optimal thickness of 1.15 $\mu$m). With this methodology, we confirm the suitability of CZTS in single junction solar cells, with a possible efficiency improvement of 10% enabled through the reduction of the non-radiative recombination rate. In addition, CZGS appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas CZSS might be interesting for transparent PV windows., Comment: 11 pages, 6 figures and 3 tables

A study of the electronic conduction mechanisms and electrically active defects in polycrystalline Sb2Se3 is presented. It is shown that for temperatures above 200 K, the electrical transport is dominated by thermal emission of free holes, ionized from shallow acceptors, over the intergrain potential barriers. In this temperature range, the temperature dependence of the mobility of holes, limited by the intergrain potential barriers, is the main contributor to the observed thermal activation energy of the conductivity of 485 meV. However, at lower temperatures, nearest-neighbor and Mott variable range hopping transport in the bulk of the grains turn into the dominant conduction mechanisms. Important parameters of the electronic structure of the Sb2Se3 thin film such as the average intergrain potential barrier height ϕ = 391 meV, the intergrain trap density Nt = 3.4 × 1011 cm−2, the shallow acceptor ionization energy EA0 = 124 meV, the acceptor density NA = 1 × 1017 cm−3, the net donor density ND = 8.3 × 1016 cm−3, and the compensation ratio k = 0, 79 were determined from the analysis of these measurements., P. M. P. Salomé acknowledges the funding of Fundação para Ciencêa e Tecnologia (FCT) through the project IF/00133/ ̂2015. This research is supported by the Development of novel ultrathin solar cell architectures for low-light, low-cost, and flexible optoelectronic devices project (028075) co-funded by FCT and ERDF through COMPETE2020. B. Vermang has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 715027). A. Shongalova acknowledges the funding of Erasmus + program 2016/17. This work was funded by FEDER funds through the COMPETE 2020 Programme and by FCTPortuguese Foundation for Science and Technology under the projects UID/CTM/50025/2013. The financial support from Brazilian funding agencies CNPq, CAPES, and FAPEMIG is also acknowledged.

In this work, hafnium oxide layer is investigated as rear surface passivation layer for ultra-thin (550 nm) CIGS solar cells. Point contact openings in the passivation layer are realized by spin-coating potassium fluoride prior to absorber layer growth. Contacts are formed during absorber layer growth and visualized with scanning electron microscopy (SEM). To assess the passivating qualities, HfOx was applied in a metal-insulator-semiconductor (MIS) structure, and it demonstrates a low interface trap density in combination with a negative density of charges. Since we used ultra-thin devices that are ideal to probe improvements at the rear, solar cell results indicated improvements in all cell parameters by the addition of 2 nm thick HfOx passivation layer with contact openings.

Numerical models are proposed that are able to describe the current-voltage (I-V) behaviour of two Cu2ZnGeSe4(CZGSe) solar cells measured under different illuminations at 300 K. In the model, the doping density of the CZGSe layer and the mobility of carriers are determined by capacitance-voltage (C-V) profiling and AC field Hall effect measurement, respectively. Some of the other parameters in the solar cells, including the series and the shunt resistance, the metal work function of the back contact, defect properties and absorption coefficient in the absorber layer, are determined using a differential evolution algorithm by fitting of the model with the experimentally measured I-V curves. Sensitivity analysis of our proposed model with SCAPS-1D demonstrates that the low metal work function, which results in a hole barrier at the back contact, the low shunt resistance and the high series resistance of the cell can explain the low fill factor and the low efficiency in these cells. This work is supported by the Special Research Fund (BOF) of Ghent Univeristy and the European Union 'Horizon 2020 research' and Innovation programme under Grant Agreement No. 640868. Yang, S (corresponding author), Univ Ghent, Dept Elect & Informat Syst, Technol Pk 126, B-9052 Zwijnaarde, Belgium. Sheng.Yang@ugent.be

We review the stability and reliability results of kesterite (Cu2ZnSn(S,Se)4, CZTSSe)-based solar cells and we complete the reviewed data with additional as yet unpublished data on these matters. We also review published and new data on upscaling and the possible technological applications for this material. Kesterite material is composed of mainly earth-abundant elements and is therefore very attractive for large-scale applications. Stability data are so far quite scarce and the main results are the accelerated aging tests carried out for CZTSSe monograin technology, as well as yet unpublished data on long indoor and outdoor irradiance tests carried out on thin-film CZTSSe technology deposited by a wet processing method. On upscaling and technological applications we point out the works on three main large-scale photovoltaic technologies (monograin, in-line vacuum thin film, and wet-deposited thin film), as well as some work on water-splitting applications. IMRA Europe and crystalsol thank their numerous co-workers for contributing to this work. The work carried out at cyrstalsol was partly supported by the European Union through the European Regional Development Fund and Archimedes/DoRa project TK141. IMRA Europe, Midsummer and Ayesa acknowledge the European Commision for the funding of the kesterite research from 2017 by the H2020 program under the project STARCELL (H2020-NMBP-03-2016-720907).

This review summarizes all studies which used dielectric-based materials as a passivation layer at the rear surface of copper indium gallium (di)selenide, Cu(In,Ga)Se2, (CIGS)-based thin film solar cells, up to 2019. The results regarding the kind of dielectric materials, the deposition techniques, contacting approaches, the existence of additional treatments, and current–voltage characteristics (J–V) of passivated devices are emphasized by a detailed table. The techniques used to implement the passivation layer, the contacting approach for the realization of the current flow between rear contact and absorber layer, additional light management techniques if applicable, the solar simulator results, and further characterization techniques, i.e., external quantum efficiency (EQE) and photoluminescence (PL), are shared and discussed. Three graphs show the difference between the reference and passivated devices in terms of open-circuit voltage (Voc), short-circuit current (Jsc), and efficiency (η), with respect to the thicknesses of the absorber layer. The effects of the passivation layer at the rear surface are discussed based on these three graphs. Furthermore, an additional section is dedicated to the theoretical aspects of the passivation mechanism. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027

We have studied the crystallization reaction of polycrystalline Cu2ZnGeSe4 solar cell absorbers fabricated by H2Se selenization of sequentially deposited metal layer stacks. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent X-ray diffraction patterns. We have found that mainly Cu3Ge and ZnSe phases form very rapidly at temperatures below 350 °C. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds. Putting the Ge layer in the bottom and the Cu layer on top leads to a very fast nucleation reaction of Cu2ZnGeSe4, leading to small grains that have obtained their final size already after 3 min of selenization at 460 °C. The inverse stack with Ge on top and Cu in the bottom delays the nucleation of Cu2ZnGeSe4, leading to a somewhat slower formation reaction and larger Cu2ZnGeSe4 grains, which obtain their final grain size only after 15 min of selenization at 460 °C. This research is partially funded by the Flemish government, Department Economy, Science and Innovation. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 640868. B. Vermang acknowledges the financial support of the Flemish Research Foundation FWO (mandate 12O4215N).

Kesterite solar cells based on Cu2ZnSnS4 and Cu2ZnSnS4 (CZTSe) are potential future candidates to be used in thin-film solar cells. The technology still has to he developed to a great extent and for this to happen, high levels of confidence in the characterization methods are required, so that improvements can he made on solid interpretations. In this study, we show that the interpretations of one of the most used characterization techniques in kesterites, scanning transmission electron microscopy (STEM), might be affected by its specimen preparation when using focused ion beam (FIB). Using complementary measurements based on scanning electron microscopy and Raman scattering spectroscopy, compelling evidence shows that secondary phases of ZnSe mixed in the bulk of CZTSe are the likely cause of the appearance of voids in STEM lamellae. Sputtering simulations support this interpretation by showing that Zn in a ZnSe matrix is preferentially sputtered compared with any metal atom in a CZTSe matrix. This work was supported in part by the FEDER funds through the COMPETE 2020 Programme, in part by FCT-Portuguese Foundation for Science and Technology under Project UID/CTM/50025/2013, in part by CAPES (CAPES-INL 04/14), in part by FAPEMIG, and in part by CNPq Brazilian agencies. The work of B. Vermang was supported by the European Research Council under the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement 715027. The work of P. M. P. Salome was supported in part by the Fundacao para Ciencia e Tecnologia (FCT) under Project IF/00133/2015 and in part by the NovaCell-Development of Novel Ultrathin Solar Cell Architectures for Low-Light, Low-Cost, and Flexible Opto-Electronic Devices Project (028075) co-funded by FCT and the ERDF under Grant COMPETE2020.

Room temperature photoluminescence (PL) and intensity dependent PL measurements were used as a tool to investigate alkali treatment (KF and RbF) of thin (< 500 nm) single-stage absorber layers and compared with similar 3-stage samples. Single-stage absorber layers have a clear distinguishable PL peak below the band gap, which is absent in 3-stage absorber layers. We attribute this peak to a defect band arising due to the planar defects and the small grains. We find that this low energy peak can be largely reduced with alkali treatment, leading to similar decay times and PL yield as thin 3 stage samples. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027

Reducing the absorber layer thickness below 1 μm for a regular copperindiumgalliumdi-selenide (CIGS)solar celllowers the minimum quality requirements for the absorber layer due to shorter electron diffusion length. Additionally, it reduces material costs and production time. Yet, having such a thin absorber reduces the cell efficiency significantly. This is due to incompletelight absorptionand high Molybdenum/CIGS rear-surface recombination [1]. The aim of this research is to implement some innovative rearsurface modificationson a 430 nm thick CIGS absorber layer to reduce both these affects: analuminium oxidepassivationlayer to reduce the back-surface recombination and point contact openings usingnano-particlesfor electrical contact. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analyzed in this paper.

In this study, CdS chemical bath deposition is investigated to improve the performance of thin film solar cell based on Cu2ZnGeSe4/CdS heterojunction. The influence of both the bath temperature and the dipping duration on the CdS thin film properties are explored thanks to the combination of scanning electron microscopy (SEM) and Raman spectroscopy, while the photovoltaic parameters of the resulting solar cells are discussed from current-voltage (I-V) and external quantum efficiency (EQE) measurements. The highest efficiency achieved herein (without antireflection coating) is 7.6%. Although it represents 35% relative improvement compared to previous best efficiency, this champion device is still limited by interface recombination. Different strategies are finally proposed to further increase the performance of these solar cells. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 640868.

Thin film solar cells based in Cu(In,Ga)Se-2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%. P.M.P.S. acknowledges the funding of Fundacao para Ciencia e Tecnologia (FCT) through the project IF/00133/2015. B.V. has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 715027). CAPES (CAPES-INL 04/14), CNPq, and FAPEMIG funding agencies are acknowledged for financial support. The European Union's Horizon 2020 research and innovation programme (grant agreement no. 720887) ARCIGS-M project is acknowledged. M.J.M. acknowledges funding from FCT through the grant SFRH/BPD/115566/2016. This project was partially supported by FEDER funds, through the COMPETE 2020 Program, and national funds, through the FCT, under the projects POCI-01-0145-FEDER-007688 (Reference UID/CTM/50025) and ALTALUZ (Reference PTDC/CTM-ENE/5125/2014). The authors also acknowledge partial funding from the European Project BET-EU (H2020-TWINN-2015, grant 692373). The NanoFabrication department at INL, namely Joao Gaspar and Helder Fonseca, are recognized for the help in the development of the nanopatterning process.

This work proposes gallium oxide grown by plasma-enhanced atomic layer deposition, as a surface passivation material at the CdS buffer interface of Cu(In,Ga)Se-2 (CIGS) solar cells. In preliminary experiments, a metal-insulator-semiconductor (MIS) structure is used to compare aluminium oxide, gallium oxide, and hafnium oxide as passivation layers at the CIGS-CdS interface. The findings suggest that gallium oxide on CIGS may show a density of positive charges and qualitatively, the least interface trap density. Subsequent solar cell results with an estimated 0.5nm passivation layer show an substantial absolute improvement of 56mV in open-circuit voltage (V-OC), 1mAcm(-2) in short-circuit current density (J(SC)), and 2.6% in overall efficiency as compared to a reference (with the reference showing 8.5% under AM 1.5G). The work published in this paper was supported by the European Research Council (ERC) under the Union's Horizon 2020 research and innovation programme (grant agreement No 715027). The authors would also like to thank Dr. Marcel Simor (Solliance) for the CIGS layer fabrication and Prof. Johan Lauwaert (Universtiy of Ghent) for his guidance on DLTS measurements.

Using the thermal annealing of evaporated metallic precursors in successive H2Se and H2S atmospheres, it was possible to reproducibly manufacture kesterite absorber material for solar cell applications with a sulfur content varying from 30% to 100%. Respective band gaps for these sulfur inclusions were measured at approximately 1.45 eV and 2.0 eV. A recipe was devised for which results could be reproduced within an error margin of +/- 5% and the influence of the H2S pressure during the post sulfurization was negligible on all measurable and observable parameters. The evolution of the S/Se ratio in the sample was observed to be linearly dependent on the annealing time. It was also observed that at very early stages of the post-sulfurization, both the original Cu-2(Zn,Ge)Se-4 (CZGSe) and a primary Cu-2(Zn,Ge)(S,Se)(4) (CZGSSe) phase with a sulfur inclusion of similar to 30% coexist in the sample. The (112) x-ray diffraction (XRD) reflection of the CZGSe phase progressively disappears in favor for the first mixed CZGSSe phase. Using grazing incidence-XRD, the S/Se ratio was shown to be in homogeneous. Indeed, the XRD measurement of the top layers led to the calculation of higher sulfur inclusions than was the case when measuring the bulk material. Top-scanning electron microscopy (SEM) as well as cross-SEM measurements were taken in order to determine the impact of the sulfur inclusion on the crystal growth and the overall quality of the produced absorber layers. The obtained images revealed a reduction in crystal size and the appearance of numerous holes in the layer as the S/Se ratio is increased. This work was performed within the framework of the SWInG project funded by the European Union's Horizon 2020 research and innovation program, grant agreement No. 640868. M. Neuwirth gratefully acknowledges financial support by the German Federal Ministry of Education and Research (BMBF, FKZ: 03SF0530B) and the Karlsruhe School of Optics and Photonics (KSOP).

In this work, the addition of molar concentrations of Na+ and/or K+ in the form of fluorides during the processing of Cu2ZnSnSe4 absorber layers was investigated. The beneficial effect of alkali salts on the performance of CZTSe solar cells has been demonstrated through physical and electrical characterization. A maximum power conversion efficiency of 8.3% has been obtained through controlled addition of Na+ and K+ in the CZTSe absorber as opposed to 6.7% from uncontrolled addition of both. The open circuit voltage in both cases was very much the same but the devices with controlled addition showed lower p-type doping, a low density of charged defects and less band tailing compared to the rest. This research is partially funded by the Flemish government, Department Economy, Science and innovation. This research has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 640868.

Ultrathin Cu(In,Ga)Se2 solar cells are a promising way to reduce costs and to increase the electrical performance of thin film solar cells. In this work, we develop an optical lithography process that can produce sub-micrometer contacts in a SiO2 passivation layer at the CIGS rear contact. Furthermore, an optimization of the patterning dimensions reveals constrains over the features sizes. High passivation areas of the rear contact are needed to passivate the CIGS interface so that high performing solar cells can be obtained. However, these dimensions should not be achieved by using long distances between the contacts as they lead to poor electrical performance due to poor carrier extraction. This study expands the choice of passivation materials already known for ultrathin solar cells and its fabrication techniques. published

Interface quality plays a key role in solar cell applications. Interface recombination at the front and rear surfaces, which determine this quality, have significant effects on open circuit voltage and fill factor values. In this work, several surface treatments were applied on Cu(In,Ga)Se2 (CIGS) surfaces to improve the interface quality. Besides, the passivation layer implementation was investigated to reduce interface recombination between the buffer and absorber layers. European Union’s H2020 research and innovation program under grant agreement No. 715027.

We investigated the use of ternary and quaternary chalcogenide compounds based on Cu, Zn, Sn and Si for use as high band gap absorber layers in thin film photovoltaics. We have investigated ;the fabrication of Cu2Zn(Sn,Si)Se-4, Cu2Si(S,Se)(3) and Cu8Si(S,Se)(6) thin film layers. Whereas, Cu2Zn(Sn,Si)Se-4 and Cu2Si(S,Se)(3) appeared to be difficult to fabricate, because the Si did not intermix well with the rest of the elements at the typical process temperatures used for glass substrates, Cu2ZnSiSe4 and Cu8Si(S,Se)(6) could be formed. The fabricated layers were polycrystalline with a typical thickness of about 1 mu m. We also fabricated solar cells with the different absorber materials, using a standard Mo back contact and CdS/ZnO buffer layer combination, but despite very bright photoluminescence response of the Cu8SiS6 and Cu8SiSe6 layers at an energy of about 1.84 and 1.3 eV respectively, the measured efficiencies remained below 0.1% due to particularly low photocurrents. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim This research is partially funded by the Flemish government, Department Economy, Science and Innovation. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 640868.

We have fabricated Cu2ZnSnSe4 (CZTSe) solar cells with different absorber layer thickness. Absorber layers with different thicknesses were fabricated by changing the thickness of e-beam evaporated Sn/Zn/Cu precursor stacks and then selenization in a rapid thermal processing system. Scanning electron microscopy revealed that by increasing the thickness the morphology of CZTSe films improves substantially and energy dispersive spectrometry measurements showed that the Cu to Sn ratio increased with increasing the film thickness, despite a similar Cu to Sn ratio in the starting layers. A longer minority carrier lifetime and higher open circuit voltage were achieved for solar cells with thicker absorber layers. A maximum conversion efficiency of 7.8% (without anti reflection coating) was achieved for a solar cell with 1.7 mu m thickness in which a low doping density of the order of 10(15) cm(-3) was measured, leading to a wide space charge region of about 300 nm. (C) 2016 Elsevier B.V. All rights reserved. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 640868. This research is partially funded by the Flemish government, Department Economy, Science and innovation. This work is also funded by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT - Portuguese Foundation for Science and Technology under the project UID/CTM/50025/2013. Samaneh Ranjbar acknowledges the financial support of the Portuguese Science and Technology Foundation (FCT) through PhD grant SFRH/BD/78409/2011. B. Vermang acknowledges the financial support of the Flemish Research Foundation FWO (mandate 1204215N).

Al2O3 rear surface passivated ultra-thin Cu(In,Ga)Se-2 (CIGS) solar cells with Mo nano-particles (NPs) as local rear contacts are developed to demonstrate their potential to improve optical confinement in ultra-thin CIGS solar cells. The CIGS absorber layer is 380 nm thick and the Mo NPs are deposited uniformly by an up-scalable technique and have typical diameters of 150 to 200 nm. The Al2O3 layer passivates the CIGS rear surface between the Mo NPs, while the rear CIGS interface in contact with the Mo NP is passivated by [Ga]/([Ga] + [In]) (GGI) grading. It is shown that photon scattering due to the Mo NP contributes to an absolute increase in short circuit current density of 3.4 mA/cm(2); as compared to equivalent CIGS solar cells with a standard back contact. (C) 2014 The Authors. Published by Elsevier B.V. This work is funded by the European Commission via FP7 MarieCurie IEF 2011 Action No. 300998, the Knut and Alice Wallenberg Foun-dation through Grant No. 2012.0083 and the Swedish Research Councilunder Grant No. 2008-6572 via the Linköping Linnaeus EnvironmentLiLi-NFM. Lastly, F.H.would like to thankthe European and Wallonia Re-gion FEDER Grant ECP12020011678F (MINATIS project) for thefinan-cial support

Ultra-thin Cu(In,Ga)Se-2 (CIGS) solar cells with an Al2O3 rear surface passivation layer between the rear contact and absorber layer frequently show a roll-over effect in the J-V curve, lowering the open circuit voltage (V-OC), short circuit current (J(SC)) and fill factor (FF), similar to what is observed for Na-deficient devices. Since Al2O3 is a well-known barrier for Na, this behaviour can indeed be interpreted as due to lack of Na in the CIGS absorber layer. In this work, applying an electric field between the backside of the soda lime glass (SLG) substrate and the SLG/rear-contact interface is investi-gated as potential treatment for such Na-deficient rear surface passivated CIGS solar cells. First, an electrical field of +50 V is applied at 85 degrees C, which increases the Na concentration in the CIGS absorber layer and the CdS buffer layer as measured by glow discharge optical emission spectroscopy (GDOES). Subsequently, the field polarity is reversed and part of the previously added Na is removed. This way, the J -V curve roll-over related to Na deficiency disappears and the V-OC (+25 mV), J(SC)(+2.3 mA/cm(2)) and FF (+13.5% absolute) of the rear surface passivated CIGS solar cells are optimized.

Random Al back surface field (BSF) p-type Si solar cells are presented, where a stack of Al2O3 and SiNx is used as rear surface passivation layer containing blisters. It is shown that no additional contact opening step is needed, since during co-firing local Al BSFs are induced at the location of these blisters. The best fill factors and short circuit currents are obtained in the case of (i) a hydrophobic pre-passivation cleaning, since it leads to a small density of larger blisters, and (ii) 10 nm of Al2O3, where the blistering size still increases during firing thanks to additional out-gassing. There is an apparent gain in J(SC) and V-OC of, respectively, 1.3 mA/cm(2) and 5 mV for the best random Al BSF cells compared to full Al BSF reference cells, because of better rear internal reflection and rear surface passivation. (C) 2012 Elsevier B.V. All rights reserved

26th European Photovoltaic Solar Energy Conference and Exhibition; 1052-1057, We observe a relationship between bias dependence and the process flow applied to produce passivated emitter rear contacted (PERC) cells. The first process flow utilizes a deposited silicon oxide layer, for both diffusion mask and passivation purposes. The second process flow uses the same silicon oxide diffusion mask, but it is removed after diffusion and a freshly deposited silicon oxide layer is used for passivation purposes. Cells that have been passivated with fresh silicon oxide result in a large bias dependence at ultra low bias levels below 10-1 sun, but saturate performance beyond 10-1 sun. Cells that have been passivated with the silicon oxide diffusion mask, an increase bias increases performance. For this process, we also observe a relationship between illumination level dependence and the diffusion recipe which densifies the oxide. With a 90 ohm/sq emitter there is an increase in collection in the UV wavelength range compared to a 60ohm/sq emitter. What is unexpected is that, in the case of the 90 ohm/sq device, there is also an increase in collection of wavelengths above 700nm under no bias compared to the 60ohm/sq device. The cells studied in this paper are fabricated using 148.25 cm2, 160μm p-type Cz-Si wafers with screen printed Ag front contacts and are rear-side passivated with a deposited rear silicon oxide/silicon nitride stack and area locally contacted with Al. The highest efficiency of the cells studied is 19.2 % with a Voc of 637mV, Jsc of 38.2 mA/cm2 and a fill factor of 79.1%

Atomic layer deposited (ALD) Al2O3 films on Cu(In, Ga)Se-2 (CIGS) surfaces have been demonstrated to exhibit excellent surface passivation properties, which is advantageous in reducing recombination losses at the rear metal contact of CIGS thin-film solar cells. Here, we report, for the first time, experimentally extracted electronic parameters, i.e. fixed charge density (Q(f)) and interface-trap charge density (D-it), for as-deposited (AD) and post-deposition annealed (PDA) ALD Al2O3 films on CIGS surfaces using capacitance-voltage (C-V) and conductance-frequency (G-f) measurements. These results indicate that the AD films exhibit positive fixed charges Q(f) (approximately 10(12) cm(-2)), whereas the PDA films exhibit a very high density of negative fixed charges Q(f) (approximately 10(13) cm(-2)). The extracted D-it values, which reflect the extent of chemical passivation, were found to be in a similar range of order (approximately 10(12) cm(-2) eV(-1)) for both AD and PDA samples. The high density of negative Q(f) in the bulk of the PDA Al2O3 film exerts a strong Coulomb repulsive force on the underlying CIGS minority carriers (n(s)), preventing them to recombine at the CIGS/Al2O3 interface. Using experimentally extracted Q(f) and D-it values, SCAPS simulation results showed that the surface concentration of minority carriers (n(s)) in the PDA films was approximately eight-orders of magnitude lower than in the AD films. The electrical characterization and estimations presented in this letter construct a comprehensive picture of the interfacial physics involved at the Al2O3/CIGS interface. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. This work is supported by FRS-FNRS Belgium, the Swedish Science Foundation (VR), and, the Swedish Energy Agency. B. Vermang acknowledges the financial support of the Flemish Research Foundation FWO (mandate 12O4215N). R. Kotipalli would like to thank the teams of the nanofabrication-shared facility WINFAB and the electrical characterization WELCOME platforms at UCL for their technical support

We have fabricated and characterized Cu8SiS6 and Cu8SiSe6 thin film layers for optoelectronic applications. The layers were fabricated using a two step process by first evaporating a bilayer of Cu/Si on a Mo/glass substrate, followed by an anneal at temperatures in excess of 480 degrees C in a H2S or H2Se containing atmosphere. Different process conditions with different Cu starting layer thickness, different H2S partial pressure and different anneal temperatures and times were evaluated. The best fabricated layers were thin polycrystalline layers with a relatively low amount of secondary phases. Relatively intense photoluminescence signals with a full width at half maximum of about 150 meV could be measured on the samples with a peak position of 1.84 and 1.33 eV for the Cu8SiS6 and Cu8SiSe6 materials respectively. The absorption coefficient of the Cu8SiS6 layer was determined from absorption and reflection measurements and was larger than 10(4) cm(-1) at energies above 1.8 eV. Solar cells could not be fabricated so far, because the material appears to be very highly doped, leading to large ohmic leakage behavior in the fabricated solar cell structures. (C) 2016 Elsevier B.V. All rights reserved. Samira Khelifiand Johan Lauwaert are acknowledged for the Hallmeasurements of sample A2(G). This research is partially fundedby the Flemish government, Department Economy, Science and Innova-tion. This project has received funding from the European Union'sHorizon 2020 research and innovation program under grant agreementNo 640868. B. Vermang acknowledges thefinancial support of theFlemish Research Foundation FWO (mandate 12O4215N)

It is well established that the addition of sodium (Na) to chalcopyrite or kesterite based solar cells markedly increases the solar cell performance. In this work, we explore the effect of Na and other alkali metals like potassium (K), rubidium, caesium and lithium (Li) - on pure selenide Cu2ZnSnSe4 (CZTSe) solar cells. We demonstrate the deposition of alkali metals using spin coating on e-beam evaporated metal precursors. The stack of metal precursors with alkali layer was then selenised at high temperatures to obtain a good quality CZTSe absorber. The diffusion of alkali metals into the absorber layer was confirmed using glow discharge optical emission spectroscopy. Samples doped with Na or K have shown improvement in the open circuit voltage. A maximum power conversion efficiency of 8.3% (without anti-reflection coating) and a top open circuit voltage 430 mV was achieved for combination of K and Na. Amongst the rest of alkali metals, Li looks the most promising dopant as far as optoelectronic properties are concerned. (C) 2016 Elsevier B.V. All rights reserved. European Union's Horizon 2020 research and innovation program [640868]; Flemish Government, Department Economy, Science and Innovation; Flemish Research Foundation FWO [12O4215N]

We present a (1-D) SCAPS device model to address the following: (i) the surface passivation mechanisms (i.e. field-effect and chemical), (ii) their impact on the CIGS solar cell performance for varying CIGS absorber thickness, (iii) the importance of fixed charge type (+/-) and densities of fixed and interface trap charges, and (iv) the reasons for discrete gains in the experimental cell efficiencies (previously reported) for varying CIGS absorber thickness. First, to obtain a reliable device model, the proposed set of parameters is validated for both field-effect (due to fixed charges) and chemical passivation (due to interface traps) using a simple M-I-S test structure and experimentally extracted values (previously reported) into the SCAPS simulator. Next, we provide figures of merits without any significant loss in the solar cell performances for minimum net- Qf and maximum acceptable limit for Dit, found to be similar to 5 x 10(12) cm(-2) and similar to x 10(13) cm(-2) eV(-1) respectively. We next show that the influence of negative fixed charges in the rear passivation layer (i.e. field-effect passivation) is more predominant than that of the positive fixed charges (i.e. counter-field effect) especially while considering ultra thin (

Modern solar cell designs include an always larger variety of elements and additional layers. This approach is usually successful and leads to the development of increasingly efficient materials. However, scientifically, it makes the drawing of accurate conclusions always more challenging, due to the growing number of elements, possible defects, interfaces, and barriers. To try and remedy this problem, we developed a novel way to investigate solar cells by representing bias dependent admittance spectroscopy (CVf) measurement data in the form of a 2D loss map. In this contribution, we elaborate how this technique can be used experimentally, present some concrete results we obtained from measurements on ultra-thin CIGS solar cells and explain how they can be interpreted. We identify 3 major response domains on our typical CVf loss maps. One at high frequency that covers most of the bias range and can be related to series resistance. One at low frequency, mostly visible at strong positive and negative biases, which is related to shunt and dissipation. The last response domain, close to 100kHz and impacting most of the bias range, can be identified as a defect response of the material. Using CVf measurements on samples with KF postdeposition treatment, known for its grain boundarypassivationproperties, and samples that were previously submitted to accelerated lifetime testing in damp heat conditions, the impact of bulk defects, grain boundaries andconduction bandoffsets are investigated.

This work presents a novel method of local contact openings formation in an aluminum oxide (Al2O3) rear surface passivation layer by the selenization of the lithium fluoride (LiF) salt on top of the Al2O3 for ultra-thin copper indium gallium (di)selenide (CIGS) solar cells (SCs). This study introduces the potentially cost-effective, fast, industrially viable, and environmentally friendly way to create the nano-sized contact openings with the homogeneous distribution in the thick, i.e. up to 30 nm, Al2O3 passivation layer. The passivation layer is deposited by atomic layer deposition, while the LiF layer is spin-coated. Selenization is done in the H2Se atmosphere and the optimal process parameters are deduced to obtain nano-sized and uniformly allocated openings as confirmed by scanning electron microscopy images. The contact openings were produced in the different thicknesses of the alumina layer from 6 nm to 30 nm. Furthermore, the Al2O3 rear surface passivation layer with the contact openings was implemented into ultra-thin CIGS SC design, and one trial set was produced. We demonstrated that the created openings facilitate the effective current collection through the dielectric Al2O3 layer up to 30 nm thick. However, the upper limit of Al2O3 thickness in which the contact openings can be created by the described method is not established yet. The produced passivated CIGS SCs show increased external quantum efficiency response due to the optical enhancement of the passivated cells. However, the production of SCs on the Al2O3 passivation layer with the openings created by selenization of LiF is not optimized yet. This work received funding from the European Union's H2020 research and innovation program under Grant Agreement No 715027. Birant, G (corresponding author), Hasselt Univ Partner Solliance, Inst Mat Res IMO, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; Imec Div IMOMEC Partner Solliance, Wetenschapspk 1, B-3590 Diepenbeek, Belgium ; EnergyVille, Thorpk,Poort Genk 8310 & 8320, B-3600 Genk, Belgium. gizem.birant@imec.be

Passivation of the Cu(In,Ga)Se-2 (CIGS)/Mo back contact using AlOx is studied to reduce the recombination at this interface. Herein, RbF postdeposition treatment (RbF-PDT), a well-established method to improve absorber and front interface properties is used on back-passivated solar cells. It is found that this combination deteriorates the performance due to formation of an injection barrier at the front and reduced acceptor concentration. Photoluminescence yield and decay times show no indication of increased defect recombination, as both are improved. With time-of- flight secondary ion mass spectroscopy, in-depth and lateral alkali profiles are measured. It is shown that the Na concentration is higher at the AlOx/ Mo back contact and that Rb accumulates at the CdS/CIGS interface. It is hypothesized that Na at the back is released during the RbF-PDT and inhibits Rb diffusion into the CIGS layer. Rb remains at the front and acceptor concentration is reduced. Modeling of dark and light current-voltage characteristics shows that the injection barrier and low doping are responsible for the reduced Voc and fill factor (FF). It is suggested that the commonly observed FF losses upon heavier alkali PDT can be eliminated by adapting the initial Na amount. This work was supported by the European Research Council (ERC) under the Union's Horizon 2020 research and innovation program (grant agreement no. 715027). The authors thank Photon etc. for the hyperspectral PL measurements. R.T.E. and J.M. were supported by the Research Foundation – Flanders (FWO) (grant no. G031416N). ToF-SIMS was rendered possible due to a grant awarded to UHasselt and IMEC by Hercules Foundation (now FWO, grant no. ZW/13/07, awarded to J.M., among others).

Herein, using the spin-coating process as a sodium pre-deposition technique for Cu(In,Ga)Se-2, ultrathin film solar cells is discussed. Throughout this study, several challenges are detected to overcome, such as unevenly deposited salt layers and unplanned clusters (even islands) of sodium salts due to accumulation. The solar cell performances with the optimal and nonoptimal amounts of sodium are shared to assess the effect of varying sodium amounts on performance. The hyperspectral photoluminescence (PL) mapping and imaging and reflection measurements are also used to emphasize the importance of homogeneity of the sodium layer. All the authors thank to Laura-Isabelle Dion-Bertrand (Photon etc.) for hyperspectral measurement. This work received funding from the European Union's H2020 research and innovation program under Grant Agreement no. 715027.