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The incorporation of Ge into kesterite thin-film absorbers for photovoltaic (PV) applications is thoroughly reviewed. Kesterite materials constitute a promising and critical raw material-free alternative to other inorganic thin-film PV compounds such as Cu(In,Ga)Se2and CdTe. The interest in Ge alloying is to solve the critical open-circuit voltage (Voc) deficit for kesterite solar cells, which is still unresolved and under strong debate. First, the substitution of Sn with Ge from 0 to 100% in the composition of Cu2Zn(Sn1−x,Gex)(Sy,Se1−y)4absorbers is largely discussed, with a complete overview on the existing literature. It is concluded that fine composition tuning is essential to ensure the Ge-induced enhancement of morphology and single-phase growth through a modified reaction pathway. In this regard, choosing between vacuum- and non vacuum-based deposition methods also plays a crucial role, since the former allows further up-scaling whereas the latter leads to higher thin-film quality. Second, at the solar cell level, Se-rich devices with less than 40% Ge currently exhibit the lowestVocand fill factor deficits with the highest efficiencies beyond 13%. This is mainly due to defect and band tail compensation as well as a graded bandgap and enlarged crystalline grains. This study unveils encouraging prospects for Ge-boosted kesterite PV devices.

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.

Relatively fast achievements in the kesterite solar cell technology have been made over the last decade, but the experimental efficiency is still ∼13%. One proposed reason is an inappropriate band alignment with Cu2ZnSnS4 (CZTS) and CdS that results in strong interface recombination losses. Results of this work show that the temperature and duration of air annealing of the CZTS/CdS heterojunction are essential for device performance. Soft annealing slightly improved the device efficiency due to the elemental intermixing at the interface. On the other hand, extended annealing increased absorber band gap energy, resulting in higher VOC values, indicating the improved Cu–Zn ordering in the CZTS structure, which also could be expected to have a beneficial influence on the device performance. However, interface analysis revealed that the CZTS absorber surface layer was Cu-rich, providing the reason for the reduction in CZTS solar cell performance. The effect of annealing on the interface defects was analyzed by the capacitance–frequency–voltage (C–V–f) analysis combined with SCAPS simulations. C–V–f-based loss maps showed that air annealing modifies the density distribution of asymmetrical interface states at the CZTS/CdS interface, which becomes fully symmetrical for longer annealing times at 200 °C.

Tandem solar cells present a promising solution to overcome the Schottky–Queisser efficiency limit of single-junction solar cells. In this article, an all–thin-film tandem solar cell based on perovskite (PK) top cell and chalcopyrite Cu (In, Ga) Se2 (CIGS) bottom cell is researched. Device optical simulations are validated on the top and bottom cells and employed for the analysis of PK/CIGS tandem cells. In particular, the optical effects of introduced laser reduced graphene oxide (rGO) layers at two positions in the tandem cell: 1) at the position between the top PK and bottom CIGS cell and 2) underneath the front transparent electrode. The purpose of introducing rGO layers is to improve the optoelectrical properties of the device, based on the tunable electronic and optical characteristics of rGO layers. Optical simulation results show that the parasitic absorption in rGO layers may noticeably affect the optical performance of the tandem cell if the layers are not optimized. The use of a thin and a few nanometer-thick rGO is suggested from the analysis if its parasitic absorption is not reduced. Directions for further optimization of optical rGO, including the reduction of parasitic absorption and tuning of the real part of the refractive index, are performed.

The implementation of anoxide layerwith contact openings to the Cu(In,Ga)Se2(CIGS) interfaces has become especially popular to reduce the recombination losses, resulting in a higher open-circuit voltage. While implementing this approach on the back surface is straightforward, it is more complicated for the front surface due to the roughness of the absorber as well as material selection constraints imposed by further processing steps. In this contribution, an AlOx/HfOxmulti-stack oxide layer with contact openings is applied between the CIGS and CdS layers. Two different approaches to create contact openings in multi-stack oxide layer are presented, and their advantages and disadvantages are investigated. In the bottom-up approach (BU), a NaCl salt pattern is created on the CIGS absorber surface, while in the top-down approach (TD), the pattern is created on the AlOxlayer surface. Time-resolved photoluminescence (TR-PL) shows that the multi-stack design improves the PL decay time regardless of the approach, but when the CdS layer is deposited, the PL decay time decreases. A more severe decrease is observed in case of the BU approach. Capacitance-Voltage measurements show that there is no impact on doping level when the NaCl pattern is created on an AlOxsurface. This work shows that any oxide material, even if not chemically resistant to the chemical bath deposition, can be applied at the CIGS/buffer layer interface, and additionally, any template can be used to create contact openings while preventing interaction between the material used for patterning and the CIGS, using the TD approach.

In the present contribution, we have measured and simulated room temperature bias- and frequency-dependent capacitances of thin-film solar cell devices. The results of both the simulations and experimental measurements are represented as 2-D contour plots showing the derivative of the capacitance with respect to the frequency multiplied by the frequency. These plots are called “loss maps,” because responses in these contour plots correspond to responses of different nonidealities in the devices. Using a 1-D drift-diffusion solver (SCAPS), we have simulated the responses of different nonidealities of the solar cell devices, such as series resistance, bulk defects, interface defects, back contact barrier, and absorber-buffer barrier. We have shown that some nonidealities have a quite recognizable trace in the loss map. Other nonidealities on the other hand show responses that look quite similar in the bias voltage and frequency space, making exact conclusions on the nature and position of the defect responses in thin-film solar cells most of the times difficult. We have compared the simulated results with experimental measurements of one of our Cu(In,Ga)Se2 (CIGS) solar cell devices and came to the conclusion that there is likely a bulk defect or a spike-like barrier at the CIGS-CdS interface present in our particular device. The loss map can, in some cases, be useful in order to analyze admittance spectroscopy data in a graphical and relatively intuitive way.

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

Thin (640 mV were achieved after the KF treatment. From SCAPS simulations and temperature dependent current voltage-measurements, it is concluded that the V-oc of these cells are limited by backcontact surface recombination and, thus, further improvements require passivation of the back contact. This work was supported by the European Union’s H2020 Research and Innovation Program under Grant 715027.

Herein, using the spin-coating process as a sodium pre-deposition technique for Cu(In,Ga)Se2, 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.

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.

This report highlights the combination of the MicroTime 100 upright confocal fluorescence lifetime microscope with a Single Quantum Eos Superconducting Nanowire Single-Photon Detector (SNSPD) system as a powerful tool for photophysical research and applications. We focus on an application in materials science, photoluminescence imaging, and lifetime characterization of Cu(InGa)Se-2 (CIGS) devices intended for solar cells. We demonstrate improved sensitivity, signal-to-noise ratio, and time-resolution in combination with confocal spatial resolution in the near-infrared (NIR) range, specifically in the 1000-1300 nm range. The MicroTime 100-Single Quantum Eos system shows two orders of magnitude higher signal-to-noise ratio for CIGS devices' photoluminescence imaging compared to a standard NIR-photomultiplier tube (NIR-PMT) and a three-fold improvement in time resolution, which is now limited by the laser pulse width. Our results demonstrate the advantages in terms of image quality and time resolution of SNSPDs technology for imaging in materials science.(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). PicoQuant and Single Quantum thank the European Union’s Horizon 2020 research and innovation program [ATTRACT Phase 2 (Grant Agreement No. 101004462), project: MicroQuaD] for the financial support of this work.

Interface recombination is one of the factors limiting the performance of Cu(In,Ga)Se2(CIGS). Especially in the absence of band grading at the front and rear surface, interface passivation approaches become important to improve device performance. The integration of an oxide layer as passivation layer at the front surface of the CIGS requires meticulous considerations in order not to impact the further steps of the solar cell production. In this article, a novel approach is reported to try to tackle the problem of interface recombination at the front surface of CIGS without affecting further solar cell production steps. In this approach, an Al2O3/HfO2multi-stack layer with contact openings is applied. NaCl template patterning with preliminarily selected parameters was used to create a homogeneous pattern of contact opening on the CIGS surface and allow the current flow in the device. After the removal of the NaCl islands, the holes in the multi-stack (openings) were visualized by scanning electron microscopy. In addition, energy-dispersive X-ray spectroscopy (EDS) was performed before and after chemical bath deposition of the buffer layer. The EDS result confirmed that the undesired etching of the Al2O3layer during buffer layer deposition was prevented by using a thin HfO2layer. Solar cells were produced by using preliminarily selected parameters for the multi-stack design. As a result, without having a significant negative impact on the solar cell parameters, a device design was achieved which is almost comparable with the reference device. In addition, options for future improvement and development are discussed.

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]

Presented (oral presentation) at the Virtual Workshop on Transparent PV: Challenges and Opportunities that was co-organised on June 11th 2021 by the Impressive and Tech4Win European projects. The presentation reviewed the main results achieved in Tech4Win (at June 2021) on the development of inorganic UV multifunctional coatings combining UV filtering and PV functionalities

Presented (as oral contribution) at the 11th European Kesterite Workshop, 26-27 November2020. Transparent photovoltaics (PV) could be a solution for integrating large amount of PV in buildings. A possible approach could be a tandem cell using the combination of IR-absorbing organic bottom cells with UV-absorbing top cells. The UV top cell would then serve as a solar cell, converting UV light into electrical energy, as well as a UV filter to protect the bottom organic PV cell from degradation from the UV light. We have developed three potential UV absorber materials: ZnS, ZnSe and Zn(S,Se). All layers were fabricated by annealing an evaporated or sputtered Zn layer in an atmosphere of H2S for sulfurization, H2Se for selenization or a mix of the two for the mixed sulfo-selenization. Typical annealing temperatures and times for this crystallization process are 450°C and 10 minutes. In this presentation we report on the optoelectronic properties of the layers and we show that the combined Zn(S,Se) shows the ideal band gap of about 3 eV combining both decent colour and good UV filtering properties. Analysis of the conductivity of the layers is also presented, together with some initial solar cell structures with transparent front and back contacts.

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.

A simplified Cu(In, Ga)Se-2 (CIGS) solar cell structure based on a 500 nm thin CIGS layer is presented. The absorber layers are grown with a single-stage coevaporation process, and various KF post-deposition treatments (KF-PDT) are performed. The KF-PDT leads to an efficiency increase from 7% to 12%. For all cells an increase in open circuit voltage (V-oc) and fill factor is measured, which is attributed to an improved pn junction. By changing the annealing conditions, an additional V-oc increase is measured. This increase is attributed to the reduction of light-induced defects at the CIGS/CdS interface in addition to the improved pn junction. A reduction of defects is confirmed by reduced sub band gap emission in the photoluminescence spectra, an increased decay time, and increased quasi Fermi level splitting. With SCAPS the results are simulated, and it is concluded that after KF-PDT the V-oc is limited to 640 mV due to recombination at the back contact. A higher V-oc can then only be achieved by applying a passivation layer at the back. There are no indications that the single-stage process is limiting the efficiency, revealing the potential of the proposed simplified CIGS structure and the importance of interfaces for ultrathin CIGS solar cells. J. de Wild, D.G. Buldu, T. Kohl, G. Birant, and B. Vermang received funding from the European Union’s H2020 research and innovation program under grant agreement no. 715027.

This work reports on developments in the field of wide band gap Cu2ZnXY4 (with X = Sn, Si or Ge, and Y = S, Se) kesterite thin film solar cells. An overview on recent developments and the current understanding of wide band gap kesterite absorber layers, alternative buffer layers, and suitable transparent back contacts is presented. Cu2ZnGe(S,Se)(4) absorbers with absorber band gaps up to 1.7 eV have been successfully developed and integrated into solar cells. Combining a CdS buffer layer prepared by an optimized chemical bath deposition process with a 1.36 eV band gap absorber resulted in a record Cu2ZnGeSe4 cell efficiency of 7.6%, while the highest open-circuit voltage of 730 mV could be obtained for a 1.54 eV band gap absorber and a Zn(O,S) buffer layer. Employing InZnOx or TiO2 protective top layers on SnO2:In transparent back contacts yields 85-90% of the solar cell performance of reference cells (with Mo back contact). These advances show the potential as well as the challenges of wide band gap kesterites for future applications in high-efficiency and low-cost tandem photovoltaic devices. This project has received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement No. 640868. The synchrotron radiation experiments were performed at the SPring-8 beamline BL15XU with the approval of the NIMS Synchrotron X-ray Station (Proposals 2016A4600, 2016B4601, and 2017A4600) and at BESSY II with the approval of HZB. B. Vermang has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (grant agreement no. 715027).

In Cu(In,Ga)Se2(CIGS) thin-film solar cells, interface recombination is one of the most important limiting factors with respect to device performance. Herein, metal–insulator–semiconductor samples are used to investigate and compare the passivation effects of Al2O3and HfO2at the interface with CIGS. Capacitance–voltage–frequency measurements allow to qualitatively and quantitatively assess the existence of high negative charge density (Qf ≈ −1012 cm−2) and low interface-trap density (Dit ≈ 1011 cm−2 eV−1). At the rear interface of CIGS solar cells, these, respectively, induce field-effect and chemical passivation. A trade-off is highlighted between stronger field-effect for HfO2and lower interface-trap density for Al2O3. This motivates the usage of Al2O3to induce chemical passivation at the front interface of CIGS solar cells but raises the issue of its processing compatibility with the buffer layer. Therefore, an innovative Al2O3/HfO2multistack design is proposed and investigated for the first time. Effective chemical passivation is similarly demonstrated for this novel design, suggesting potential decrease in recombination rate at the front interface in CIGS solar cells and increased efficiency. 300 °C annealing in N2environment enable to enhance passivation effectiveness by reducingDitwhile surface cleaning may reveal useful for alternative CIGS processing methods.

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

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 Al2O3for 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, Al2O3passivation 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 Al2O3rear 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 Al2O3layer up to 30 nm thick. However, the upper limit of Al2O3thickness 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 Al2O3passivation layer with the openings created by selenization of LiF is not optimized yet.

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.

To reduce the prominent VOC-deficit that limits kesterite-based solar cells efficiencies, Ge has been proposed over the recent years with encouraging results, as the reduction of the non-radiative recombination rate is considered as a way to improve the well-known Sn-kesterite world record efficiency. To gain further insight into this mechanism, we investigate the physical behaviour of intrinsic point defects both upon Ge doping and alloying of Cu2ZnSnS4 kesterite. Using a first-principles approach, we confirm the p-type conductivity of both Cu2ZnSnS4 and Cu2ZnGeS4, attributed to the low formation energies of the VCu and CuZn acceptor defects within the whole stable phase diagram range. Via doping of the Sn-kesterite matrix, we report the lowest formation energy for the substitutional defect GeSn. We also confirm the detrimental role of the substitutional defects XZn (X=Sn,Ge) acting as recombination centres within the Sn-based, the Ge-doped and the Ge-based kesterite. Finally, we highlight the reduction of the lattice distortion upon Ge incorporation resulting in a reduction of the carrier capture cross section and consequently a decrease of the non-radiative recombination rate within the bulk material., Comment: 14 pages, 6 figures, Journal of Materials Chemistry A (2022)

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

The incorporation of nanostructures in optoelectronic devices for enhancing their optical performance is widely studied. However, several problems related to the processing complexity and the low performance of the nanostructures have hindered such actions in real-life devices. Herein, a novel way of introducing gold nanoparticles in a solar cell structure is proposed in which the nanostructures are encapsulated with a dielectric layer, shielding them from high temperatures and harsh growth processing conditions of the remaining device. Through optical simulations, an enhancement of the effective optical path length of approximately four times the nominal thickness of the absorber layer is verified with the new architecture. Furthermore, the proposed concept in a Cu(In,Ga)Se2 solar cell device is demonstrated, where the short-circuit current density is increased by 17.4%. The novel structure presented in this work is achieved by combining a bottom-up chemical approach of depositing the nanostructures with a top-down photolithographic process, which allows for an electrical contact., This work was funded in part by the Fundação para a Ciência e a Tecnologia (FCT) under Grants IF/00133/2015, PD/BD/142780/2018 and SFRH/BD/ 146776/2019. The authors also want to acknowledge the European Union’s Horizon 2020 Research and Innovation Programme through the ARCIGS-M project under Grant 720887, the Special Research Fund (BOF) of Hasselt University, the FCT through the project NovaCell (PTDC/CTM-CTM/28075/ 2017), and InovSolarCells (PTDC/FISMAC/29696/2017) co-funded by FCT and the ERDF through COMPETE2020. The authors also want to acknowledge Sandra Maya for the production of images used in this work.

In ultrathin Cu(In,Ga)Se2(CIGS) film solar cells, the CdS/CIGS interface may become one of the limiting factors for efficiency. The first step toward reducing the impact of this problem could be a surface treatment process to improve the quality of the front interface. The purpose of this study is to have a better understanding of the effect of wet chemical surface treatment, using ammonium sulfide ((NH4)2S), on CIGS thin film layers with different Cu/(Ga + In) (CGI) ratios. Herein, photoluminescence (PL) and time-resolved PL (TRPL) studies are conducted on bare CIGS, ammonium sulfide–treated CIGS thin films, and samples with CdS. In bare CIGS, CGI ratio–dependent changes in PL are observed on both a low-energy (defect related transition) and a high-energy peak (band-to-band transition). After the surface treatment, the PL maximum increases by factors ranging from 4 to 11 depending on the CGI ratio, accompanied by a slower decay. Trends with similar improvement as in the PL study are observed in the performance of the solar cells. It is shown that the impact of the surface treatment is beneficial independently of the CGI ratio of the absorber layers. In all cases, the treatment is shown to improve the efficiency.

Current state-of-the-art Cu2ZnSn(S,Se)4kesterite solar cells are limited by low open-circuit voltages (VOC). In order to evaluate to what extent the substitution of Sn by Ge is able to result in higherVOCvalues, 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 °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 theVOCof kesterite solar cells.

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.

To determine if Cu(In, Ga)Se-2 (CIGS) can be submitted to the constraints that will be imposed on the solar materials of the future by an ever growing demand and necessity, we produced ultrathin (500 nm) single-stage coevaporated CIGS solar cells doped with varying amounts and types of alkali atoms. We subjected them to accelerated lifetime testing. In the present work, we observed the seeping of water into the grain boundaries which could be identified as one of the main culprits for performance degradation. In addition, this phenomenon could only be observed when alkali atoms were present in the absorber layer. This work received funding from the European Union’s H2020 research and innovation program under grant agreement 715027. We acknowledge the use of a LEAP 5000 HR at APT Flanders, KU Leuven (Hercules Foundation under contract ZW/13/09)

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

In this work, an industrially viable and novel rear surface passivation approach for Copper Indium Gallium di-Selenide, Cu(In,Ga)Se2, CIGS, ultra-thin (500nm) solar cells is developed. The passivation layer was deposited by atomic layer deposition (ALD), and an alkali treatment was applied via spin coating. It was observed that selenization of the samples is required to create contact openings. The openings were visualized by SEM, and these results were supported by EDS. The impact of the oxide layer’s thickness, as well as the alkali solution’s molarity, was studied. Solar cells were produced for the optimal combination of these two parameters. As a result, with a relative 13% increase, the highest Vocof 623mV was achieved. Hence, the efficiency of the passivated solar cell was relatively increased by one-third, by using an industrially feasible, fast, and repeatable technique.

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).

In this work we present a method to grow Sb 2 Se 3 thin films with a potential use as absorber layers in solar cell structures. The films were grown on several substrates: soda-lime glass, Mo coated soda-lime glass and Si. The Sb-Se precursor's films were deposited by RF magnetron sputtering and then selenized under a H 2 Se gas flow. Different selenization temperatures were tested and analysed. Compositional and morphological analyses were performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Phase identification and structural characterization were done by X-ray Diffraction and Raman scattering spectroscopy showing that Sb 2 Se 3 is the dominant phase with an orthorhombic crystalline structure. Traces of rhombohedral and amorphous Se secondary phases were also observed supported by their Se-rich compositions. Visible-NIR reflectance measurements allowed to extract a direct bandgap with a value close to 1.06 eV. Photoluminescence spectroscopy shows an emission with a broad band at 0.85 eV for samples selenized at lower temperatures and an intense peak at 0.75 eV for the sample selenized at higher temperatures. Electrical characterization shows low free hole concentrations and mobilities. At low temperatures, the nearest neighbour hopping is the dominant mechanism for the electronic transport for the analysed samples. Both electrical and optical properties are influenced by the type of defects present on samples. A discussion is made on the properties that need to be improved in order that these films can be integrated into thin film solar cells.

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.

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.

The fabrication and properties of a Ge-based Kesterite Cu 2 ZnGeSe 4 (CZGSe) solar cell are discussed. The existence of the quaternary compound has been verified by physical methods such as X Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The Cu 2 ZnGeSe 4 solar cell has a power conversion efficiency (PCE) of 5.5% under AM1.5G illumination which is among the highest reported for pure Ge substitution. Detailed low temperature current-voltage and time-resolved photoluminescence measurements show that the Cu 2 ZnGeSe 4 absorber has less bulk defects and less band tailing in contrast to the typical characteristics of Cu 2 ZnSnSe 4 devices. These beneficial opto-electronic properties also resulted in a high open circuit voltage (V oc ) of 744 mV which is amongst the highest values reported for Kesterite materials.