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This work provides a rapid overview for the current state of surface passivation layer schemes for thin film solar cells: From its fundamentals to solar cell applications, and their perspective. It provides an overview of important literature and prospect considerations based on simulations. ; European Research Council (ERC) under the European Union [715027]

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]

Room temperature photoluminescence (PL) is a powerful technique to study the properties of semiconductors. However, the interpretation of the data can be cumbersome when non-ideal band edge absorption takes place, as is the case in the presence of potential fluctuations. In this study, PL measurements are modeled to quantify potential fluctuations in Cu ( In , Ga ) Se 2 (CIGS) absorber layers for photovoltaic applications. Previous models have attributed these variations to either bandgap fluctuations (BGFs) or electrostatic fluctuations (EFs). In reality, these two phenomena happen simultaneously and, therefore, affect the PL together. For this, the unified potential fluctuation (UPF) model is introduced. This model incorporates the effect of both types of fluctuations on the absorptance of the material and subsequently the PL spectra. The UPF model is successfully used to fit both single- and three-stage co-evaporated ultrathin (around 500 nm) CIGS samples, showing a clear improvement with respect to the previous BGF and EF models. Some PL measurements show possible interference distortions for which an interference function is used to simultaneously correct the PL spectra of a sample measured with several laser excitation intensities. All the models used in this work are bundled into a user-friendly, open-source Python program.& nbsp;Published under an exclusive license by AIP Publishing ; Dr. J. de Wild and Professor B. Vermang received funding from the European Union's H2020 research and innovation programme under Grant Agreement No. 715027 for this work.

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.

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.

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.

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.

In this study, an ultra-thin MoO3 layer synthesized by a solution-based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open-circuit voltage compared to the reference solar cells. Temperature dependent current density-voltage measurement indicated that the activation energy (E-A) of the main recombination is higher (approximate to 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where E-A approximate to 770meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved. ; This project has received funding from the European Union's Horizon 2020 research and innovation program 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. This work is also supported by JSPS Core-to-Core Program, A. Advanced Research Networks. S. 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 12O4215N).

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).

On the way to achieving high efficiencies for CuIn0.7Ga0.3Se2 (CIGS) thin film solar cells, alkali doping has become a necessary step. However, it is expected that a migration of the various alkali atoms over the lifetime of the cell will lead to efficiency losses [1]. Logically, ensuring the reliability of a solar cell material is a paramount step towards the commercialization of a product. Indeed, it is necessary for companies to able to guarantee the efficiencies of the "as-sold" panels over periods of time that expand well over 20 or even 25 years. Experiments studying the reliability of the CIGS solar cells and their different components have already been performed in the past [1-4], nevertheless, the exact effects of alkali, and more generally, the precise reasons for, and effect of, the degradation, are still widely unknown. In order to evaluate the viability of some of the most common techniques used to produce highly efficient thin-film solar cells, ultra-thin coevaporated CIGS solar cell absorber material is produced and doped with various alkali atoms (Na, K, .). The absorber material is deposited in ultra-thin (400-500nm) layers on soda-lime glass (SLG) equipped with a Si(O,N) diffusion barrier. By reducing the amount of bulk material, the aim is to reduce the concentration of defects in the material and limit their effects as much as possible. The barrier prevents the diffusion of alkali atoms from the SLG into the CIGS and allows for a better control of the concentration of alkali atoms in the layer. Additionally, it allows for a better isolation of the effects of the different alkalis by preventing an uncontrolled mix of alkali from being present in the absorber material. All alkalis are deposited in their fluoride form (NaF, KF, etc.) either by evaporation, prior to CIGS deposition, or by a post deposition treatment. We submitted the produced cells to highly degrading conditions in a damp heat environment. This aggressive setting is expected to simulate standard outdoor condition equivalent to 20 years in an experimentally reasonable timespan of a 1000 hours. We monitored the effects of the degradation by regular current-voltage (IV) measurements all along the 1000h experiment. These measurements showed a clear, and alkali concentration dependent, degradation of the solar cells as time increases. Using Atom Probe Tomography (APT) on degraded as well as on undegraded (reference) cells, it was possible to show that in the absence of a potential, the alkali atom migration is minimal, and that K can be mostly found in the grain boundary region of the absorber material. Given the lack of the expected alkali migration, the main reason for the degradation of the cells seems to be the presence of water that seeps into the grain boundaries of the solar cell material, which could also be observed using APT. The possibility to reverse that degradation mechanisms involving water inclusion is currently under investigation. ; This work received funding from the European Union's H2020 research and innovation program under grant agreement No. 715027

Producing the green energy of tomorrow will require highly efficient as well as energy -, and cost-effective solar cells in addition to having reasonable lifetimes To determine if CIGS can be made to submit to these constraints, we produced ultra thin (500nm ) single stage coevaporated CIGS solar cells. We doped these cells with varying amounts and t ypes of a lkali atoms a nd submitted them to accelerated lifetime testing Results showed definite effect of the alkali concentration on the degradation of the cells but showed limited migration. Instead, the seeping of water into the grain boundaries was identified as the main culprit for performance degradation. ; This work received funding from the European Union's H2020 research and innovation program under grant agreement No. 715027

Reducing the thickness of Cu(In,Ga)Se2 (CIGS) absorber layers has potential to decrease its cost significantly, but has drawbacks like incomplete absorption and increased back-contact recombination, both resulting in power conversion efficiency losses. One solution is to implement a rear surface passivation layer, which has potential to reduce rear surface recombination velocity and increase rear internal reflection [1]. Alumina (Al2O3) is such an ideal passivation layer, but unfortunately also acts as an electron and diffusion barrier layer, and thus prevents current flow and sodium (Na) diffusion. As is discussed below, our novel approach to generate point contact openings in this passivation layer overcomes both problems at once. The proposed method is to use sodium fluoride (NaF) on top of the Al2O3 passivation layer, which will generate contact openings during selenization. This applied approach is industrially viable, as compared to proven methods – e.g. using nanoparticles, e-beam or nano-imprint lithography – which are too expensive, time-consuming or not applicable for larger areas. D. Ledinek et al. previously proposed using a very thin layer of Al2O3 rear surface passivation in combination with NaF deposition to enhance the electrical characteristics of CIGS solar cells. In their study this surface passivation is claimed to allow tunneling [2]. In our study, we prove that point contacts have been generated in this Al2O3 surface passivation layer. Atomic layer deposition (ALD) was used to deposit very thin Al2O3 layers on molybdenum (Mo) rear contact, and NaF was deposited on these layers by spin coating. Thereafter CIGS layers (±500 nm thick) were grown by single stage co-evaporation at 550°C, followed by standard solar cell process described in [3]. To detect the contact openings in the passivation layer, scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) spectroscopy was applied. Glass/Mo/Al2O3/NaF characterization samples were used for this analysis, where the co-evaporation process was mimicked by using a selenization step. After the selenization, point openings in the thin Al2O3 passivation layers were determined by SEM, and supported by EDX measurement. To investigate the impact of Al2O3 thickness on passivation of the CIGS rear surface, time resolved photo-luminescence (TR-PL) measurements on finished absorber layers (capped with CdS) were used. According to these TR-PL results, a 6 nm thick passivation layer gave the highest free charge carrier lifetime and PL response in all sets of samples. This was confirmed in full solar cell devices, where a significant increase in power conversion efficiency, and gain in open-circuit voltage, current density and fill-factor values was measured for the 6 nm Al2O3 rear passivated CIGS cells, as compared to unpassivated reference cells. Hence, it can be concluded that by using a simple, cost-effective and fast way, i.e. ALD for Al2O3 and spin coating for NaF, we succeeded to make point contact openings to passivate the back surface of ultra-thin CIGS solar cells. The ongoing work focuses on different aspects: (i) At present the point openings are still submicron size, now we try to make them nano-size. (ii) We also concentrate on controlling the density of the contact openings, e.g. by changing the temperature of ALD process or the molarity of NaF solution. (iii) This approach also enables us to investigate different metal oxides as passivation layers, such as HfO2 and TiO2, in combination with the same contact opening approach. (iv) And finally, we are investigating this rear surface passivation approach for other absorber layer deposition/growth techniques, e.g. a 2-step sputtering + selenization method. ; This work received funding from the European Union's H 2020 research and innovation program under grant agreement No 715027.

In this work, we used a solution-processed TiO2 layer between Cu2ZnSnSe4 and CdS buffer layer to reduce the recombination at the p-n junction. Introducing the TiO2 layer showed a positive impact on VOC but fill factor and efficiency decreased. Using a KCN treatment, we could create openings in the TiO2 layer, as confirmed by transmission electron microscopy measurements. Formation of these openings in the TiO2 layer led to the improvement of the short-circuit current, fill factor, and the efficiency of the modified solar cells. ; This work was supported in part by the European Union's Horizon 2020 research and innovation program under Grant 640868, in part by the Flemish government, Department Economy, Science and Innovation, in part by the FEDER funds through the COMPETE 2020 Programme, and in part by the National Funds through FCT - Portuguese Foundation for Science and Technology under the project UID/CTM/50025/2013. The work of S. Ranjbar was supported by the Portuguese Science and Technology Foundation through Ph.D. grant SFRH/BD/78409/2011. The work of B. Vermang was supported by the Flemish Research Foundation FWO (mandate 12O4215N). (Corresponding author: Samaneh Ranjbar.)

Current state-of-the-art Cu2ZnSn(S,Se)(4) kesterite solar cells are limited by low open-circuit voltages (V-OC). In order to evaluate to what extent the substitution of Sn by Ge is able to result in higher V oc values, this article focuses on Cu2ZnGeSe4 "CZGSe" devices. To reveal their full potential, different strategies are explored that, in particular, aim at the optimization of the CZGSe/buffer heterojunction. Here, employing hard X-ray photoelectron spectroscopy, it is evidenced that only a combination of different surface treatments is able to remove all detrimental secondary phases. Further improvements are achieved by establishing a solar cell heat treatment in air. A systematic study of the impact of different annealing temperatures and durations determines the best heat treatment parameters to be 60 min at 200 degrees C. Also, Zn(O,S,OH) as a more transparent alternative to the heavy-metal compound CdS buffer layer has been realized. Combining all of the strategies, solar cells with 8.5 and 7.5% total area efficiency have been prepared, which is a record for Sn-free kesterite solar cells and any kesterite solar cell with a Zn(O,S,OH) buffer, respectively. Beyond these records, this work clearly confirms the emerging trend that Ge-for-Sn substitution is a successful strategy to improve the V-OC of kesterite solar cells. ; This project has received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement no. 640868. ; Choubrac, L (corresponding author), Univ Nantes, UMR6502, CNRS, Inst Mat Jean Rouxel IMN, F-44300 Nantes, France; Helmholtz Zentrum Berlin Mat & Energie GmbH, Dept Struct & Dynam Energy Mat, D-14109 Berlin, Germany. leo.choubrac@helmholtz-berlin.de

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