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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.

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.

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.

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 ...

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.

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.

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.

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.

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.

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.

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.

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.

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

We report here on the latest research developments of tandem micromorph (amorphous/ microcrystalline) silicon solar cells in our laboratory. High conversion efficiency for micromorph cells requires both a dedicated light management to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. We will show that an improved cell design based on the use of silicon oxide doped layers permits to achieve high efficiencies on substrates that are usually considered as inappropriate for μc-Si:H because of their roughness. Furthermore, recently, new front contacts based e.g. on bi-layers or Ultraviolet nanoimprint lithography were developed, leading to very promising results in micromorph solar cells. While efficiencies of 12.7% initial and 11.3% stable could be achieved with only 1.1 μm of bottom cell on a front rough Low Pressure Chemical Vapor Deposited (LPCVD) ZnO, a remarkable 12% initial was also reached on textured replica (as on the master). Emphasis is also laid in our lab on increasing the deposition rate of mc-Si:H while maintaining high quality material. This is done by reducing the interelectrode gap while working at high deposition pressure, in "powder free" processes at 40 MHz. We could observe that high pressure-low hydrogen dilution process conditions lead to dense high quality material. So far, conversion efficiencies up to 8.5% have been achieved at 1 nm/s for single junction μc-Si:H solar cells with 1.8 μm thick absorber layer. We also report a promising micromorph tandem initial efficiency of 11.9% with the μc-Si:H i-layer at 0.9 nm/s. High efficiency micromorph solar cells could thus be fabricated under conditions that are favorable to industrial, low-cost, fabrication of micromorph modules. Recent results of tandems combining original substrates and improved deposition processes suggest that stabilised efficiencies close to 13% can be expected in a near future.

The electric field E within the i-layer of hydrogenated amorphous silicon (a-Si:H) solar cells strongly affects the cell performances, and, specifically, the fill factor FF. It governs the drift length L drift = μTE which is the crucial parameter limiting charge collection. Ideally, a constant electric field is assumed across the i-layer, whereas in real devices, it is deformed by charged band tail states and dangling bonds. If the i-layer is too thick or has a high density of charged defects, E is deformed and reduced. To determine theoretically the charge states of band tails and dangling bonds, we must know the carrier density profiles within the i-layer. Here, the SunShine program is used to determine carrier generation profiles within i-layers of pin-cells on TCO-covered glass substrates. A classical model for transport and electron/hole capture is employed to determine charge conditions of band tail states and dangling bonds. Results are: (a) charged dangling bonds are predominant for the electric field deformation, affecting the output performance of the cell; (b) this effect is very pronounced especially in degraded cells; (c) it is independent of light intensity; (d) it accounts for performance breakdown of thick, degraded a-Si:H cells. Calculated results are confronted with experimental observations (measurements of FF, collection voltage V coll and external quantum efficiency EQE) on pin-type solar cells of 100, 200, 300, and 400 nm thickness produced at IMT Neuchâtel, in initial and degraded state. L drift is evaluated via V coll , determined here with the method of variable intensity measurements (VIM). Trends observed are explained to full satisfaction.

Thin-film hydrogenated amorphous silicon solar (a-Si:H) cells are known to have better temperature coefficients than crystalline silicon cells. To investigate whether a-Si:H cells that are optimized for standard conditions (STC) also have the highest energy yield, we measured the temperature and irradiance dependence of the maximum power output (Pmpp), the fill factor (FF), the short-circuit current density (Jsc), and the open-circuit voltage (Voc) for four series of cells fabricated with different deposition conditions. The parameters varied during plasma-enhanced chemical vapor deposition (PE-CVD) were the power and frequency of the PE-CVD generator, the hydrogen-to-silane dilution during deposition of the intrinsic absorber layer (i-layer), and the thicknesses of the a-Si:H i-layer and p-type hydrogenated amorphous silicon carbide layer. The results show that the temperature coefficient of the Voc generally varies linearly with the Voc value. The Jsc increases linearly with temperature mainly due to temperature-inducedbandgap reduction and reduced recombination. The FFtemperature dependence is not linear and reaches a maximum at temperatures between 15 °C and 80 °C. Numerical simulations show that this behavior is due to a more positive space-charge induced by the photogenerated holes in the p-layer and to a recombination decrease with temperature. Due to the FF(T) behavior, the Pmpp (T) curves also have a maximum, but at a lower temperature. Moreover, for most series, the cells with the highest power output at STC also have the best energy yield. However, the Pmpp (T) curves of two cells with different i-layer thicknesses cross each other in the operating cell temperature range, indicating that the cell with the highest power output could, for instance, have a lower energy yield than the other cell. A simple energy-yield simulation for the light-soaked and annealed states shows that for Neuchâtel (Switzerland) the best cell at STC also has the best energy yield. However, for a different climate or cell configuration, this may not be true.

The boron-tailing effect in hydrogenated amorphous silicon (a-Si:H) solar cells describes the reduced charge collection specifically in the blue part of the spectrum for absorber layers deposited above a critical temperature. This effect limits the device performance of state-of-the art solar cells: For enhanced current density (reduced bandgap), the deposition temperature should be as high as possible, but boron tailing gets detrimental above 200°C. To investigate this limitation and to show potential paths to overcome it, we deposited high-efficiency a-Si:H solar cells, varying the deposition temperatures of the p-type and the intrinsic absorber (i) layers between 150 and 250°C. Using secondary ion mass spectroscopy, we study dedicated stacks of i-p-i layers deposited at different temperatures. This allows us to track boron diffusion at the p-i and i-p interfaces as they occur in the p-i-n and n-i-p configurations of a-Si:H solar cells for different deposition conditions. Finally, we prove step-by-step that the common explanation for boron tailing—boron diffusion from the p layer into the i layer leading to enhanced recombination—is not generally true and propose an alter-native explanation for the experimentally observed drop in the external quantum efficiency at short wavelengths.

When it comes to parasitic absorption in thin-film silicon solar cells, most studies focus on one electrode only, most of the time the substrate (in n -i -p configuration) or superstrate (in p -i -n configuration). We investigate here simultaneously the influence of the absorption in both front and back electrodes on the current density of tandem micromorph solar cells in p -i -n configuration. We compare four possible combinations of front and back electrodes with two different doping levels, but identical sheet resistance and identical light-scattering properties. In the infrared part of the spectrum, parasitic absorption in the front or back electrode is shown to have a similar effect on the current generation in the cell, which is confirmed by modeling. By combining highly transparent front and back ZnO electrodes and high-quality silicon layers, a micromorph device with a stabilized efficiency of 11.75% is obtained.

We demonstrate self-patterned insulating nanoparticle layers to define local electrical interconnects in thin-film electronic devices. We show this with thin-film silicon tandem solar cells, where we introduce between the two component cells a solution-processed SiO2 nanoparticle layer with local openings to allow for charge transport. Because of its low refractive index, high transparency, and smooth surface, the SiO2 nanoparticle layer acts as an excellent intermediate reflector allowing for efficient light management.

In Thin Film Silicon (TF-Si) solar cells light harvesting schemes must guarantee an efficient light trapping in the thin absorber layers without decreasing the silicon layers quality and consecutively the p-i-n diodes electrical performance. TF-Si solar cells resilience to the substrate roughness is reported to be possibly improved through optimizations of the cell design and of the silicon deposition processes. By further tailoring the superstrate texture, amorphous silicon / microcrystalline silicon (a-Si:H/mu c-Si:H) tandem solar cells with an initial efficiency up to 13.7 % and a stabilized efficiency up to 11.8 % are demonstrated on single-scale textured superstrates. An alternative approach combining large and smooth features nanoimprinted onto a transparent lacquer with small and sharp textures from as-grown LPCVD ZnO is then shown to have a high potential for further increasing TF-Si devices efficiency. First results demonstrate up to 14.1 % initial efficiency for a TF-Si tandem solar cell.

We evaluate the performance of thin-film silicon micromorph tandem solar cells deposited on transparent superstrates with embossed micrometer-scale 2-D gratings. Once coated with a thin conductive layer of hydrogenated indium oxide, the textured superstrates can be used as 2-D periodic single-texture front electrodes. Combining these almost loss-free front electrodes with a highly transparent, random self-textured zinc oxide layer (with a thickness 1 um) deposited by low-pressure chemical vapor deposition (LPCVD), we obtain double-texture transparent front electrodes. The potential of both single- and double-texture front electrodes is estimated by varying the illumination spectrum of the solar simulator, thereby assessing the maximum efficiency of the tandem cells under optimal current-matching conditions. Our results demonstrate the complementary roles of the 2-D gratings and the LPCVD-ZnO layers in double textures: Cell efficiencies as high as with our state-of-the-art 2.3-um-thick LPCVD-ZnO front electrode are obtained with significantly reduced ZnO layer thicknesses. Additionally, we show that equivalent efficiencies are also within reach with 2-D periodic single textures if the proper cell configuration is applied.

We give an overview on recent progress in the synthesis, fabrication and integration of advanced nanostructured materials for efficient light trapping in high-efficiency thin-film silicon solar cells.

High-efficiency amorphous silicon (a-Si:H) solar cells were deposited with different thicknesses of the p-type amorphous silicon carbide layer on substrates of varying roughness. We observed a light-induced open-circuit voltage (Voc) increase upon light soaking for thin p-layers, but a decrease for thick p-layers. Further, the Voc increase is enhanced with increasing substrate roughness. After correction of the p-layer thickness for the increased surface area of rough substrates, we can exclude varying the effective p-layer thickness as the cause of the substrate roughness dependence. Instead, we explain the observations by an increase of the dangling-bond density in both the p-layer—causing a Voc increase—and in the intrinsic absorber layer, causing a Voc decrease. We present a mechanism for the light-induced increase and decrease, justified by the investigation of light-induced changes of the p-layer and supported by Advanced Semiconductor Analysis simulation. We conclude that a shift of the electron quasi-Fermi level towards the conduction band is the reason for the observed Voc enhancements, and poor amorphous silicon quality on rough substrates enhances this effect.

This contribution reviews some of the latest achievements and challenges in thin-film silicon photovoltaic (PV) technology based on amorphous and nanocrystalline silicon and their alloys. We address material and device developments, including (i) improved plasma deposition processes to achieve high-quality dense absorber materials; (ii) absorber layers based on silicon tetrafluoride, which lead to enhanced absorption in the near-infrared and yield outstanding short-circuit current densities; (iii) dedicated optimization of the interfaces and device architecture, as well as (iv) enhanced light harvesting by means of multi-scale textured substrates and reduced parasitic absorption in the non-active layers. This paper will describe how, by combining all of these advances along with precise control of plasmas over large areas, key results have been achieved in recent years, at both the cell and large-area module level, with stabilized efficiencies of over 13 and 12%, respectively.

High-efficiency thin-film silicon triple-junction solar cells in p-i-n configuration have been fabricated using amorphous silicon top cell absorber layers, as well as microcrystalline silicon middle and bottom cell absorbers. The triple-junction cells were fabricated on boron doped zinc oxide (ZnO) films with different surface morphologies. To this end, the naturally grown rough ZnO surfaces were flattened using an Ar plasma for three different treatment times. For the shortest time, we achieved a summed current density over 30 mA/cm(2) and initial and stabilized conversion efficiencies of 13.5% and 12.5%, respectively. For the medium treatment time, we obtained the highest efficiencies (13.7% initial and 12.8% stable), whereas the longest treatment time led to the highest open-circuit voltage (V-OC) of 1.91 V but lower current densities, leading to efficiencies of 12.9% initial and 12.2% stable, respectively. These results were obtained by combining various recently developed features and approaches: first of all, we implemented high-quality mu c-Si: H cells with novel buffer layers, leading to very high efficiencies. Second, we applied randomly textured pyramids on the front glass to improve light in-coupling, and finally, we used very thin (similar to 140 nm) top cells that led to a low light-induced degradation (5%-7% relative loss in efficiency).

We demonstrate the integration of single-crystalline barium titanate thin films on silicon substrates, with Pockels coefficients of rEff ~ 148 pm/V. We further show the implementation of such layers into silicon photonic devices such as ring resonators.

The use of intrinsic silicon oxide as a buffer layer at the p-i interface of thin-film silicon solar cells is shown to provide significant advantages. For microcrystalline silicon solar cells, when associated with highly crystalline i-layers deposited at high rates, all electrical parameters are improved. Larger efficiency gains are achieved with substrates of increased roughness. For cells with an improved i-layer material quality, there is mainly a gain in short-circuit current density. An improvement in carrier collection in the blue region of the spectrum is systematically observed on all the cells. The presence of a silicon oxide buffer layer also promotes the nucleation of the subsequent intrinsic microcrystalline silicon layer. In amorphous silicon solar cells, the silicon oxide buffer layer is proven to act as an efficient barrier to boron cross-contamination, eliminating the need for additional processing steps (e.g. water vapor flush), while providing a wide bandgap material at the interface. The implementation of silicon oxide buffer layers for both types of cells thus provides a decisive improvement, as it allows extremely fast deposition of the full p-i-n stack of layers of the cell in a single-chamber configuration while providing a high-quality substrate-resilient p-i interface. (C) 2013 Elsevier B.V. All rights reserved.

We present and validate a finite-element model for coupled charge and heat transport in monolithically interconnected thin-film solar modules. Using measured current-voltage ( I-V) and lock-in thermography (LIT) measurements of amorphous silicon minimodules, we experimentally validate our model. The entire module volume is represented by two planes (front and back electrodes) which are coupled in vertical direction using 1-D models, leading to a large reduction of the degrees of freedom in the numerical model and contributing to an efficient solution approach. As compared to 3-D models, the vertical coupling of the charge transport is represented by local temperature-dependent I-V curves. These can be obtained by drift-diffusion calculations, single-cell measurements or, as presented here, by an analytical solar cell diode model. Inhomogeneous heat sources such as Joule's heating in the electrodes lead to nonuniform temperature distributions. The explicit temperature dependence in the local I-V curve, therefore, mediates the feedback of the thermal transport on the local electrical cell characteristics. We employ measured I-V curves under partial illumination and analytical solutions for the potential distribution to validate this approach. Further, with LIT measurements of the same modules with and without artificially induced electrical shunts, we verify the computed temperature distributions.

Understanding the electron scattering mechanisms dominating transport in high and low mobility transparent conducting oxides can provide insight into improving the properties of this crucial photovoltaic device layer. Contributions from ionized impurity scattering, phonon scattering, and grain boundary scattering were measured and quantified in hydrogenated indium oxide thin films, which had varying composition and grain size, resulting in a wide range of carrier densities $(\sim 10^{1S}\mathrm {c}\mathrm {m}^{-3}-\sim 10^{20}\mathrm {c}\mathrm {m}^{-3})$ and carrier mobilities $(\sim 10 \mathrm {c}\mathrm {m}^{2}/\mathrm {V}\mathrm {s} - \gt100 \mathrm {c}\mathrm {m}^{2}/\mathrm {V}\mathrm {s})$. We used temperature-dependent Hall measurements from 5–300 Kelvin to characterize electrical properties of the sputtered films. Grain boundary scattering dominated films with the lowest mobility, whereas films with $\gt100 \mathrm {c}\mathrm {m}^{2}/\mathrm {V}\mathrm {s}$ mobilities were dominated by contributions from ionized impurity scattering and polar optical phonon scattering.