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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.

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

The open circuit voltage (VOC) deficit of Cu2ZnSn(S,Se)4 (CZTSSe) kesterite solar cells is higher than that of the closely related Cu(InGa)Se2 solar cells. One of the most promising strategies to overcome the large VOC deficit of kesterite solar cells is by reducing the recombination losses through appropriate cation substitution. In fact, replacing totally or partially Zn or Cu by an element with larger covalent radius one can significantly reduce the concentration of I–II antisite defects in the bulk. In this study, an investigation of the impact of partial substitution of Cu by Ag in CZTSSe solid solution monograins is presented. A detailed photoluminescence study is conducted on Ag-incorporated CZTSSe monograins and a radiative recombination model is proposed. The composition and structural quality of the monograins in dependence of the added Ag amount are characterized using Energy Dispersive X-ray Spectroscopy and X-Ray Diffraction method, respectively. The Ag-incorporated CZTSSe monograin solar cells are characterized by temperature dependent current-voltage and electron beam induced current methods. It was found, that low Ag contents (x ≤ 0.02) in CZTSSe lead to higher solar cell device efficiencies.

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.

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.

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).

A new multi-stack approach with contact openings was investigated for front surface of Cu(In,Ga)Se 2 thin film solar cells. In this multi-stack design, a thin HfO x layer was used to protect a thicker AlO x layer from the presence of ammonia in the chemical bath deposition. The contact openings were created by using alkali solution. The lifetime decay was improved by implementing multi-stack passivation layer between CIGS/buffer layer interface. It was shown that open circuit voltage was increased by up to 30mV.

In this contribution, a cost- and time- effective contacting approach is presented for dielectric-based rear surface passivation layers up to 30 nm. Nano-size and homogenously distributed openings are realized in AlOx passivation layer by selenization of LiF salt on top, and monitored via SEM. Further, this layer was implemented into sub-micron thick CIGS solar cells. Consequently, contact openings facilitate the current collection through the dielectric layer up to 30 nm thick. However, solar cell parameters remain the same, even worse than the reference. Thus, this approach is still open for optimizations.

In modern Cu(In,Ga)Se2 (CIGS) solar cell production, alkali-poor substrates are becoming increasingly common. Thus, understanding the effect of alkali atoms on the growth of the CIGS absorber has become crucial. We studied this impact by comparing the bias dependent admittance spectroscopy response of CIGS solar cells grown with varying amounts of sodium using two different growth processes. The absence of sodium led to an intense response which is progressively replaced by other response domains as the sodium supply increases. We linked these new domains to Na accumulation and a resulting increase in band bending at the CIGS/CdS interface.

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

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).

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.

This research reveals the missing or often undisclosed experimental data on processing conditions of Cu(In,Ga)(S,Se)2 absorbers by the two-step sequential technique. Accordingly, by employing a one-variable-at-a-time approach- where each factor is exclusively changed with respect to a defined center point, we identified a handful of significant parameters which play the most significant role in an absorber's properties and consequently, prospective high-performing solar cells. This was done by analyzing their impact on a few most important metrics. Hence, the vast processing window with numerous factors and levels for development of Cu(In,Ga)(S,Se)2 by this method becomes constrained and feasible to optimize in future endeavours.

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.

In this contribution, we report on the impact of ammonium sulfide ((NH4)(2)S-x) chemical treatments done at room temperature on the surface properties of Cu2ZnSnSe4 (CZTSe) thin film. The first approach is based on the immersion of the absorber layer in a (NH4)(2)S-x solution (20 wt.%). This method results in the preferential etching of Se, Sn and Zn atoms from the absorber surface. After a treatment time of 1 min, the performances of the solar cells are found to be improved. The second approach consists in the exposure of the CZTSe layer to ammonium sulfide vapors. In this case, it was found that the progressive sulfurization of the surface of the absorber leads to the decomposition of the CZTSe phase into Sn(S,Se)(x) and Se-0 secondary phases. As a consequence, the short circuit current of the corresponding solar cells is reduced. (C) 2016 Elsevier B.V. All rights reserved. This work refers to the patent EP 2 871 683 A1 (date of publication: 13th May 2015). This research is partially funded by the Flemish government, Department Economy, Science and Innovation. AGC is acknowledged for providing Mo-coated soda lime glass substrates. Dr. Carole La (University of Nantes, France) is acknowledged for her technical support for the ICP-AES measurements.

Room temperature photoluminescence measurement has been carried out on 1-stage co-evaporated Cu(In,Ga)Se 2 thin film in order to investigate the effect of ammonium sulfide (NH 4 ) 2 S surface treatment. The defect related PL peak in the as-deposited CIGS thin film changes with CGI ratio. The surface treatment leads to an increase in the main PL intensity by a factor of ~10. The lifetime of minority carriers is slightly increased after surface treatment, and it is observed that a low-Cu region is more affected than a high-Cu region.

Producing the green energy of tomorrow will require highly efficient as well as energy --, and cost eff ective solar cells in addition to having reasonable lifetimes To determine if CIGS can be made to submit to these constraints , we produced ultrathin (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 a lkali concen tration on the degradation of the cells b ut showed limited mig ration. Inste ad, the seeping of water into the gra in bounda rie s was identified as the main culprit for perf ormance degradati on. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027

Thin-film solar cells consist of several layers. The interfaces between these layers can provide critical recombination paths and consequently play a vital role in the efficiency of the solar cell. One of the main challenges for polycrystalline semiconductor absorber materials is the absorber-buffer interface. The Cu(In, Ga)Se-2 system is particularly interesting in this context, since Cu-rich absorbers are dominated by recombination at the interface, while Cu-poor ones are not. This paper unveils the root cause of the challenge in the interface of Cu-rich solar cells in terms of a Se-related defect with an activation energy of 200 +/- 20 meV. This defect causes interface recombination and is responsible for the deficiency of open-circuit voltage in Cu-rich cells. Moreover, this paper demonstrates that the origin of this defect is due to the etching step necessary to remove secondary phases. Postdeposition surface treatments or modified buffer layers are shown to passivate this defect, to reduce interface recombination, and to increase the efficiency. This paper has been funded by the Luxembourgish Fonds National de la Recherche in the framework of the Cu-rich CIS treated with potassium (K) (CURI-K), Surface passivation for thin film photovoltaics (SURPASS), and Materials for Sensing and Energy Harvesting (Massena) projects, which are gratefully acknowledged. Christian Andreas and Stephan Bucheler from EMPA are acknowledged for providing CdS buffer layer deposition.

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).

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.

This paper provides an overview of the physical vapor technologies used to synthesize Cu2ZnSn(S,Se)4 thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films have been fabricated using sequential stacking or co-sputtering of precursors as well as using sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition, pulsed laser deposition of composite targets and monograin growth by the molten salt method were developed as alternative methods for kesterite layers deposition. This review presents the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical description of the main issues limiting this efficiency and of the experimental pathways designed to prevent or limit these issues is provided and discussed as well. Afinal section is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiency, such as alkali doping and bandgap grading, 1. R Caballero and M León acknowledge financial support via the Spanish Ministry of Science, Innovation and Universities project (WINCOST, ENE2016-80788-C5-2-R) and thank H2020 EU Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968). 2. S Canulescu and J Schou acknowledge the support from Innovation Fund Denmark. 3. D-H Kim acknowledges financial support via the DGIST R&D Program of the Ministry of Science and ICT, KOREA (18-BD-05). 4.C. Malerba acknowledges the support from the Italian Ministry of Economic development in the framework of the Operating Agreement with ENEA for the Research on the Electric System. 5.A Redinger acknowledges financial support via the FNR Attract program, Project : SUNSPOT, Nr.11244141. 6. E Saucedo thanks H2020 EU Programme under the projects STARCELL (H2020-NMBP-03-2016-720907) and INFINITE-CELL (H2020-MSCA-RISE-2017-777968), the Spanish Ministry of Science, Innovation and Universities for the IGNITE project (ENE2017-87671-C3-1-R), and the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the ‘Generalitat de Catalunya’ (Ref. 2017 SGR 862). 7. Taltech acknowledges financial support via the Estonian Ministry of Education and Research funding project IUT19-28 and the European Union Regional Development Fund, Project TK141. 8. 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)

Ultrathin single- and three-stage Cu(In,Ga)Se-2 absorber layers were analyzed with room temperature photoluminescence (PL) spectra. An anomalous blueshift was observed upon increasing carrier injection for both samples. This blueshift was attributed to the presence of bandgap fluctuations that are of the same order as the minority carrier diffusion length. From time resolved measurements, a diffusion length of a few 100nms was deduced. The single-stage spectrum consists of two peaks, and the sample was, therefore, also analyzed by hyperspectral imaging, providing lateral PL and reflectance data with 1 mu m resolution. Marginal variations were observed in the PL yield and spectra. This homogeneity could again be attributed to an intermediate scale of the bandgap fluctuation with an upper limit of 1 mu m for the scale of the lateral bandgap fluctuations. The two peaks in the PL spectra of the single-stage sample could be attributed to interference, and correction methods were applied. The bandgap fluctuations were extracted for the three-stage and single-stage sample and were 45meV and 72 +/- 3meV, respectively. It is suggested that this difference is attributed to the smaller grains and larger amount of grain boundaries in the single-stage sample. This work received funding from the European Union's H2020 Research and Innovation Program under Grant Agreement No. 715027. We acknowledge Photon etc. and Professor Delphine Bouilly's research group from IRIC for the hyperspectral measurements and Erik Spaans for the python program.

In ultrathin Cu(In,Ga)Se-2(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)(2)S), 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. This work received funding from the European Union's H2020 research and innovation program under grant agreement No. 715027. Buldu, DG (corresponding author), Hasselt Univ Partner Solliance, Inst Mat Res IMO, Agoralaan Gebouw H, 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. Dilara.Gokcen.Buldu@imec.be

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.

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.

Solution processed CH 3 NH 3 PbI x Cl 3– x based planar heterojunction perovskite solar cells with power conversion efficiency (PCE) above 14% are reported. The devices benefit from a phenyl-C 61 -butyric acid methyl ester (PCBM)/ZnO double electron transport layer (ETL) as well as a short air-aging step. The role of the additional ZnO ETL is studied by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and secondary ions mass spectroscopy (SIMS). Apart from improving the energy level alignment, the ZnO layer blocks the reactions between the metal electrode and perovskite components, increasing the air stability of the device. A crucial step in our processing is a short air-aging step for the device, which significantly increases the device performance by reducing the recombination process. Since the ZnO nanoparticle layer requires no thermal annealing, the maximum temperature to fabricate the device can be kept below 100 °C, making this structure compatible with roll-to-roll processing on plastic films.

Cu2ZnSi(S,Se)4 and Cu2Si(S,Se)3 are potential materials to obtain cost effective high band gap absorbers for tandem thin film solar cell devices. A method to synthesize Cu2SiS3, Cu2SiSe3and Cu2ZnSiSe4thin film absorbers is proposed. This method is based on a multistep process, using sequential deposition and annealing processes. X-ray diffraction analysis performed on the final thin films have confirmed the presence of the Cu2Si(S,Se)3 and Cu2ZnSiSe4phases. Scanning electron microscopy images revealed the formation of polycrystalline layers with grains size up to 1 µm. The band gap of the ternary Cu2SiSe3 and Cu2SiS3, and quaternary Cu2ZnSiSe4 based thin films as determined from optical and photoluminescence measurements are found to be close to their theoretical values. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

We report on the electrical, optical and physical properties of Cu2ZnSnSe4 solar cells using an absorber layer fabricated by selenization of sputtered Cu, Zn and Cu10Sn90 multilayers. A maximum active-area conversion efficiency of 10.4% under AM1.5G was measured with a maximum short circuit current density of 39.7 mA/cm(2), an open circuit voltage of 394 mV and a fill factor of 66.4%. We perform electrical and optical characterization using photoluminescence spectroscopy, external quantum efficiency, current-voltage and admittance versus temperature measurements in order to derive information about possible causes for the low open circuit voltage values observed. The main defects derived from these measurements are strong potential fluctuations in the absorber layer as well as a potential barrier of the order of 133 meV at the back side contact. (C) 2014 Elsevier B.V. All rights reserved.

One of the promising low cost and non-vacuum approaches for the fabrication of semiconductor CuInSe 2 and Cu(In,Ga)(S,Se) 2 thin film absorbers is the printing of precursor materials followed by a sintering/selenization process. The selenization process parameters such as temperature, duration, and selenium vapor pressure strongly influence the morphology and electronic properties of the absorber film. In this study, the effect of pre-annealing in an inert atmosphere and selenization on printed mechanically synthesized CuInSe 0.5 alloy nanopowder precursor films was investigated. 1–2 μm thick CuInSe 0.5 alloy nanopowder layers were deposited on a Mo-sputtered glass substrate by means of doctor blade coating of a nanopowder based precursor suspension. Pre-annealing was performed on a hot plate inside a nitrogen gas filled glove box. Selenization was performed in a home-made rapid thermal processing (RTP) furnace with two RTP heating zones for independent temperature control of the selenium source and the coated substrate. The temperature of the selenium source was fixed at 390–410 °C during the selenization to provide a constant supply of selenium vapor. A two-step process, i.e., a pre-annealing in nitrogen atmosphere at 400 °C for 30 min followed by selenization at 530 °C for 15 min was found to result in better densification and grain growth of the CuInSe 2 phase, compared to a single step selenization at 530 °C for 15 min. The solar cell fabricated by the two-step process had an efficiency of 5.4% and a fill factor of 52%, while the device fabricated by the single step selenization had an efficiency of 1.1% and a fill factor of 31%.

Cu 2 ZnGeSe 4 (CZGeSe) can be considered as a potential alternative for wide band gap thin film devices. In this work, CZGeSe thin films were deposited on Mo-coated soda lime glass substrates by sequential deposition of sputtered Cu, Zn and e-beam evaporated Ge layers from elemental targets followed by annealing at high temperature using H 2 Se gas. We report on the effect of the precursor stack order and composition and the impact of the annealing temperature on the physical properties of CZGeSe thin films. The optimal layer morphology was obtained when using a Mo/Cu/Zn/Ge precursor stack annealed at 460 °C. We have observed that the formation of secondary phases such as ZnSe can be prevented by tuning the initial composition of the stack, the stack order and the annealing conditions. This synthesis process allows synthesizing CZGeSe absorber with an optical band gap of 1.5 eV.

Cu2nSnSe4 (CZTSe) thin film solar cells are promising emergent photovoltaic technologies based on low-bandgap absorber layer with high absorption coefficient. To reduce optical losses in such devices and thus improve their efficiency, numerical simulations of CZTSe solar cells optical characteristics can be performed based on individual optical properties of each layer present in the cell structure. In this contribution, we have first determined the optical coefficients of individual thin films (i.e., (n, k) of the absorber, buffer, and window layers) to build a realistic model simulating the optical behavior of the whole cell stack we propose. Optical characterization was performed using two approaches, one based on ellipsometry measurements for characterizing thin flat cadmium sulfide (CdS) and zinc oxide (ZnO) layers and the other relying on reflectance and transmission (R/T) analysis for the rough CZTSe absorber. Then, we performed numerical simulations using as input experimental optical parameters predicting optimal CZTSe cell structure minimizing optical losses. The impact of each layer's thickness on the cell's short-circuit current has been studied. A set of optimal thicknesses of each of the active layers was proposed. Finally, the proposed optical optimization was experimented practically leading to CZTSe cells with 9.7% and 10.4% efficiencies. cop. 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this paper, we examine photoluminescence spectra (PL) of Cu2ZnSnSe4/CdS/ZnO solar cells, based on an absorber layer fabricated by selenization of sputtered Cu, Zn, Sn multilayers, via temperaturedependent and illumination power-dependent measurements. We observe anomalous emission behavior: the PL peak initially decreases with increasing temperature (red shift) in the temperature range 10– 60 K, followed by a blue shift at higher temperature in the range 60–180 K. A recombination model is proposed that is able to explain both temperature dependent PL as well as power-dependent PL. The important aspect of the proposed model is taking into account the presence of strong potential

We have investigated different nonidealities in Cu2ZnSnSe4–CdS–ZnO solar cells with 9.7% conversion efficiency, in order to determine what is limiting the efficiency of these devices. Several nonidealities could be observed. A barrier of about 300 meV is present for electron flow at the absorber–buffer heterojunction leading to a strong crossover behavior between dark and illuminated current–voltage curves. In addition, a barrier of about 130 meV is present at the Mo–absorber contact, which could be reduced to 15 meV by inclusion of a TiN interlayer. Admittance spectroscopy results on the devices with the TiN backside contact show a defect level with an activation energy of 170 meV. Using all parameters extracted by the different characterization methods for simulations of the two-diode model including injection and recombination currents, we come to the conclusion that our devices are limited by the large recombination current in the depletion region. Potential fluctuations are present in the devices as well, but they do not seem to have a special degrading effect on the devices, besides a probable reduction in minority carrier lifetime through enhanced recombination through the band tail defects.

Modification of the absorber surface properties by Cd2+ treatment (Cd2+ partial electrolyte) results in the following: formation of Cd(OH)2 on the absorber surface, deposition of thinner chemical bath-deposited CdS buffer layer, and a smaller space charge region. The impact on electrical performances is as follows: decrease of the series resistance (RS), increase of the fill factor, increase of the efficiency (η), and reduction of the crossover between the dark and light current density-voltage curves. The present contribution aims at determining the impact of modifying the properties of the absorber/buffer layer interface on the electrical performance of Cu2ZnSnSe4 (CZTSe) thin-film solar cells, by using a Cd2+ partial electrolyte (Cd PE) treatment of the absorber before the buffer layer deposition. In this work, CZTSe/CdS solar cells with and without Cd PE treatment were compared with their respective Cu(In,Ga)Se2 (CIGSe)/CdS references. The Cd PE treatment was performed in a chemical bath for 7 min at 70 °C using a basic solution of cadmium acetate. X-ray photoemission spectroscopy measurements have revealed the presence of Cd at the absorber surface after the treatment. The solar cells were characterized using current density-voltage (J-V), external quantum efficiency, and drive-level capacitance profiling measurements. For the CZTSe-based devices, the fill factor increased from 57.7% to 64.0% when using the Cd PE treatment, leading to the improvement of the efficiency (η) from 8.3% to 9.0% for the best solar cells. Similar observations were made on the CIGSe solar cell reference. This effect comes from a considerable reduction of the series resistance (RS) of the dark and light J-V, as determined using the one-diode model. The crossover effect between dark and light J-V curves is also significantly reduced by Cd PE treatment.