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In this contribution, the impact of thermal stress on Cu(In,Ga)Se $_{2}$ (CIGSe) thin film photovoltaic devices is investigated. The tolerance of such devices to high temperatures is of particular interest for processing transparent conductive oxides (TCOs) in order to further close the gap to the theoretical efficiency limit and for their potential use as bottom devices in tandem applications in order to overcome the theoretical efficiency limit of single junction solar cells. When CdS-buffered CIGSe high efficiency solar cells are subjected to thermal stress, elemental interdiffusion of Na and Cd between the absorber and the window layers as well as chemical reactions at the CIGSe/CdS interface result in a degraded power conversion efficiency (PCE). Here, we compare the degradation mechanisms of CdS and GaO $_{x}$ buffered CIGSe solar cells under thermal stress. A model explaining the observed degradation behaviors is proposed.

We present a comprehensive device model for Cu(In,Ga)Se $_{2}$ (CIGSe) thin-film solar cells based on numerical SCAPS-1D simulations. The model reproduces the experimentally determined current-voltage and capacitance-voltage characteristics of a Rb-free reference device, a sample that underwent an RbF-treatment, and a sample based on a CIGSe/RbInSe $_{2}$ -stack. According to this model, and in agreement with experimental findings, the main consequences of both Rb-conditionings are an increased doping-density and a defect passivation in the CIGSe as well as the formation of a photocurrent barrier at the hetero interface. With the numerical model established, fundamental aspects of the Rb-conditioning, e.g., the differentiation between its effect on bulk and interface recombination are discussed. Additionally, temperature dependent current-voltage analysis is employed in order to test the model's predictions regarding the interaction of Rb with an injection-current barrier at the back contact of the device. Both the simulation and the temperature dependent current-voltage measurements lead to the result that the RbF-PDT is increasing the height of this barrier, while the deposition of RbInSe $_{2}$ is decreasing it.

Individual grains of chalcopyrite solar cell absorbers can facet in different crystallographic directions at their surfaces. To gain a deeper understanding of the junction formation in these devices, we correlate variations in the surface facet orientation with the defect electronic properties. We use a combined analytical approach based on scanning tunneling spectroscopy (STS), scanning electron microscopy, and electron back scatter diffraction (EBSD), where we perform these experiments on identical surface areas as small as 2 × 2 µm2 with a lateral resolution well below 50 nm. The topography of the absorber surfaces indicates two main morphological features: micro-faceted, long basalt-like columns and their short nano-faceted terminations. Our STS results reveal that the long columns exhibit spectral signatures typical for the presence of pronounced oxidation-induced surface dipoles in conjunction with an increased density of electronic defect levels. In contrast, the nano-faceted terminations of the basalt-like columns are largely passivated in terms of electronic defect levels within the band gap region. Corresponding crystallographic data based on EBSD experiments show that the surface of the basalt-like columns can be assigned to intrinsically polar facet orientations, while the passivated terminations are assigned to non-polar planes. Ab-initio calculations suggest that the polar surfaces are more prone to oxidation and resulting O-induced defects, in comparison to non-polar planes. Our results emphasize the correlation between morphology, surface facet orientations and surface electronic properties. Furthermore, this work aids in gaining a fundamental understanding of oxidation induced lateral inhomogeneities in view of the p-n junction formation in chalcopyrite thin-film solar cells.

International audience; In Cu(In,Ga)Se 2 (CIGS) thin-film solar cells, laterally inhomogeneous distributions of point defects may induce electrostatic potential fluctuations and thus reduce the open-circuit voltage (V oc). In the present work, we investigate possible origins of fluctuating potentials and estimate the amplitude of fluctuations and V oc losses in solar cells with various [Ga] in the CIGS absorber, with different buffer layers and with different durations of an RbF postdeposition treatment (PDT). Electron-beam-induced current measurements were employed to study the local difference in the width of the space-charge region (w SCR). It is shown that the amplitude of fluctuations in the w SCR depends significantly on the choice of buffer system and on the duration of the RbF PDT. In addition, energy-dispersive X-ray spectroscopy and cat-hodoluminescence measurements reveal that band-gap fluctuations do not have substantial impact on the device performance. Finally, some of the investigated cells were exposed to light soaking, which was found to be a means to reduce the detected electrostatic potential fluctuations and also to increase the effective electron diffusion length in the CIGS absorber for a part of the investigated cells.

We demonstrate a monolithic perovskite CIGS tandem solar cell with a certified power conversion efficiency PCE of 24.2 . The tandem solar cell still exhibits photocurrent mismatch between the subcells; thus optical simulations are used to determine the optimal device stack. Results reveal a high optical potential with the optimized device reaching a short circuit current density of 19.9 mA cm 2 and 32 PCE based on semiempirical material properties. To evaluate its energy yield, we first determine the CIGS temperature coefficient, which is at amp; 8722;0.38 K 1 notably higher than the one from the perovskite subcell amp; 8722;0.22 K 1 , favoring perovskite in the field operation at elevated cell temperatures. Both single junction cells, however, are significantly outperformed by the combined tandem device. The enhancement in energy output is more than 50 in the case of CIGS single junction device. The results demonstrate the high potential of perovskite CIGS tandem solar cells, for which we describe optical guidelines toward 30 PCE

RbInSe2 is attracting growing interest as a secondary semiconductor compound in Cu(In,Ga)Se2-based solar cells by virtue of the recent investigations on absorber post-deposition treatments with alk...

In this contribution, the effectiveness of an RbF post deposition treatment (PDT) is evaluated in dependence on the Cu content of the absorber layer of Cu(In,Ga)Se $_{2}$ solar cells. It is shown that the PDT only acts beneficially on the open-circuit voltage and fill factor ( FF ) on samples with rather high Cu content, while it deteriorates all parameters of the solar cells in samples with low Cu content. In order to clarify the behavior of the open-circuit voltage, the well-known exchange mechanism of Rb and Na during the PDT is analyzed as a function of the Cu content of the absorber layers and discussed in regard to theoretical publications. Furthermore, a model explaining the observed effect on the FF based on the formation of an RbInSe $_{2}$ (RIS) layer during the RbF-PDT is proposed. The model supposes that the RIS layer acts as a barrier for the photocurrent and therefore lowers the FF . It is evaluated theoretically in dependence of the properties of the RIS layer using one-dimensional solar cell capacitance simulator (SCAPS) simulations. Finally, the proposed model is also tested and confirmed experimentally by directly depositing RIS onto untreated Cu(In,Ga)Se $_{2}$ layers.

Cu(In,Ga)Se2 (CIGS)-based photovoltaic cells were fabricated under varying deposition conditions for the Mo back contact and CIGS absorber layers. Specifically, O2 concentration and sputtering pressure were varied during the Mo deposition, as well as the final Mo-layer thickness. The maximum deposition temperature was then varied during the subsequent deposition of the CIGS-layer by thermal coevaporation. The JV response curve of the finished devices were recorded and used to calculate the usual solar performance characteristics (efficiency, J sc, V oc, fill-factor, R oc, and R sc). The effects of the varied fabrication conditions on these characteristics were then investigated, and CIGS-deposition temperature alone was found to have had a significant influence on J sc. Following this, the entire JV -curve was analyzed using functional data analysis (FDA). The curve shape was revealed to be affected not only by temperature, but by O2 conc. and Mo sputter pressure as well. The effect of these fabrication parameters was greatest in regions of the curve larger than V oc. External quantum efficiency measurements on the finished devices were also studied using FDA, indicating a strong influence of temperature in reducing collection across most of the visible spectrum. Mechanisms that aim to explain these observations are proposed. This work details, according to the authors’ best knowledge, the first-ever use of FDA in the analysis of photovoltaic devices.

Perovskite based tandem solar cells can increase the power conversion efficiency PCE of conventional single junction photovoltaic devices. Here, we present monolithic perovskite CIGSe tandem solar cells with a perovskite top cell fabricated directly on an as grown, rough CIGSe bottom cell. To prevent potential shunting due to the rough CIGSe surface, a thin NiOx layer is conformally deposited via atomic layer deposition on the front contact of the CIGSe bottom cell. The performance is further improved by an additional layer of the polymer PTAA at the NiOx perovskite interface. This hole transport bilayer enables a 21.6 stabilized PCE of the tandem device at amp; 8764;0.8 cm2 active area. We use TEM EDX measurements to investigate the deposition uniformity and conformality of the NiOx and PTAA layers. By absolute photoluminescence measurements, the contribution of the individual subcells to the tandem VOC is determined, revealing that further fine tuning of the recombination layers might improve the tandem VOC. Finally, on the basis of the obtained results, we give guidelines to improve monolithic perovskite CIGSe tandems toward predicted PCE estimates above 30

Ambient-pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF postdeposition treatments (KF-PDT, RbF-PDT) on the electronic features of Cu(In,Ga)Se2 (CIGSe) thin films and the CdS/CIGSe interface in a CdS thickness series that has been sequentially prepared during the chemical bath deposition (CBD) process depending on the deposition time. We observe distinct features correlated to the CBD-CdS growth stages. In particular, we find that after an initial CBD etching stage, the valence band maximum (VBM) of the CIGSe surface is significantly shifted (by 180-620 mV) toward the Fermi level. However, VBM positions at the surface of the CIGSe are still much below the VBM of the CIGSe bulk. The CIGSe surface band gap is found to depend on the type of postdeposition treatment, showing values between 1.46 and 1.58 eV, characteristic for a copper-poor CIGSe surface composition. At the CdS/CIGSe interface, the lowest VBM discontinuity is observed for the RbF-PDT sample. At this interface, a thin layer with a graded band gap is found. We also find that K and Rb act as compensating acceptors in the CdS layer. Detailed energy band diagrams of the CdS/CIGSe heterostructures are proposed.

Ambient-pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF postdeposition treatments (KF-PDT, RbF-PDT) on the electronic features of Cu(In,Ga)Se

In conventional Cu(In,Ga)Se2 (CIGSe) solar cells a chemical bath deposited CdS thin film is used as a buffer layer. However, it is desired to replace CdS due to the toxicity of cadmium and the rather narrow bandgap energy of CdS. Zn(O,S) is considered to be one of the most attractive candidates as an alternative, non-toxic buffer layer with a larger bandgap. This paper aims to compare the properties of the CdS/CIGSe and the Zn(O,S)/CIGSe interfaces depending on the absorber composition and the application of an RbF post deposition treatment (PDT). Synchrotron-based hard X-ray photoelectron spectroscopy revealed a strong correlation of Cd diffusion and concentration of VCu in CIGSe before the PDT. Additionally, the RbF-PDT enhanced the Cd diffusion into the CIGSe. On the other hand, it was found that Zn atoms are not as easily incorporated into the CIGSe as Cd atoms. As a result, we consider the formation of donor-like ZnCu+ defects at the interface to be less likely than the formation of CdCu+ defects. Moreover, we observed that the Zn(O,S)/CIGSe interface is less sensitive to changes of the CIGSe composition and to the RbF-PDT compared to the CdS/CIGSe interface.

Due to their tunable bandgap energy, Cu(In,Ga)Se2 (CIGSe) thin‐film solar cells are an attractive option for use as bottom devices in tandem configurations. In monolithic tandem devices, the thermal stability of the bottom device is paramount for reliable application. Ideally, it will permit the processing of a top device at the required optimum process temperature. Here, we investigate the degradation behavior of chemical bath deposited (CBD) CdS‐buffered CIGSe thin‐film solar cells with and without Na incorporation under thermal stress in ambient air and vacuum with the aim to gain a more detailed understanding of their degradation mechanisms. For the devices studied, we observe severe degradation after annealing at 300°C independent of the atmosphere. The electrical and compositional properties of the samples before and after a defined application of thermal stress are studied. In good agreement with literature reports, we find pronounced Cd diffusion into the CIGS absorber layer. In addition, for Na‐containing samples, the observed degradation can be mainly explained by the formation of Na‐induced acceptor states in the TCO front contact and a back contact barrier formation due to the out‐diffusion of Na. Supported by numerical device simulation using SCAPS‐1D, various possible degradation models are discussed and correlated with our findings.

Photovoltaic devices based on Cu(In,Ga)Se2 (CIGSe) absorbers are among the most attractive non-Si alternatives. The key to their steadily increasing efficiency is a post-deposition treatment (PDT) with alkali salts. For co-evaporated CIGSe, a RbF-PDT was demonstrated as the most efficient, however, the mechanism of the RbF influence on the CIGSe absorber is not completely understood. Here, we focus on the impact of RbF on the surface of co-evaporated CIGSe absorbers in dependence on their bulk composition. Surface-as well as bulk-sensitive methods with overlapping information depths are used to examine an overall depth profile of RbF-free and RbF-treated CIGSe samples with different Cu contents. We show a gradual depletion of copper towards the surface for the as-deposited CIGSe absorber layers. The following RbF-PDT sharpens this effect especially for the sample grown with the overall lower Cu content. Under the Cu-depleted surface layer, the composition of the CIGSe gradually changes until it reaches the respective bulk composition. As a result of the RbF-PDT, Ga diffuses towards the surface and Rb gets incorporated at the surface where GaF3 and RbInSe2 secondary phases are formed, respectively. A higher Cu content leads to less surface oriented Ga diffusion, less Rb incorporation and to a thinner RbInSe2 layer. A thinner RbInSe2 barrier layer, in turn, maintains the gain in open-circuit voltage and prevents the fill factor loss of the CIGSe device.

In this contribution the impact of thermal stress on Cu(In,Ga)Se 2 (CIGSe) thin film photovoltaic devices is investigated. The tolerance of such devices to high temperatures is of particular interest for their potential use as bottom devices in tandem applications in order to overcome the theoretical efficiency limit of single junction solar cells. When CdS-buffered CIGSe high efficiency solar cells are subjected to thermal stress, elemental interdiffusion of Na and Cd between the absorber and the window layers as well as chemical reactions at the CIGSe/CdS interface result in a degraded power conversion efficiency (PCE). Here, we compare the degradation mechanisms of CdS and amorphous GaO x buffered CIGSe solar cells under thermal stress. A model explaining the observed degradation behaviours is proposed.

A comprehensive device model based on SCAPS-1D simulations is presented that reproduces the experimentally determined current-voltage and capacitance-voltage characteristics of a Rb-free reference, a sample that underwent an RbF-treatment, and a sample based on a CIGSe/RbInSe 2 -stack. According to this model, and in agreement with experimental findings, the main consequences of both Rb-conditionings are an increased doping-density and a defect passivation in the CIGSe as well as the formation of a photocurrent-barrier at the hetero-interface. With the numerical model established, fundamental aspects of the Rb-conditioning, as e.g. the differentiation between its effect on bulk and interface recombination are discussed.

This article presents an approach for industrial roll-to-roll (R2R) deposition of Cu(In,Ga)Se2 (CIGSe) thin films on stainless steel foil for use as absorber layers in flexible thin-film solar cells. The hybrid-CIGSe-deposition-process combines magnetron sputtering with evaporation techniques and allows the realization of a double graded CIGSe layer, reaching a conversion efficiency of currently up to 14.0% (w/o antireflective coating). The influence of different power settings of the individual magnetrons on the gallium profile is described. For a deeper understanding of differences between a static and an R2R process, a calculated flux diagram is presented. In the second part of this contribution, the homogeneity of the industrial scale samples is discussed by comparing the change in CGI and GGI over the foil width of 30 cm. The hybrid-CIGSe-deposition-process excels in a straightforward scalability for industrial usage due to the application of planar magnetron sputtering sources as metal vapor supply.

We show a monolithic perovskite/CIGS tandem solar cell with a certified PCE of 24.2% and a high V OC of 1.77 V, enabled by reduction of interface recombination. Using optical simulations, we further optimize the structure of the cell and compare it to perovskite/silicon tandem solar cells. The yearly energy yield of the perovskite/CIGS tandem is only 6% lower than that of the perovskite/silicon tandem, showing great potential of the perovskite/CIGS technology.

Summary Monolithic [Cs0.05(MA0.17FA0.83)0.95]Pb(I0.83Br0.17)3/Cu(In,Ga)Se2 (perovskite/CIGS) tandem solar cells promise high performance and can be processed on flexible substrates, enabling cost-efficient and ultra-lightweight space photovoltaics with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based multijunctions. However, to become a viable space technology, the full tandem stack must withstand the harsh radiation environments in space. Here, we design tailored operando and ex situ measurements to show that perovskite/CIGS cells retain over 85% of their initial efficiency even after 68 MeV proton irradiation at a dose of 2 × 1012 p+/cm2. We use photoluminescence microscopy to show that the local quasi-Fermi-level splitting of the perovskite top cell is unaffected. We identify that the efficiency losses arise primarily from increased recombination in the CIGS bottom cell and the nickel-oxide-based recombination contact. These results are corroborated by measurements of monolithic perovskite/silicon-heterojunction cells, which severely degrade to 1% of their initial efficiency due to radiation-induced recombination centers in silicon., Graphical Abstract, Highlights • Halide perovskite sub-cells exhibit strong proton irradiation resiliency • Novel operando characterization distinguishes degradation of individual sub-cells • Perovskite/CIGS tandem solar cells retain 85% of their initial efficiency after irradiation • Perovskite/SHJ tandem solar cells degrade to 1% of their initial efficiency after irradiation, Context & Scale Monolithic perovskite/silicon and perovskite/CIGS tandem solar cells could facilitate large-scale decarbonization of the power sector, provided their long-term stability is proven. In this work, we test the stability of both technologies under high-energy proton irradiation. While this mimics the radiation environment in space, our versatile operando and ex situ methodology is also suitable for studying the long-term stability of multijunction solar cells for terrestrial applications. We find that perovskite/silicon tandem solar cells are unsuitable for space, whereas perovskite/CIGS tandems are radiation-hard, promising cheap, flexible, and ultra-lightweight space photovoltaics. Both the growing demand for smaller, cheaper satellites and the privatization of space exploration are revolutionizing space economics, providing an ideal niche for the commercialization of this new technology until the levelized cost-of-electricity can compete with current terrestrial photovoltaics., We propose and test monolithic perovskite/CIGS tandem solar cells for readily stowable, ultra-lightweight space photovoltaics. We design operando and ex situ measurements to show that perovskite/CIGS tandem solar cells retain over 85% of their initial power-conversion efficiency after high-energy proton irradiation. While the perovskite sub-cell is unaffected after this bombardment, we identify increased non-radiative recombination in the CIGS bottom cell and nickel-oxide-based recombination layer. By contrast, monolithic perovskite/silicon-heterojunction cells degrade to 1% of their initial efficiency due to radiation-induced defects in silicon.

RbInSe2 is attracting growing interests as a secondary semiconductor compound in Cu(In,Ga)Se2-based solar cells in virtue of the recent investigations on absorber post-deposition treatments with alkali metal salts that have resulted in significant efficiency improvements. However, the detection of the RbInSe2 phase on the surface of chalcopyrite absorbers is very challenging due to its nanometric thickness and to the limited information available about its fundamental properties. In this context, this work expounds a detailed analysis of the vibrational properties of RbInSe2 that combines first-principle calculations with multiwavelength Raman scattering spectroscopy and provides a methodology for the detection and identification of very thin layers of this material employing solely optical measurements. As a result, we present here the classification of the different vibrational modes together with the fingerprint Raman spectra of RbInSe2 thin films measured under five different excitations (close to and far from resonance). The employment of a 442 nm excitation wavelength is found to be the most adequate strategy for the detection and characterization of the RbInSe2 phase in view of its resonance with the bandgap of the material and its low penetration depth. Additionally, the purity of the deposited thin films as well as the possible influence of the subjacent layers on the Raman spectra of the compound are also investigated by analyzing the presence of secondary phases and by measuring RbInSe2 thin films deposited onto Mo-coated soda-lime glass, respectively. These results set the basis for the future evaluation of the suitability of Raman spectroscopy as a fast and non-destructive characterization technique for a reliable identification and characterization of the nanometric layers of RbInSe2 in Cu(In,Ga)Se2-based solar cells., The authors would like to thank B. Bunn and K. Mayer-Stillrich for the preparation of the Mo-covered SLG substrates. This research was supported by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the Spanish Ministry of Science, Innovation and Universities under the WINCOST (ENE2016-80788-C5-1-R), and by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). Authors from IREC and the University of Barcelona belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the "Generalitat de Catalunya" (Ref. 2017 SGR 862). The authors would like to acknowledge financial support from the German Bundesministerium für Wirtschaft und Energie (BMWi) for the speedCIGS project (0324095C and 0324095D). RKMR, TDK, and HM would like to acknowledge the Paderborn Center for Parallel Computing (PC 2) for computing time on OCuLUS and FPGA-based supercomputer NOCTUA. MG This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and the Government of Catalonia's Agency for Business Competitiveness(ACCIÓ).

Thin film solar cells based on Cu-poor Cu(In,Ga)Se2 absorbers grown by a three-stage co-evaporation process have been shown to strongly benefit from a Cu-rich intermediate growth stage and the presence of alkali elements such as Na, K, and Rb, pushing the current record efficiency to 22.6%. However, some details of the mechanisms underlying these improvements are not yet fully understood, particularly on a very local scale. We therefore used extended X-ray absorption fine structure spectroscopy to study element-specific local structural parameters of Cu(In,Ga)Se2 thin films with varying final Cu content, varying history of the Cu content, and varying alkali treatment. We find that the bulk structure on a subnanometer scale is unaffected by the presence of alkali elements, confirming that their beneficial effect mostly stems from improving the electronic properties of the material. In contrast, local structural disorder clearly increases with decreasing Cu content but does not depend on whether or not the material has passed through a Cu-rich intermediate stage. While the latter is known to improve film morphology and microstructure, it obviously has no effect on the subnanoscale structure, which exhibits a fully reversible correlation with the final Cu content of the Cu(In,Ga)Se2 absorber.

Rb, In and Se were deposited in a co-evaporation chamber at various substrate temperatures (TSub = 280°C, 350°C, 450°C, 550°C) on Mo coated soda-lime glass. It was found that the RbInSe2 crystal phase is already detectable for nominal TSub = 350°C. During calibrated in-situ high-temperature grazing incidence X-ray diffraction on the sample grown at the lowest temperature, crystallization of monoclinic RbInSe2 can be observed at 215°C ± 3°C. The morphology of the layers shows distinct differences for increasing TSub, while the elemental composition of the deposited thin films only exhibits minor changes. The deposited layers show a decrease of (X-ray) amorphous constituents and changes in the preferred crystal orientation of the RISe layer with increasing TSub. The influence of Na supply on the layer growth was investigated by applying a SiNxOy diffusion barrier layer in between the glass substrate and the Mo layer on half of the studied samples.

A remarkable discrepancy between the optically active bandgap of Cu(In,Ga)(S,Se) 2 absorber layers for thin film solar cells and the minimum bandgap as determined via elemental depth profiling has been observed in this study. This behavior occurs in absorbers sequentially grown by sulfurization after selenization of metal precursors and is demonstrated and explored in the following, using glow discharge optical emission spectroscopy, external quantum efficiency and Raman scattering. Furthermore this mismatch is explained by investigating the microscopic elemental distributions using transmission electron microscopy. It turns out, that sulfur is - on a microscopic scale - inhomogeneously distributed in the bulk of the absorber and solely present in areas near the absorber surface itself and at inner surfaces, e.g. in voids in the bulk.

Sequential Cu(In,Ga)Se 2 fabrication with a thermally activated reaction of Cu-In-Ga metal precursor layers in chalcogen atmosphere is an industrially attractive route for preparation of Cu(In,Ga)Se 2 absorber based thin film solar cells. Recent results show that controlling the selenium supply during rapid thermal processing has a huge impact on absorber growth. Especially a two stage process applying a first annealing step with or without Se at temperatures up to 400 °C was shown to have a positive effect on the elemental in-depth distribution. However, during this annealing, lateral phase separation, dewetting and coarsening may occur in the metal phase, leading to lateral non-uniformity of the absorber. In this study we show how the dewetting can be strongly decreased by adjusting the precursor architecture, applying faster heating rates and NaF addition on top of a precursor. In contrast, NaF deposited underneath the precursor increases the dewetting effect. Further we show that lateral phase separation during annealing increases with temperature and leads to phase domain sizes of several micrometers at 580 °C.

We study the Cu(In,Ga)Se2 absorber fabrication by fast atmospheric pressure selenization of metal precursor films utilizing elemental selenium vapor from independent sources. We find that a high Se supply during selenization leads to the generally observed Ga accumulation at the back contact, while a reduced Se supply leads to a more homogeneous Ga distribution within the absorber. By optimizing the precursor as well as the Ga in‐depth distribution in the CIGSe layer, a conversion efficiency of 15.5% was achieved.

The change of electrical conductivity in chalcopyrite (i.e., Cu(Inx, Ga1−x)Se2 or CIGSe) solar cells induced by nanosecond laser pulses is investigated as a function of the elemental composition and its spatial distribution. The underlying laser induced phase transformation process, which results in a decomposition of the CIGSe semiconductor and a modification of its elemental composition, is utilized to form the monolithic series interconnection between front and back contact in CIGSe based thin film solar cells. The results show a dependence of the composition of the CIGSe layer and the resulting series resistance on the applied laser fluence. Lower series resistance is primarily related to an enhanced fraction of copper, gallium and zinc in the laser transformed zone resulting from selective vaporization of absorber elements. For intermediate laser fluences (~0.36 J/cm2) a patterning process is established that allows reliable and high-quality series interconnection. Both, lower and higher laser fluences result in high series resistances due to incomplete phase transformation or damages of the back contact, respectively.

In this contribution we are comparing the commonly used RbF post deposition treatment (PDT) of CIGS absorber layers to an alternative RbCl-PDT. The RbCl-PDT leads to similar improvements of the cell efficiency as the fluorine treatment does. While the effect of the used procedure on the open circuit voltage is not as strong as it is in case of the RbF-PDT, we could reach an even higher fill factor. Furthermore we are investigating the impact of the RbCl treatment on the degradation of already RbF-treated absorber layers. We find that the additional RbCl-PDT can decrease degradation by air-, moisture- and low temperature-exposure. A complete Na-depletion of the surface of the absorber layer is found as a possible reason for this reduced degradation. Furthermore the RbCl-PDT suppresses the formation of an additional oxide-species, which is found on the surface of degraded absorber layers.

Determining elemental distributions dependent on the thickness of a sample is of utmost importance for process optimization in different fields e.g. from quality control in the steel industry to controlling doping profiles in semiconductor labs. Glow discharge optical emission spectrometry GD OES is a widely used tool for fast measurements of depth profiles. In order to be able to draw profound conclusions from GD OES profiles, one has to optimize the measurement conditions for the given application as well as to ensure the suitability of the used emission lines. Furthermore a quantification algorithm has to be implemented to convert the measured properties intensity of the emission lines versus sputtering time to more useful parameters, e.g. the molar fractions versus sample depth depth profiles . In this contribution a typical optimization procedure of the sputtering parameters is adapted to the case of polycrystalline Cu In,Ga S,Se 2 thin films, which are used as absorber layers in solar cell devices, for the first time. All emission lines used are shown to be suitable for the quantification of the depth profiles and a quantification routine based on the assumption of constant emission yield is used. The accuracy of this quantification method is demonstrated on the basis of several examples. The bandgap energy profile of the compound semiconductor, as determined by the elemental distributions, is compared to optical measurements. The depth profiles of Na the main dopant in these compounds are correlated with measurements of the open circuit voltage of the corresponding devices, and the quantification of the sample depth is validated by comparison with profilometry and X ray fluorescence measurements

Buried wurtzite structures composed by stacking faults of the 111 planes in zinc blende and 112 planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of stacking fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co evaporation of Cu In,Ga Se2 a photovoltaic absorber material showing record efficiencies of up to 22.9 for thin film solar cells a reduction of stacking faults occurs at the transition from a Cu poor to a Cu rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X ray diffraction during annealing of Cu poor CuInSe2 thin films that stacking faults can be strongly reduced through annealing, without passing through a Cu rich film composition. We simulate the evolution of the X ray diffraction stacking fault signal with a simple numerical model of grain growth driven by stacking fault energy and grain boundary curvature. The results support the hypothesis that the stacking fault reduction can be explained by grain growth. The model is used to make predictions on annealing times and temperatures required for stacking fault reduction and could be adapted for polycrystalline thin films with similar morphology

In this contribution we are analyzing and comparing the impact of two different alkali-fluorine post deposition treatments (KF and RbF) on the growth of chemical-bath-deposited CdS buffer layers on Cu ( In , Ga ) Se 2 absorber layers for thin film solar cells. By combining Raman scattering, scanning electron microscopy, current-voltage analysis and measurements of the internal quantum efficiency we provide a comprehensive picture of this issue on the material and device level. We find that both PDTs lead to a better CdS-coverage of the surface of the CIGS, which leads to an improved junction quality at early growth stages compared to untreated devices. Furthermore the growth rate of the CdS is enhanced on KF-treated absorber layers while it is decreased on those treated with RbF (compared to the reference). This leads to a more stable behavior of RbF-treated devices after longer duration of the CdS deposition, while the KF-treated devices suffer from reduced fill factor and open circuit voltage. Furthermore we show that not only both PDTs but also the growth of the CdS lead to a reduction of the amount of the so called ordered defect compound, which is initially present at the surface of our absorber layers. This behavior indicates either the formation of Cd Cu -anti-sites or of a secondary phase at the interface.

The rapid rise of perovskite solar cells (PSCs) is increasingly limited by the available charge-selective contacts. This work introduces two new hole-selective contacts for p–i–n PSCs that outperform all typical p-contacts in versatility, scalability and PSC power-conversion efficiency (PCE). The molecules are based on carbazole bodies with phosphonic acid anchoring groups and can form self-assembled monolayers (SAMs) on various oxides. Besides minimal material consumption and parasitic absorption, the self-assembly process enables conformal coverage of arbitrarily formed oxide surfaces with simple process control. The SAMs are designed to create an energetically aligned interface to the perovskite absorber without non-radiative losses. For three different perovskite compositions, one of which is prepared by co-evaporation, we show dopant-, additive- and interlayer-free PSCs with stabilized PCEs of up to 21.1%. Further, the conformal coverage allows to realize a monolithic CIGSe/perovskite tandem solar cell with as-deposited, rough CIGSe surface and certified efficiency of 23.26% on an active area of 1 cm2. The simplicity and diverse substrate compatibility of the SAMs might help to further progress perovskite photovoltaics towards a low-cost, widely adopted solar technology.

During 3-stage thermal co-evaporation of Cu(In,Ga)Se2 (CIGSe), alkali aluminosilicate (AAS) glass has proven to be stable at process temperatures Tmax of up to 650 °C. This is considerably higher than the maximum endurable Tmax for soda-lime glass (SLG), which is in the range of 530–550 °C. As the Na-content of the AAS glass used here is comparable to that of SLG, growing CIGSe at elevated Tmax is expected to promote Na diffusion from the glass substrate through the Mo back contact (BC) into the absorber layer. An increased Na concentration in the CIGSe absorber would be expected to result in an improved device performance as it is mostly associated with an increased open-circuit voltage (Voc) and fill factor (FF). This paper discusses the influence of varying Tmax from 530 to 650 °C during the 2nd and the 3rd stage of the 3-stage process on the morphological, electrical, and photovoltaic performance of CIGSe solar cells. Upon raising Tmax from 530 to 650 °C, the short-circuit current density (Jsc) decreased due to bandgap (Eg) widening. Glow discharge optical emission spectrometry analysis reveals that the Na concentration in the absorber is gradually decreased when elevating Tmax. As will be seen, we attribute this in part to a densification of the Mo BC during the high temperature process. A maximum conversion efficiency (η) was realized at Tmax = 600 °C. An increased Voc at Tmax = 600 °C is due to the wider Eg and to an increased carrier concentration despite the fact that the Na concentration in the CIGSe thin film was low compared to lower Tmax. Admittance spectroscopy analysis is performed to access information on defect energy level and density in the finished devices. In the light of present findings, ways to further improve η at elevated Tmax are suggested.

The superstrate configuration of Cu(In,Ga)Se2 (CIGS) photovoltaic devices exhibits the potential for improved light harvesting compared to the standard substrate configuration. Due to the limited power conversion efficiency of superstrate devices caused by the poor hetero-interface between the p-type CIGS and the n-type component, these advantages have not been fully considered so far. This work shows that the superstrate configuration can significantly improve the light management within CIGS devices. Experimentally, it is shown that the electric and optical properties of ZnO window layer strongly improve during the CIGS deposition, allowing the use of structured ZnO layers for efficient light-trapping. A highly reflective and scattering back contact is realized by using the high work function material MoO3−x for the hole-transport layer in combination with a low work function Ag metal layer. Combined with the unique Ga gradient achievable in the superstrate configuration, simulation results show that a strong reduction of the CIGS layer thickness is possible while increasing the power conversion efficiency. This proof of concept highlights the appeal of the superstrate configuration for the future of cost-effective CIGS solar cell production.

The present work investigates the potential of ion beam analysis (IBA) techniques such as the Rutherford backscattering spectrometry (RBS) and particle induced X-ray emission (PIXE) using helium ions to provide quantitative in-depth elemental analysis of various types of Cu(In,Ga)Se 2 thin films. These films with a thickness of about 2 μm are used as absorber layers in photovoltaic devices with continuously increasing the performance of this technology. The preparation process generally aims to obtain an in-depth gradient of In and Ga concentrations that optimizes the optoelectronic and electrical properties of the solar cell. The measurements were performed at directly accessible single or double layered CIGS absorbers and at buried absorbers in completed thin film solar cells. The IBA data were analyzed simultaneously in order to derive best fitted profiles that match all experimental RBS and PIXE spectra. For some samples elemental profiles deduced form synchrotron based, reference free grazing incidence X-ray fluorescence analysis were compared with the IBA results and an overall good agreement was observed within quoted uncertainties.

A new industrial approach for the production of CuInSe2 (CISe) absorber layers in a roll-to-roll process is described. This process is used by Sunplugged GmbH and combines magnetron sputtering with thermal coevaporation. A CISe-based device with a conversion efficiency of more than 10% has been fabricated from absorbers grown on polyimide at low temperature. The characteristic properties of a copper-poor ([Cu]/[In] = 0.73) CISe layer, deposited by Sunplugged's industrial process, are compared to those of a state-of-the-art, more copper-rich ([Cu]/[In] = 0.93) layer, grown in a molecular beam epitaxy system at Helmholtz-Zentrum Berlin. The presence of a so called vacancy-compound in low [Cu]/[In] CISe devices exhibiting an increased bandgap energy is visible by external quantum efficiency measurements, Raman scattering, and scaps simulations.

Alkali metals play a crucial role in the performance and growth of polycrystalline CIGSe solar cells. Here solar cells are investigated, that were grown in a high substrate temperature co-evaporation process with a focus on the depth distribution of Na at very high process temperatures. Devices with and without RbF post deposition treatment are characterized with the goal of understanding how the two alkali metals interact. It can be shown that at high process temperatures only very little Na is present in the films and located only at the absorber surface. After a RbF-PDT even the superficial Na disappears in all finished devices. The strong influence of the temperature on the Na content in the films needs to be accounted for and optimized for high efficiency solar cells grown above 600°C.

To gain an in-depth understanding of the mechanisms and driving forces responsible for the microstructural changes during the Cu-Se deposition of the three-stage Cu(In,Ga)Se 2 co-evaporation process and its effect on the absorber quality, we have performed a series of in-situ studies as well as complementing ex-situ studies in the recent past. Our studies provide insights into several correlating changes of the structural and physical properties of the absorber layer during Cu-Se deposition or subsequent annealing, such as defect annihilation, stress relaxation, texture changes, and grain growth. In this contribution, we combine these various observations and discuss how they can be explained by a model of abnormal grain growth, driven by differences in the defect densities of neighboring grains.

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses.

Latest record efficiencies of Cu In,Ga Se2 CIGSe solar cells were achieved by means of a rubidium fluoride RbF post deposition treatment PDT . To understand the effect of the RbF PDT on the surface chemistry of CIGSe and its interaction with sodium that is generally present in the CIGSe absorber, we performed an X ray photoelectron spectroscopy XPS study on CIGSe thin films as deposited by a three stage co evaporation process and after the consecutive RbF PDT. The sample transfer from the deposition to the XPS analysis chamber was performed via an ultra high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali treated absorbers. Beside an expected reduction of Cu and Ga content at the surface of RbF treated CIGSe films, we find that Rb penetrates the CIGSe and, contrary to fluorine, it is not completely removed by subsequent ammonia etching. The remaining Rb contribution at 110.0 amp; 8239;eV binding energy, which appears after the RbF PDT is similar to the one detected on a co evaporated RbInSe2 reference sample and together with a new Se 3d contribution may hence belong to an Rb In Se secondary phase on the CIGSe surface. In addition, Na is driven towards the surface of the CIGSe absorber as a direct result of the RbF PDT. This proves the ion exchange mechanism in the absence of moisture and air oxygen between heavy Rb atoms incorporated via PDT and lighter Na atoms supplied by the glass substrate. A remaining XPS signal of Na 1 amp; 8239;s is observed after etching the vacuum transferred RbF CIGSe sample, indicating that Rb and or F are not as much a driving force for Na as oxygen usually is

Either metallic Na or NaF were deposited onto Cu(In,Ga)Se2 surfaces and studied by photoelectron spectroscopy and surface photovoltage spectroscopy without breaking the ultra-high vacuum. The deposition of elemental Na at room temperature led to the formation of an intermediate Cu and Ga rich layer at the CIGSe surface, whereas for NaF the composition of the CIGSe surface remained unchanged. A metal like surface induced by an inverted near surface region with a reduced number of defect states was formed after the deposition of Na. Under the chosen experimental conditions, the near surface layer was independent on the amount of Na and stable in time. In contrast, the usage of NaF weakened the inversion and led to an increased band bending compared to the untreated CIGSe sample. The SPV signals decreased with proceeding time after the deposition of NaF.

The continuous improvement of Cu(In,Ga)Se2 (CIGSe) solar cells relies considerably on advanced characterization of individual layers in the solar-cell stacks as well as of completed CIGSe devices. The present contribution provides an overview of corresponding efforts performed by various research groups at Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH. In-situ growth monitoring of CIGSe absorber layers by means of energy-dispersive X-ray spectrometry and light scattering is described, as well as structural analyses by means of X-ray and neutron diffraction. In addition, the characterization of surfaces and interfaces by soft X-ray and electron spectroscopy, the microscopic analysis by means of correlative electron microscopy, and optoelectronic characterization by optical spectroscopy are highlighted. The present contribution shows which substantial efforts in a research network are necessary in order to obtain deeper insight into materials properties and potentially limiting factors for the device performance, as well as to be able to control these factors during the solar-cell production.

Atomic-scale structural parameters of Cu(In,Ga)Se2 powders and polycrystalline thin films were determined as a function of the In and Cu contents using X-ray absorption spectroscopy. No difference in the two sample types is observed for the average bond lengths demonstrating the strong tendency towards bond length conservation typical for tetrahedrally coordinated semiconductors. In contrast, the bond length variation is significantly smaller in the thin films than in the powders, particularly for Cu-poor material. This difference in the nanostructure is proposed to originate from differences in the preparation conditions, most prominently from the different history of Cu composition.