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In this work, recombination mechanisms are investigated in Cu(In, Ga)Se 2 solar cells based on numerical modelling and verified by regression analysis of the dark and light current density–voltage (J–V) curves. Loss mechanisms such as a back contact barrier, deep level defects in the absorber layer are determined by fitting the simulated cell performance with the measurements using global optimisation algorithm differential evolution. The cell performance in the fitting process includes J–V curves recorded at different temperatures and open-circuit voltage(V o c ) under different illumination intensities at 300K. The results show that for CIGSe solar cells with different preparation methods and absorber thickness, the main loss mechanisms are different. Based on the proposed numerical model and the observed loss mechanisms, suggestions are given for further improving the solar cell efficiency in each cell.

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.

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

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.

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.

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

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

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

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

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

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

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.

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.

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

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

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.

Chapter 4 gives a summary based on one of the authors' earlier articles (Vermang (2012)), and consistent with more general overview papers. First, Section 4.1 describes recombination at Si surfaces theoretically to clarify two key passivation approaches: chemical and field effect passivation. Then, Section 4.2 quantifies the chemical and field effect passivation of Al2O3, i.e., its interface trap density and fixed charge density, respectively. Finally, Section 4.3 provides a larger picture by comparing Al2O3 to other Si surface layers, and showing that Al2O3 surface passivation can also be applied to passivate thin film photovoltaics.

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

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).

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

An optimized test structure to study rear surface passivation in Cu(In,Ga)Se-2 (CIGS) solar cells by means of photoluminescence (PL) is developed and tested. The structure - illustrated in the abst ...

The benefits of gallium (Ga) grading on Cu(In,Ga)Se2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell’s open-circuit voltage Voc. The parameter most constraining the cell’s Voc is found to be the deep-defect density close to the space charge region (SCR). In ungraded devices, high deep-defect concentrations (4.2 × 1016 cm–3) were observed near the SCR, offering a source for Shockley–Read–Hall recombination, reducing the cell’s Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 × 1015 cm–3) for back surface graded devices, and almost two orders of magnitude (8.6 × 1014 cm–3) for double surface graded devices, enhancing the cell’s Voc. In compositionally graded devices, the free-carrier density in the absorber’s bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band.

Bi-facial solar cells with ultrathin CIGS solar cells are fabricated to investigate the influence of back contact passivation. Solar cells with CIGS thicknesses of 300 and 500 nm and with an ultrathin transparent Mo layer are characterized using EQE measurements from both the front and the rear side as well as with I-V measurements. Back contact passivation consisting of Al 2 O 3 deposited by atomic layer deposition and nano-sized point contact openings is used. The results are compared to cells with only the transparent Mo layer as back contact. We find a significant effect of the passivation manifested as an increase in the current density of the solar cells with the passivation.

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

Reducing absorber layer thickness below 500nm in regular Cu(In,Ga)Se-2 (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF2 coated with a thin atomic layer deposited Al2O3 layer, or direct current magnetron sputtering of Al2O3 are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al2O3/CIGS rear interface. (MgF2/)Al2O3 rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells. (c) 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd. B. Vermang acknowledges the financial support of the European Commission via FP7 Marie Curie IEF 2011 Action No. 300998. Furthermore, this work is partly funded by the Swedish Science Foundation (VR) and the Swedish Energy Agency. Lastly, F. Henry would like to thank the European and Wallonia Region FEDER grant ECP12020011678F (MINATIS Project) for financial support.

An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (VOC) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of Al2O3 is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured Al2O3 rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in VOC compared with unpassivated reference cells.

For the first time, a novel rear contacting structure for copper indium gallium (di)selenide (CIGS) thin film solar cells is discussed theoretically, developed in an industrially viable way, and demonstrated in tangible devices. The proposed cell design reduces back contacting area by combining a rear surface passivation layer and nano-sized local point contacts. Atomic layer deposition (ALD) of Al2O3 is used to passivate the CIGS surface and the formation of nano-sphere shaped precipitates in chemical bath deposition (CBD) of CdS to generate point contact openings. The Al2O3 rear surface passivated CIGS solar cells with nano-sized local rear point contacts show a significant improvement in open circuit voltage (Voc) compared to unpassivated reference cells. Comparing the passivated devices to solar cell capacitance simulator (SCAPS) modeling indicates that this increase is attributed to a decrease in rear surface recombination of a few orders. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved. B. Vermang acknowledges the financial support of the European Commission via FP7 Marie Curie IEF 2011 Action no. 300998. Furthermore, this work is partly funded by the Swedish Science Foundation (VR) and the Swedish Energy Agency. Finally, the authors would like to thank Timo Wätjen for the TEM picture, used as Graphical abstract.

Copper indium gallium selenide/sulfide (CIGS) and copper zinc tin selenide/sulfide (CZTS) are two thin film photovoltaic materials with many similar properties. Therefore, three new processing steps - which are well-known to be beneficial for CIGS solar cell processing - are developed, optimized and implemented in CZTS solar cells. For all these novel processing steps an increase in minority carrier lifetime and cell conversion efficiency is measured, as compared to standard CZTS processing. The scientific explanation of these effects is very similar to its CIGS equivalent: the incorporation of alkali metals, ammonium sulfide surface cleaning, and Al2O3 surface passivation leads to electrical enhancement of the CZTS bulk, front surface and reduced front interface recombination, respectively. This work is funded by the Flemish government (Department Economy, Science and Innovation) and the European Union (Horizon 2020 grant agreement No 640868). B. Vermang acknowledges the financial support of the Flemish Research Foundation FWO (mandate 12O4215N).

Atomic layer deposition (ALD) of thin Al2O3 (≤10 nm) films is used to improve the rear surface passivation of large-area screen-printed p-type Si passivated emitter and rear cells (PERC). A blister-free stack of Al2O3/SiOx/SiNx is developed, leading to an improved back reflection and a rear recombination current (J0,rear) of 92 ± 6 fA/cm2. The Al2O3/SiOx/SiNx stack is blister-free if a 700°C anneal in N2 is performed after the Al2O3 deposition and prior to the SiOx/SiNx capping. A clear relationship between blistering density and lower open-circuit voltage (VOC) due to increased rear contacting area is shown. In case of the blister-free Al2O3/SiOx/SiNx rear surface passivation stack, an average cell efficiency of 19.0% is reached and independently confirmed by FhG-ISE CalLab. Compared with SiOx/SiNx-passivated PERC, there is an obvious gain in VOC and short-circuit current (JSC) of 5 mV and 0.2 mA/cm2, respectively, thanks to improved rear surface passivation and rear internal reflection. Copyright © 2012 John Wiley & Sons, Ltd.

Atomic layer deposition (ALD) of thin Al2O3 (

Deep Level Transient Spectroscopy (DLTS) has been applied to study the interface states of 5 nm Atomic Layer Deposited (ALD) Al2O3 films on p-type Si. In addition, the passivation of the interface states by Forming Gas Anneal (FGA) and Firing has been explored. It is shown that the near mid-gap density of interface states (Dit) is successfully reduced by both treatments. From a comparison with 5 nm SiO2 reference capacitors it can be concluded that similar interface defects are observed, suggesting that they are dominated by the thin interfacial SiO2 layer formed during ALD.

Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2(Zn,Sn)(S,Se)4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage ( $V_{{\rm OC}}$ ; +17%rel.), short-circuit current ( $J_{{\rm SC}}$ ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.

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

This Letter discusses an important difference between positively charged SiO2 and negatively charged Al2O3 rear-passivated p-type Si solar cells: their illumination level dependency. For positively charged SiO2 rear-passivated p-type Si solar cells, a loss in short circuit current (JSC) and open circuit voltage (VOC) as a function of illumination level is mainly caused by parasitic shunting and a decrease in surface recombination, respectively. Hence, the relative loss in cell conversion efficiency, JSC, and VOC as a function of the illumination level for SiO2 compared to Al2O3 rear-passivated p-type Si solar cells has been measured and discussed. Subsequently, an exponential decay fit of the loss in cell efficiency is applied in order to estimate the difference in the energy output for both cell types in three different territories: Belgium (EU), Seattle and Austin (US). The observed trends in the difference in energy output between both cells, as a function of time of the year and region, are as expected and discussed. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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

Surface passivation of Si solar cells is typically achieved by deposition of a dielectric layer. Via the investigation of Al2O3 passivation layers, we show that care must be taken when performing capacitance–voltage (C –V) measurements in order to obtain results that are meaningful at solar cell level. The passivation properties of a dielectric are not only affected by post-deposition treatments but also by the presence and the nature of a metal covering the dielectric. Consequently, this Letter emphasizes how important it is to perform C –V measurements on a device structure that resembles as closely as possible that of the finished solar cell, using the same metal, deposition technique and thermal budget. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this study, an ultra-thin MoO3 layer synthesized by a solution-based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10 nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open-circuit voltage compared to the reference solar cells. Temperature dependent current density–voltage measurement indicated that the activation energy (EA) of the main recombination is higher (∼ 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where EA ∼ 770 meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved.

This work characterizes p-type Silicon surface passivation using a high-k material (Al 2 O 3 or HfO 2 ) combining capacitance voltage (CV) and lifetime measurements.

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

A next generation material for Si surface passivation is atomic layer deposited (ALD) Al 2 O 3 . However, conventional time-resolved ALD is limited by its low deposition rate. Initially, a high-deposition-rate prototype ALD reactor based on the spatially-separated ALD principle has been developed, with Al 2 O 3 deposition rates up to 1.2 nm/s. Later, the spatial ALD technique has been transferred to an actual in-line process development tool (PDT) for commercial high-throughput ALD of Al 2 O 3 , resulting in a deposition rate of 30 nm/min. The passivation quality and uniformity of the spatially-separated ALD Al 2 O 3 films are evaluated on p- and n-type Si, applying quasi-steady-state photo-conductance, carrier density imaging and infrared lifetime mapping. In all cases, a spatial ALD Al 2 O 3 layer of only 10 nm reached an excellent passivation quality and uniformity, comparable to reference wafers passivated by equivalent temporal plasma-assisted or thermal ALD Al 2 O 3 . Effective surface recombination velocities as low as 1.1 or 2.9 cm/s were obtained after annealing at 350 °C or firing, respectively. Using spatial ALD Al 2 O 3 passivated local Al back surface field p-type Si solar cells, the sufficient passivation of this high-throughput Al 2 O 3 layer is evaluated: an average gain in open circuit voltage as compared to SiO x rear passivated i-PERC cells is obtained.

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

For further reduction of the crystalline Silicon solar cell cost/Wp, a dual approach is required: Further reduction of the silicon material by using thinner wafer and further increasing the conversion efficiency. Considering wafer thicknesses of 150µm and below the standard process with Ag screen-printed contacts on 50–60Ω/sq emitter and full Al BSF cannot provide the necessary efficiency increase. The reason for that is the increasing influence of the rear surface recombination current, which becomes a limited current loss mechanism. Within the Photovoltaic department in IMEC a research program has been launched with the goal of providing industrial processes for the next generation thin crystalline silicon solar cells. In this paper we are reporting on the development of a process toolbox that allows overcoming the full Al-BSF and the Ag-screen printing front-side metallization limitations. The next step towards higher efficiency targets is the implementation of novel emitter schemes and consequently advanced front-side metallization like electro-plating of copper for further photocurrent and fillfactor increase. By implementing Cu-plating as a front-side metallization, large area cells with efficiencies up to 18.4% have been fabricated. These are the initial steps for a cell concept that potentially can reach 19% efficiency in an industrial process flow. The progress towards an industrial Passivated Emitter and Rear Locally doped cell concept (i-PERL) is presented.