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CuInSe2 (CIS) has been proposed as an attractive bottom cell candidate in tandem solar cells. However, to justify the coupling with high-performance top cells (e.g., perovskites, GaAs), significant...

Strain and interactions at grain boundaries during solid-phase crystallization are known to play a significant role in the functional properties of polycrystalline materials. However, elucidating three-dimensional nanoscale grain morphology, kinetics, and strain under realistic conditions is challenging. Here, we image a single-grain growth during the amorphous-to-polycrystalline transition in technologically relevant transparent conductive oxide (TCO) film of In2O3:Zr with in-situ Bragg coherent X-ray diffraction imaging and transmission electron microscopy. We find that the Johnson-Mehl-Avrami-Kolmogorov theory, which describes the average kinetics of polycrystalline films growth, can be applied to the single grains as well. The quantitative analysis stems directly from imaging results. We elucidate the interface-controlled nature of the single-grain growth in thin films and reveal the surface strains which may be a driving force for anisotropic crystallization rates. Our results bring in-situ imaging with coherent X-rays towards understanding and controlling the crystallization processes of TCOs and other polycrystalline materials at the nanoscale.

IEEE journal of photovoltaics 9(6), 1790-1799 (2019). doi:10.1109/JPHOTOV.2019.2942487, Strain within grains and at grain boundaries (GBs) in polycrystalline thin-film absorber layers limits the overall performance because of higher defect concentrations and band fluctuations. However, the nanoscale strain distribution in operational devices is not easily accessible using standard methods. X-ray nanodiffraction offers the unique possibility to evaluate the strain or lattice spacing at nanoscale resolution. Furthermore, the combination of nanodiffraction with additional techniques in the framework of multimodal scanning X-ray microscopy enables the direct correlation of the strain with material and device parameters such as the elemental distribution or local performance. This approach is applied for the investigation of the strain distribution in CdTe grains in fully operational photovoltaic solar cells. It is found that the lattice spacing in the (111) direction remains fairly constant in the grain cores but systematically decreases at the GBs. The lower strain at GBs is accompanied by an increase of the total tilt. These observations are both compatible with the inhomogeneous incorporation of smaller atoms into the lattice, and local stress induced by neighboring grains., Published by IEEE, New York, NY

Copper is a traditional dopant for many types of polycrystalline thin-film CdTe photovoltaic devices. However, Cu can easily distribute through the depth and breadth of the device, segregating at interfaces or grain boundaries and leading to metastability of the device. Directly correlating Cu-related defect species to the local (i.e. nanoscale) charge transport in CdTe devices remains challenging due to relatively low Cu concentrations in the CdTe layer. Using nanoscale X-ray microscopy, we simultaneously probe both the elemental copper distribution and electrical performance of the device in cross-section. Complementary charge transport modelling delineates the possible defect distributions that can exist under low and high Cu loading, and how these defects interact with charge carriers at different depths of the device.

A fully encapsulated c-Si solar cell was evaluated using focused X-ray and laser beams to probe the microscopic electrical performance and composition. Particular emphasis was placed on the influence of the silver fingers on the laser (LBIC) and X-ray beam induced current (XBIC). Therefore, an uncommonly high X-ray energy of 28keV was utilized for maximum sensitivity to the Ag distribution measured by X-ray fluorescence through the back sheet. The direct comparison of LBIC and XBIC measurements yields a comprehensive picture of these techniques, illustrating the advantages and challenges of both approaches. Specifically, the effect of heavy elements acting as a secondary photon source that increase the XBIC signal is discussed and supported by Monte-Carlo simulations.

X-ray absorption spectroscopy (XAS) has been shown to be a powerful tool to unravel the chemical environment of a given atom within a matrix. When used in correlative X-ray microscopy approaches, XAS allows one to probe with nanoscale precision regions of particular interest in an absorber. Herein, we use X-ray absorption near edge structure (XANES) to evaluate the chemical environment of Cu atoms within a CdTe solar cell. The reconstruction of XANES spectra from XRF maps have unfolded 2D maps of Cu chemical structures. In this work, we found that most Cu atoms exist in Cu 2 Te and Cu 1.4 Te phase. Moreover, we found traces of CuTe, Cu 2 O, CuO, Cu 2 S, CuS, and metallic Cu phase. Investigating Cu chemical structures at different performing areas, we found no observable correlation between Cu chemical structures and electrical performance. This approach allows tracking of Cu chemical structures along with electrical performance and elemental distribution simultaneously, with high spatial resolution in a statistically practical way.

Materials 14(5), 1189 (2021). doi:10.3390/ma14051189, Published by MDPI, Basel

Coatings 10(9), 820 (2020). doi:10.3390/coatings10090820 special issue: "Advances in Thin Films for Photovoltaic Applications", The method of detecting deep defects in photovoltaic materials by Fourier-Transform Photocurrent Spectroscopy has gone through continuous development during the last two decades. Still, giving quantitative predictions of photovoltaic device performance is a challenging task. As new materials appear, a prediction of potentially achievable open-circuit voltage with respect to bandgap is highly desirable. From thermodynamics, a prediction can be made based on the radiative limit, neglecting non-radiative recombination and carrier transport effects. Beyond this, more accurate analysis has to be done. First, the absolute defect density has to be calculated, taking into account optical effects, such as absorption enhancement, due to scattering. Secondly, the electrical effect of thickness variation has to be addressed. We analyzed a series of state-of-the-art hydrogenated amorphous silicon solar cells of different thicknesses at different states of light soaking degradation. Based on a combination of empirical results with optical, electrical and thermodynamic simulations, we provide a predictive model of the open-circuit voltage of a device with a given defect density and absorber thickness. We observed that, rather than the defect density or thickness alone, it is their product or the total number of defects, that matters. Alternatively, including defect absorption into the thermodynamic radiative limit gives close upper bounds to the open-circuit voltage with the advantage of a much easier evaluation., Published by MDPI, Basel

The origin of metastability in $\text{Cu}(\text{In}_{1-x}\text{Ga}_{x})\text{Se}_{2}$ solar cells has long be a topic of study and debate. The drop in device performance after light soaking has been attributed to changes in the V Se -V Cu defect pair and to the electromigration of Cu. In this work, we use a suite of X-ray microscopy tools to study the nanoscale distribution of Cu and Se in industrially-relevant CIGS modules on stainless steel before and after accelerated stress testing from dark heat and light soaking. Changes in chemical inhomogeneity are studied through nano-scale X-ray fluorescence and correlated to electrical performance through Xray beam induced current and voltage. We observe no meaningful variation in the Cu distribution before and after light soaking but a sensible difference in the Se distribution. Average XBIC values at the nano-scale trend directly with changes in full device $J_{\text{sc}}$ both of which see a reduction after light soaking. We note that low performing areas of XBIC in both the initial and dark heat samples perform significantly worse after light soaking when compared to changes in the good performing areas.

For decades, copper has been introduced in CdTe devices to improve overall performance (open circuit voltage, fill factor, and series resistance). While multiple articles have reported on Cu-based defects, very little is known about how the local structure around the copper atom affects electrical performance. Using X-ray Absorption Near Edge Structure (XANES) coupled with X-ray microscopy we investigate good and poor performing region in Cu-doped CdTe devices. Our XANES coupled with theoretical standards by FEFF9 suggest that CU2Te phase and Cu Cd may be responsible for the high electrical performance of the regions under study. This correlation of structure-performance at the nanoscale offers a unique framework to understand and tune processes with deep implications to the overall electrical performance of the solar cell.

IEEE Photovoltaic Specialists Conference, PVSC, Virtual conference, Online only, 15 Jun 2020 - 19 Jun 2020; tbd (2020)., Scanning probe measurements are an indispensable tool of solar cell research today, and the compatibility with simultaneous acquisition of complementary measurement modes is a particular strength. However, multi-modal data acquisition is often limited by different scan-parameter requirements. As a consequence, the modalities may be assessed subsequently rather than simultaneously. In this instance, image registration serves as a tool to align two-dimensional datasets at nanoscale. Here, we showcase an example of two subsequent scanning Xray microscopy measurements of solar cells with a Cu(In,Ga)Se2 absorber, the first measurement being optimized for X-ray beam induced current and the second for X-ray fluorescence. We discuss different approaches and pitfalls of image registration and its potential combination with Gaussian filtering. This finally allows us to proceed with the investigation of point-by-point correlations.

For decades, copper has been used to improve the performance of cadmium telluride thin film solar cells. However, it has also been shown to be the main cause of metastability in CdTe. Recently a low activation energy has been reported for the thermal diffusion of Cu in CdTe explaining the ease of motion that it has under moderate temperatures. The community consensus is that copper segregates to the absorber grain boundaries, where it's either beneficial or detrimental to device performance depending on its concentration. Using nanoscale X-ray micrsocopy and a two-dimensional drift-diffusion model we present a preliminary correlation between local copper distribution and electrical performance of a single-junction CdTe/CdS solar cell.

Within the analysis of solar cells with multi-modal X-ray microscopy, X-ray fluorescence (XRF) measurements have become a reliable source for evaluating elemental distributions. While XRF measurements can unveil the elemental distribution at unparalleled sensitivity and spatial resolution, the quantitative analysis is challenged by effects such as self-absorption and further complicated by the inclusion of multiple detector modules. Here, we showcase the exemplary analysis of XRF spectra obtained from a Cu(In, Ga)Se2 solar cell utilizing four detector modules. After cataloging typical features found in XRF spectra, we demonstrate the inclusion of detector modules with individual absorption correction. This results in quantitative stoichiometric ratios of the critical absorber elements Cu, In, and Ga that are in good agreement with the nominal ratios. These results are particularly relevant in view of future measurements at diffraction-limited synchrotron beamlines: in order to profit from the boost of nano-focused photon flux, XRF measurements will require multiple detector modules, for which we demonstrate an approach of quantitative analysis.

Journal of applied crystallography 53(4), 957 - 971 (2020). doi:10.1107/S1600576720008420, Ptychographic X-ray imaging at the highest spatial resolution requires an optimal experimental environment, providing a high coherent flux, excellent mechanical stability and a low background in the measured data. This requires, for example, a stable performance of all optical components along the entire beam path, high temperature stability, a robust sample and optics tracking system, and a scatter-free environment. This contribution summarizes the efforts along these lines to transform the nanoprobe station on beamline P06 (PETRA III) into the ptychographic nano-analytical microscope (PtyNAMi)., Published by Wiley-Blackwell, [S.l.]

For decades, Cu has been the primary dopant in CdTe photovoltaic absorbers. Typically, Cu acceptor concentrations in these devices are on the order of 1 × 1014 cm−3, which has made it notoriously difficult to directly correlate nanoscale Cu distributions to the local charge transport properties of these devices. To measure and correlate these properties, measurement techniques require high sensitivity to elemental concentration, large penetration depth, and operando compatibility. Techniques such as secondary-ion mass spectroscopy and X-ray energy dispersive spectroscopy are widely adopted to measure Cu concentrations, but they are limited by penetration depth, sensitivity, or spatial resolution. Additionally, they lack the operando capabilities required to correlate one-to-one Cu concentrations to electrical performance. In this work, correlative X-ray microscopy is used to investigate the spatial distribution of Cu and its impact on charge collection through the depth and breadth of CdTe photovoltaic devices. Plan-view, nanoscale X-ray fluorescence maps clearly demonstrate the spatial segregation of copper around regions thought to be CdTe grain boundaries. Complementary cross-section imaging unveils the transition of the maximum charge-collection efficiency from the ZnTe–CdTe interface to the CdS–CdTe interface as a function of Cu incorporation. The copper concentration through the depth of the CdTe layer is characterized by slow and fast diffusion components, and cross-section charge-transport modeling shows that the experimentally obtained charge collection can be explained by the modeled acceptor distribution through the depth of the CdTe layer.

We investigate the kinetics of elemental segregation during the synthesis of Cu(In1−x,Gax)Se2 (CIGS) thin films via a rapid thermal growth process. Changes in elemental concentration are mapped rapidly via in situ X-ray fluorescence (XRF) microscopy. In situ XRF enables the quantification of Cu, In, Ga, and Se concentrations with 200 nm spatial resolution over a region of 7 μm × 7 μm, large enough to map multiple CIGS grains. Reacting elemental precursor stacks at 500, 550, and 600 °C, we find that copper is the driving force for α-CIGS phase formation at a rate of 17 nmol/cm2/min at 500 °C. Because of the thermal expansion of the substrate, cluster analysis and classification algorithms were employed to track compositionally similar regions over time and across samples measured at different temperatures. Beyond the direct application of these results for the optimization of CIGS manufacturing, these in situ experiments demonstrate the feasibility of quantitative compositional evaluation at the nanoscale ...

Unveiling the correlation among electrical performance, elemental distribution, and defects at the microscale is crucial for the understanding and improvement of the overall solar cell performance. While this is true in general for solar cells with polycrystalline absorber layers, it is particularly critical for defect engineering of the complex quaternary CuIn $_x$ Ga $_{1-x}$ Se $_2$ (CIGS) material system. Studying these relationships under standard ambient conditions can provide important insights but does not provide input on the behavior of the cell under real operating conditions. In this contribution, we take a close look at the complex temperature dependence of defects and voltage in CIGS at the microscale. We have developed correlative X-ray microscopy methods and adapted them for temperature-dependent measurements of the locally generated voltage and elemental compositions at the microscale. We have applied these techniques to industrial CIGS solar cells covering temperatures from room temperature up to 100 $^\circ \text{C}$ . We find underperforming areas spanning multiple grains that do not correlate with the elemental distribution of major absorber constituents. However, we demonstrate that low-performing areas perform better at higher temperatures relative to the high-performing areas.

The lamination process of photovoltaic modules relies on the application of high temperatures and pressures, which inherently introduces different amounts of expansion and shrinkage of the individual layers including glass, solar cells, polymeric encapsulants, and backsheet. There is no doubt that these effects translate into the cell in the form of deflection and stresses. Thus far though, only the consequences of this have been tracked in terms of failure modes—microcracks, delamination, stress points, etc. The general approaches have been applied to optimize processes with a focus on encapsulant properties, such as degree of cross-linking, moisture permeability, and their long-term lifetime, overlooking their effect on the solar cells. Module reliability is a major driver to lower the levelized cost of electricity and the bankability of projects, and more effort needs to be placed in predictive failure analysis and the optimization of the module components from the point of view of the active components—the cells. In this paper, we propose an in-house X-ray based technique as a novel approach to assess the state of the solar cell under polymeric encapsulation inside a fully assembled module. This gives access for the first time not only to the evaluation of cracks and microdefects, but also to the cell deflection and stress distribution inside the encapsulation.

There is no doubt about the potential offered by the low-cost fabrication and high efficiency of hybrid organic–inorganic perovskite solar cells. However, the service lifetimes of these devices must be increased from months to years to capitalize on their potential. The archetypal hybrid perovskite for solar cells, methylammonium lead iodide (CH3NH3PbI3, abbreviated MAPI), readily degrades in ambient atmosphere under standard operating conditions. Understanding the origin and effects of this degradation can pave the way to better engineer photovoltaic devices and the perovskite material itself. Herein we present the effects of varying pressure on the electrical performance of MAPI solar cells. Solar cell parameters, especially open circuit voltage, are significantly affected by the total ambient pressure and present an unexpected reversible behavior upon pressure cycling. We complement photoluminescence studies as a function of ambient atmosphere and temperature with first-principles density functional t...

Hydrogenated amorphous silicon (a-Si:H) has been used for decades—doped and as intrinsic absorber layers—in thin-film silicon solar cells. Whereas their effiency was improved for a long time by the deposition of higher quality absorber layers, recent improvements can be attributed to a better understanding of the interfaces, allowing for their specific engineering. In this review, we briefly resume the state-of-the-art of a-Si:H solar cell technology from growth and characterization of single layers to full solar cells and multijunction devices. Focusing on the absorber layer quality first, we highlight thereafter aspects of interface problematics and discuss the growth and role of doped microcrystalline silicon-oxide layers and approaches of 3D-solar-cell designs in more detail. Although the findings summarized in this review were obtained from thin-film solar cells, we show that a-Si:H is a very versatile material with properties that are of high interest for application in other devices such as heterojunction solar cells, detectors, or optoelectronic devices.

In situ and operando measurement techniques combined with nanoscale resolution have proven invaluable in multiple fields of study. We argue that evaluating device performance as well as material behavior by correlative X-ray microscopy with

Statistical and correlative analysis are increasingly important in the design and study of new materials, from semiconductors to metals. Non-destructive measurement techniques, with high spatial resolution, capable of correlating composition and/or structure with device properties, are few and far between. For the case of polycrystalline and inhomogeneous materials, the added challenge is that nanoscale resolution is in general not compatible with the large sampling areas necessary to have a statistical representation of the specimen under study. For the study of grain cores and grain boundaries in polycrystalline solar absorbers this is of particular importance since their dissimilar behavior and variability throughout the samples makes it difficult to draw conclusions and ultimately optimize the material. In this study, we present a nanoscale in-operando approach based on the multimodal utilization of synchrotron nano x-ray fluorescence and x-ray beam induced current collected for grain core and grain boundary areas and correlated pixel-by-pixel in fully operational Cu ( In ( 1 − x ) Ga x ) Se 2 solar cells. We observe that low gallium cells have grain boundaries that over perform compared to the grain cores and high gallium cells have boundaries that under perform. These results demonstrate how nanoscale correlative X-ray microscopy can guide research pathways towards grain engineering low cost, high efficiency solar cells.

Metal impurities are known to hinder the performance of commercial Si-based solar cells by inducing bulk recombination, increasing leakage current, and causing direct shunting. Recently, a set of photoluminescence (PL) images of neighboring multicrystalline silicon wafers taken from a cell production line at different processing stages has been acquired. Both band-to-band PL and sub-bandgap PL (subPL) images showed various regions with different PL signal intensity. Interestingly, in several of these regions a reversal of the subPL intensity was observed right after the deposition of the antireflective coating. In this paper, we present the results of the synchrotron-based nano-X-ray fluorescence imaging performed in areas characterized by the subPL reversal to evaluate the possible role of metal decoration in this uncommon behavior. Furthermore, the acquisition of a statistically meaningful set of data for samples taken at different stages of the solar cell manufacturing allows us to shine a light on the precipitation and rediffusion mechanisms of metal impurities at these grain boundaries.

Nano-X-ray fluorescence microscopy is used to correlate elemental inhomogeneity to device performance measured through X-ray beam induced current (XBIC). Unsupervised machine learning techniques can be useful in the processing and correlation of XRF/XBIC data. A case study using CuInSe 2 solar cells treated with Ag-alloying and KF-post- deposition treatment (PDT) is presented. The implementation of K-means clustering is studied for its ability to identify statistically meaningful results from XRF/XBIC data. The procedure described may be applied to datasets of a similar nature.

A multimodal in situ nanofocused X-ray microscopy approach is demonstrated and applied to a working polycrystalline thin film solar cell that revealed chemical, structural and electronic heterogeneity from a single measurement., The factors limiting the performance of alternative polycrystalline solar cells as compared with their single-crystal counterparts are not fully understood, but are thought to originate from structural and chemical heterogeneities at various length scales. Here, it is demonstrated that multimodal focused nanobeam X-ray microscopy can be used to reveal multiple aspects of the problem in a single measurement by mapping chemical makeup, lattice structure and charge collection efficiency simultaneously in a working solar cell. This approach was applied to micrometre-sized individual grains in a Cu(In,Ga)Se2 polycrystalline film packaged in a working device. It was found that, near grain boundaries, collection efficiency is increased, and that in these regions the lattice parameter of the material is expanded. These observations are discussed in terms of possible physical models and future experiments.

Ultrafast and highly efficient optical modulators that are based on the Pockels effect are key components of today's optical communication networks. For the next generation of photonic links, silicon photonic technology is used to establish a new wave of densely integrated optic components. However, this new technology cannot exploit the advantages of using the Pockels effect for optical switching for two reasons: First, silicon does not exhibit any Pockels effect, and second, attempts to combine nonlinear materials with silicon photonics have been cumbersome. Here, we demonstrate a path to integrate barium titanate thin films with strong Pockels coefficients into silicon photonic structures. We highlight various design options, discuss the actual fabrication process, and present experimental results of functional passive and active structures. Examples include couplers and interferometers, as well as active, electrically driven nonvolatilely tunable ring resonators with a tunability of 4 μW/nm. Our results represent a major advancement in the field of ultralow-power silicon photonic switches based on nonlinear oxides, and demonstrate the potential of novel applications based on the hybrid barium titanate–silicon photonic platform.

Aluminum-doped zinc oxide (ZnO:Al) layers deposited by sputtering and boron-doped zinc oxide (ZnO:B) layers deposited by low-pressure chemical vapor deposition (LPCVD) are well-established materials for front electrodes in thin-film silicon solar cells. In this study, both types of front electrodes are evaluated with respect to their inherent properties and their surface textures in micromorph tandem solar cells in the superstrate configuration. The silicon layer stack investigated here consists of a 220-nm-thick amorphous silicon top cell, a 40-nm-thick intermediate reflector and a 1.1-mu m-thick microcrystalline silicon bottom cell; for this specific silicon layer stack, the LPCVD ZnO:B provides higher power conversion efficiency than its sputtered ZnO:Al counterpart. The growth-friendly surface topography of ZnO:Al yields better electrical performance. However, tandem cells on ZnO:Al suffer from a fundamental optical limitation in terms of light trapping in the top cell. They also show a higher parasitic absorption linked to the relatively high doping concentration of the ZnO:Al layers used here. Detailed analysis of the experimental results allows us to clearly understand the opto-electrical behavior of both types of cells and envisage several possible upgrades to further improve their performance. (C) 2015 Published by Elsevier B.V.

JPhys energy 2(2), 025001 (Tbd). doi:10.1088/2515-7655/ab5fa6, The efficiency of thin-film solar cells with a CuIn1-xGaxSe2 absorber is limited by nanoscopic inhomogeneities and defects. Traditional characterization methods are challenged by the multi-scale evaluation of the performance at defects that are buried in the device structures. Multi-modal X-ray microscopy offers a unique tool-set to probe the performance in fully assembled solar cells, and to correlate the performance with composition down to the micro- and nanoscale. We applied this approach to the mapping of temperature-dependent recombination for CuIn1-xGaxSe2 solar cells with different absorber grain sizes, evaluating the same areas from room temperature to 100 C. It was found that poor performing areas in the large-grain sample are correlated with a Cu-deficient phase, whereas defects in the small-grain sample are not correlated with the distribution of Cu. In both samples, classes of recombination sites were identified, where defects were activated or annihilated by temperature. More generally, the methodology of combined operando and in-situ X-ray microscopy was established at the physical limit of spatial resolution given by the device itself. As proof-of-principle, the measurement of nanoscopic current generation in a solar cell is demonstrated with applied bias voltage and bias light., Published by IOP Publishing, Bristol

The quantification of X-ray fluorescence (XRF) microscopy maps by fitting the raw spectra to a known standard is crucial for evaluating chemical composition and elemental distribution within a material. Synchrotron-based XRF has become an integral characterization technique for a variety of research topics, particularly due to its non-destructive nature and its high sensitivity. Today, synchrotrons can acquire fluorescence data at spatial resolutions well below a micron, allowing for the evaluation of compositional variations at the nanoscale. Through proper quantification, it is then possible to obtain an in-depth, high-resolution understanding of elemental segregation, stoichiometric relationships, and clustering behavior. This article explains how to use the MAPS fitting software developed by Argonne National Laboratory for the quantification of full 2-D XRF maps. We use as an example results from a Cu(In,Ga)Se(2) solar cell, taken at the Advanced Photon Source beamline 2-ID-D at Argonne National Laboratory. We show the standard procedure for fitting raw data, demonstrate how to evaluate the quality of a fit and present the typical outputs generated by the program. In addition, we discuss in this manuscript certain software limitations and offer suggestions for how to further correct the data to be numerically accurate and representative of spatially resolved, elemental concentrations.

X-Ray Microscopy 2018, Saskatoon, Canada, 19 Aug 2018 - 24 Aug 2018; Microscopy and microanalysis 24(S2), 434 - 435 (2018). doi:10.1017/S1431927618014423, Published by Cambridge University Press

To further increase the efficiency of multijunction thin-film silicon (TF-Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high-quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light-trapping structures in multijunction TF-Si solar cells. The MST substrates comprise a micro-textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub-micron nano-textured ZnO layer grown by low-pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in-coupling and light trapping for the top amorphous silicon (a-Si:H) solar cell while minimizing the parasitic absorption losses. The crater-shaped micro-textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high-quality nanocrystalline silicon (nc-Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high-quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single-textured state-of-the-art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a-Si:H/nc-Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state-of-the-art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high-efficiency multijunction solar cells. Copyright (c) 2015 John Wiley & Sons, Ltd.

We evaluate the potential of three-dimensional (3D) thin-film silicon solar cells in the superstrate configuration deposited on hexagonally ordered arrays of ZnO nanocolumns (NCs). These nanostructures are prepared by hydrothermal growth, which is an effective and versatile method to obtain ZnO NCs of high optical and electrical quality at low temperature. For the periods P investigated, varied between 0.9 and 1.4m, 3D solar cells based on hydrogenated amorphous silicon (a-Si:H) exhibit a photocurrent (J(SC)) boost in the red wavelength range as compared to flat cells; this J(SC) gain (by more than 1.5mAcm(-2) for P=0.9m) is explained mostly by the increased effective optical thickness of the absorber layer grown on the vertical walls of the NCs. Combining this 3D concept with randomly textured interfaces, rigorous 3D optical simulations based on the finite element method predict that photocurrents significantly higher than those obtained with state-of-the-art substrates (up to 20mAcm(-2)) are within reach, if the experimental obstacles specific for 3D design are overcome.

We study amorphous silicon-germanium (a-SiGe:H) as intrinsic absorber material for thin-film silicon-based triple and quadruple junction solar cells. First, we present the development of a-SiGe:H single junction devices, in particular the Ge-content grading in the absorber layer, the influence of the Ge-content on electrical properties and (infra)red-response, and the influence of using different types of players. We subsequently show the incorporation of optimized single-junction devices in triple junction cells and discuss the interplay between Ge-content and intermediate reflector thickness. For triple junction devices with amorphous silicon (a-Si:H) top cells, a-SiGe:H middle cells and microcrystalline silicion (mu c-Si:H) bottom cells, we obtained an initial efficiency of 13.6% and an efficiency of 11.3% after light-soaking. We also present a quadruple junction device with an a-Si:H top cell, a low Ge-content a-SiGe:H second cell, and mu c-Si:H third and bottom cells. In this device configuration, we obtained an open-circuit voltage as high as 2.57 V. The performance of these cells was limited by not yet optimized current matching, leading nevertheless to an initial efficiency of 10.1%. A brief roadmap towards quadruple-junction devices with stabilized efficiencies of 14% is also outlined. (C) 2014 Elsevier B.V. All rights reserved.

The efficiency of solar cells with a polycrystalline absorber is typically limited by grain boundaries. Many questions need to be answered such as: which roles do elemental and structural inhomogeneities play with respect to electrical performance? By which mechanisms does degradation occur and how can it be mitigated? The answers to these questions lie at the nanoscale. Ideally, we would measure performance, composition, and structure in-situ and operando all at once. Correlative, synchrotron-based X-ray microscopy offers a step in this direction. Here, we present a novel technique, X-ray beam induced voltage (XBIV) measurements, which complements the set of X-ray microscopy techniques. Combining the penetration depth of visible-light microscopy with the spatial resolution of electron-beam methods, XBIV measurements shine light on recombination and absorber-layer bandgap variations at nanoscale resolution. We give experimental details and discuss the use of lock-in amplification based on first applications of XBIV measurements to $\text{CuIn}_{x} \text{Ga}_{1}-x\text{Se}_{2}$ solar cells.

Spalling is a promising kerfless method for cutting thin silicon wafers while doubling the yield of a silicon ingot. The main obstacle in this technology is the high total thickness variation of the spalled wafers, often as high as 100% of the wafer thickness. It has been suggested before that a strong correlation exists between low crack velocities and a smooth surface, but this correlation has never been shown during a spalling process in silicon. The reason lies in the challenge associated to measuring such velocities. In this contribution, we present a new approach to assess, in real time, the crack velocity as it propagates during a low temperature spalling process. Understanding the relationship between crack velocity and surface roughness during spalling can pave the way to attain full control on the surface quality of the spalled wafer.

Correlative microscopy techniques have improved tremendously in the last 5 years and enabled simultaneous high spatial resolution mapping of a variety of material parameters. As acquisition speeds and resolution increase giving us a greater density of data points and the functionality of the measurements add more dimensions to be analyzed; the handling, management and analysis of data sets becomes more and more complicated. Operando measurements as well as in-situ studies pose a new challenge. Finding correlations in the 3+ dimensional data sets that result from many of these measurements is not straight forward, and the possibility of missing correlations and trends is increasingly concerning. In the following, we evaluate the potential of two machine-learning methods, an unsupervised Gaussian-Mixture-Model and a supervised Naive Bayes model, to identify two groups of pixels in a micro-X-ray fluorescence map, given varied Innnt data sizes.

Cell cracking is one of the most common factors that limit the lifetime of PV modules. Until now electroluminescence (EL) has been the tool of choice to inspect cracks in finished modules. However, there are intrinsic limitations to the size of the cracks that this technique can resolve making it complicated to study the origins of crack formation. We also argue that the process of module assembly today is optimized from the point of view of the inactive materials (e.g. encapsulant cross-linking) offering no insights into the solar cell status. To this regard, even the correlation of module degradation to cell cracking in flat modules is only evident when cracks are of detectable size and detrimental to the electrical performance as measured by EL or PL. We have shown that in-house X-ray topography (XRT) is a unique technology that provides a nondestructive way of assessing the mechanical state of encapsulated solar cells, not only the evaluation of cracks and microdefects developed during handling and outdoor operation, but also the analysis of intrinsic deflection and stress induced by materials and processes (e.g. soldering, lamination). In this contribution, we present results on the deflection of the solar cells caused by different processing temperatures, various lamination materials, accelerated testing as well as metal ribbon.

The complex refractive index (dielectric function) of planar CH3NH3PbI3 thin films at room temperature is investigated by variable angle spectroscopic ellipsometry and spectrophotometry. Knowledge of the complex refractive index is essential for designing photonic devices based on CH3NH3PbI3 thin films such as solar cells, light-emitting diodes, or lasers. Because the directly measured quantities (reflectance, transmittance, and ellipsometric spectra) are inherently affected by multiple reflections, the complex refractive index has to be determined indirectly by fitting a model dielectric function to the experimental spectra. We model the dielectric function according to the Forouhi-Bloomer formulation with oscillators positioned at 1.597, 2.418, and 3.392 eV and achieve excellent agreement with the experimental spectra. Our results agree well with previously reported data of the absorption coefficient and are consistent with Kramers-Kronig transformations. The real part of the refractive index assumes a value of 2.611 at 633 nm, implying that CH3NH3PbI3-based solar cells are ideally suited for the top cell in monolithic silicon-based tandem solar cells.

We investigate the influence of the deposition parameters for intrinsic amorphous silicon absorber layers on light-induced degradation (LID) of thin-film silicon solar cells. The focus is on absorber layers with different bandgaps: on one side, solar cells with a wide-bandgap absorber layer that provides open-circuit voltages up to 1.04 V; on the other, cells with short-circuit current densities of 18.2 mA / cm2 with a 300-nm-thick narrow-bandgap absorber layer, and 20 mA / cm2 at reverse bias for a cell with a 1000-nm-thick absorber layer. Between these extremes, we varied the hydrogen-to-silane ratio and the deposition pressure during the absorber layer deposition. The light-induced degradation of these materials—covering the deposition regimes of low-pressure, protocrystalline, polymorphous, and high-pressure amorphous silicon—incorporated in single-junction amorphous silicon solar cells is detailed here. For each pressure, we found an optimum hydrogen dilu-tion with least LID close to the amorphous-to-microcrystalline transition. The relative LID is similar for all pressures at optimized hydrogen dilutions. Further, we present the influence of absorber layer thickness, p-layer thickness, and deposi-tion rate on the kinetics of light-induced degradation to facilitate the choice of a material for its application in several types of multi-junction thin-film silicon solar cells. We show that the degradation kinetics depends, in semi-logarithmic scale, only weakly on time but more on deposition conditions.

Non-traditional photovoltaic (PV) modules, such as solar shingles, facades and skins, are expected to increase in market share as the solar industry matures and moves to fill building integrated photovoltaics niches. The advent of building integration and the application of photovoltaics to multiple surfaces come with fabrication challenges and inherent geometrical, optical and thermal constraints. In this paper, we investigate the curved lamination process and the integration of a dichroic polymeric film within the laminate for the particular case of a large-scale optical collector called a ”PVMirror”. PVMirror is a concentrating solar power (CSP)/PV hybrid design that combines the high efficiency of PV and storage capability of CSP using a dichroic film. The film does not degrade upon lamination and exhibits good adhesion to encapsulants. Multiple lamination approaches, such as tuning the thickness of the encapsulant, are shown to significantly reduce the impact of the lamination process on the shape error of PVMirror modules, which could also be applied to the fabrication of other non-traditional PV modules.

It is well known that the addition of alkali elements such as Na and K during and after growth of Cu(In, Ga)Se 2 (CIGS) has beneficial effects on the electronic properties of bulk material, improving device performance significantly. While the device level effects have been measured and reported, a direct observations of the localization of Na including its chemical nature are missing, and the impact of Na on elemental and phase segregation during CIGS growth is not fully understood. We investigate these aspects to shine light on the role of Na in CIGS solar cells with the ultimate goal of increasing their conversion efficiency. Utilizing a suite of synchrotron based x-ray characterization techniques, we discuss the challenges and advantages of these techniques for investigating segregation of main constituents of CIGS, Na distribution, chemical bonding of Na, and collection efficiency in CIGS as well as their correlations.

Solar panel's reliability studies focus mainly on the properties of the encapsulating such as gel content and transmittance, while ignoring the impact of encapsulation process on the solar cells themselves. The harsh lamination conditions apply high temperature and pressure on the wafers, which can induce increased stress, deformation and defects. The investigation of solar cells sealed inside modules calls for a non-destructive method. In this paper, we demonstrate that transmission X-ray topography (XRT) can be used as an accurate method to evaluate bending feature of encapsulated wafers and present in detail the experimental methods from capturing diffraction data to the data analysis.

In-situ investigations of Cu(In, Ga)(S, Se) 2 (CIGS) absorber layers during growth have the potential to provide insight into the origin behind elemental segregation and defect formation at grain boundaries and in grain cores. Our study focuses on CIGS films grown by an industrially relevant precursor reaction process. An in-situ stage developed for these synchrotron studies is used to image the growth of CIGS layers with nanoscale resolution. Utilizing synchrotron based x-ray fluorescence, we are able to rapidly monitor changes in elemental distribution and particle diffusion, with better than 200 nm spatial resolution throughout the 25 minute process. In this work, we highlight some challenges associated with this type of measurements and discuss solutions identified to overcome them.

Unveiling the correlation between elemental composition, fermi-level splitting, and charge collection in perovskite solar cells (PSCs) exposed to different environments is crucial to understanding the origin of defects. This will enable defect engineering to achieve high performing and long lasting perovskite solar cells. In this contribution we measured for the first time the spatial distribution and charge collection efficiency at the nano-scale by synchrotron-based x-ray fluorescence (XRF) and x-ray beam induced current (XBIC) with sub-grain resolution, and we observe a correlation between Pb/I ratio and charge collection efficiency. In contrast to other thin-film solar cells, perovskite solar cells are highly sensitive to ambient conditions (atmosphere and illumination). As the XRF and XBIC measurements were conducted in vacuum under an x-ray source illumination, the impact of measurement conditions on the measurements need to be taken into account. Furthermore, necessary conditions for quantification of XRF/XBIC measurements are not fulfilled for perovskite solar cells. Therefore, we will discuss fundamentals of XRF/XBIC measurements of perovskite solar cells that will enable reliable quantitative, high-resolution measurements of elemental distribution and charge collection.

Hydrogen-doped indium oxide (IO:H) has recently garnered attention as a high-performance transparent conducting oxide (TCO) and has been incorporated into a wide array of photovoltaic devices due to its high electron mobility (>100 cm2/V s) and transparency (>90% in the visible range). Here, we demonstrate IO:H thin-films deposited by sputtering with mobilities in the wide range of 10–100 cm2/V s and carrier densities of 4 × 1018 cm–3–4.5 × 1020 cm–3 with a large range of hydrogen incorporation. We use the temperature-dependent Hall mobility from 5 to 300 K to determine the limiting electron scattering mechanisms for each film and identify the temperature ranges over which these remain significant. We find that at high hydrogen concentrations, the grain size is reduced, causing the onset of grain boundary scattering. At lower hydrogen concentrations, a combination of ionized impurity and polar optical phonon scattering limits mobility. We find that the influence of ionized impurity scattering is reduced with the increasing hydrogen content, allowing a maximization of mobility >100 cm2/V s at moderate hydrogen incorporation amounts prior to the onset of grain boundary scattering. By investigating the parameter space of the hydrogen content, temperature, and grain size, we define the three distinct regions in which the grain boundary, ionized impurity, and polar optical phonon scattering operate in this high mobility TCO. Published version