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We have measured the subgap optical absorption of undoped microcrystalline silicon samples with photothermal deflection spectroscopy and compared it with the spin density measured by electron spin resonance. The material was prepared using the hot wire chemical vapor deposition technique with a broad range of deposition parameters, yielding materials with a wide variety of structural, optical, and electronic properties. A nearly linear correlation between the spin densities and the subgap absorption coefficient at photon energy of 0.7 eV over three orders of magnitude indicates that the optical absorption in this region is due to the dominating paramagnetic defects, likely dangling bonds. However, the structural composition of the material, determined by Raman spectroscopy, has also some influence on the optical and electronic properties, which leads to some deviation from a straightforward relationship between optical absorption and spin or defect density. [ABSTRACT FROM AUTHOR]

Microcrystalline silicon (μc-Si:H) of superior quality can be prepared using the hot-wire chemical-vapor deposition method (HWCVD). At a low substrate temperature (TS) of 185 °C excellent material properties and solar cell performance were obtained with spin densities of 6×1015 cm-3 and solar cell efficiencies up to 9.4%, respectively. In this study we have systematically investigated the influence of various deposition parameters on the deposition rate and the material properties. For this purpose, thin films and solar cells were prepared at specific substrate and filament temperatures and deposition pressures (pD), covering the complete range from amorphous to highly crystalline material by adjusting the silane concentration. The influence of these deposition parameters on the chemical reactions at the filament and in the gas phase qualitatively explains the behavior of the structural composition and the formation of defects. In particular, we propose that the deposition rate is determined by the production of reactive species at the filament and a particular atomic-hydrogen-to-silicon ratio is found at the microcrystalline/amorphous transition. The structural, optical, and electronic properties were studied using Raman and infrared spectroscopies, optical-absorption measurements, electron-spin resonance, and dark and photoconductivities. These experiments show that higher TS and pD lead to a deterioration of the material quality, i.e., much higher defect densities, oxygen contaminations, and SiH absorption at 2100 cm-1. Similar to plasma enhanced chemical-vapor deposition material, μc-Si:H solar cells prepared with HW i layers show increasing open circuit voltages (Voc) with increasing silane concentration and best performance is achieved near the transition to amorphous growth. Such solar cells prepared at low TS exhibit very high Voc up to 600 mV and fill factors above 70% with i layers prepared by HWCVD. [ABSTRACT FROM AUTHOR]

Transparent polymer films such as polyethylene terephthalate (PET) present a cost efficient substrate material for flexible solar cells. In addition to certain process temperature limitations, these materials are permeable to moisture and/or oxygen, and additional barrier layers are required to prevent the diffusion of contaminating atoms into the photovoltaic device. In this study, the influence of a ZnSnO x barrier layer deposited on PET substrates on the properties of the subsequently grown low temperature (<140 °C) aluminum-doped zinc oxide (ZnO:Al) layers is examined. We investigate the electrical and optical properties, as well as surface topography of ZnO:Al layers grown on PET and PET/ZnSnO x substrates before and after wet-chemical etching treatment, needed to maintain the advanced light management schemes in solar cells. A-Si:H solar cells show an improvement in all photovoltaic parameters when ZnSnO x barrier layers are applied on PET substrates, leading to a remarkable increase in efficiency from 4.8 to 6.9%. [ABSTRACT FROM AUTHOR]

Abstract: Paramagnetic defects in μc-Si:H and a-Si:H with various structure compositions were investigated by electron spin resonance (ESR). The defect density was varied by high energy electron bombardment and subsequent annealing. The spin density increases by up to 3 orders of magnitude. In most cases the initial spin density can be restored upon annealing at 160°C. [Copyright &y& Elsevier]

Abstract: Improving the deposition rate for microcrystalline silicon (μc-Si:H) prepared by Hot Wire Chemical Vapour Deposition (HWCVD) under the constraint of a low substrate temperature is the subject of this study. The influence of the deposition pressure and various combinations of filament configuration and filament temperature on the deposition rate and material quality was investigated. Raman and infrared spectroscopy and solar cell J–V parameters are used to evaluate the quality of the material. Low deposition pressures and low filament temperatures at low filament–substrate distances are most suitable to obtain high quality material at improved deposition rates. Deposition rates of 4 Å/s were achieved for high quality material at a substrate temperature of 250 °C. [Copyright &y& Elsevier]

Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature TS and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at TS below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm−1 IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages Voc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high Voc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high Voc. Raman spectra show a continuously increasing amorphous peak with increasing Voc. Crystalline fractions XC ranging from 50% for the highest Voc to 95% for the lowest Voc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest XC, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency. [Copyright &y& Elsevier]

Profiled buffer layers at the interfaces of amorphous silicon-germanium (a-SiGe:H) solar cells are routinely used to avoid band-gap discontinuities and high-defect densities at the p/i and i/n interfaces. It is shown that such profiled a-SiGe:H buffer layers can be replaced by a constant band-gap a-Si:H buffer, an inverse profiled a-SiGe:H buffer, or even a 3-nm-thin (δ) buffer at some distance away from the interface without losses in the open-circuit voltage V[sub OC] and fill factor while maintaining the same short current density j[sub SC]. In view of these results, common model assumptions for a-SiGe:H solar cells have to be revised. [ABSTRACT FROM AUTHOR]

Double layers of deuterated and hydrogenated amorphous silicon (a‐Si:H) on glass are heated in the ambient by scanning with a green (532 nm) continuous wave laser. The hydrogen diffusion length in the laser spot is obtained from the deuterium (D)–hydrogen (H) interdiffusion measured by secondary ion mass spectrometry (SIMS), the temperature in the laser spot is obtained by calculation. Under certain conditions, detachment of the deuterated layer from the hydrogenated layer is observed in the SIMS depth profiles, visible by rising oxygen and carbon signals at the D/H interface attributed to in‐diffusion of atmospheric gas species like water vapor, oxygen, and carbon oxide. Stacks involving both undoped and boron‐doped a‐Si:H films show disintegration. The results suggest that the parameters leading to the disintegration effects are the presence of a plane of reduced material cohesion at the D/H interface, a sizeable H diffusion length and a rather high heating rate. Herein, it is likely considered that the observed layer disintegration process is involved in the peeling of a‐Si:H films upon fast heating. Furthermore, the results show that rapid laser heating can be used to detect planes of reduced material cohesion which may compromise the electronic properties of a‐Si:H‐based stacks. [ABSTRACT FROM AUTHOR]

Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring the silicon melting point and with calculated data using a physical model as well as published work. Since D-H interdiffusion measures hydrogen diffusion length and temperature within the silicon films by a memory effect, the method is capable of determining both quantities precisely also in multilayer structures, as is demonstrated for films underneath metal contacts. Several applications are discussed. Employing literature data of laser-induced temperature rise, laser scanning is used to measure the H diffusion coefficient at T > 500 °C in a-Si:H. The model-based high temperature hydrogen diffusion parameters are confirmed with important implications for the understanding of hydrogen diffusion in the amorphous silicon material. [ABSTRACT FROM AUTHOR]

Microcrystalline silicon carbide alloys (μc-SiC:H) have attracted attention as a potential window layer for thin film silicon solar cells and silicon heterojunction solar cells. Thanks to the combination of high transparency and conductivity. At present, identification of electronically active impurities and defects is highly relevant for application of μc-SiC:H. Electron paramagnetic resonance has been successfully applied to investigate defects and impurities in crystalline SiC. Continuous wave X-band EPR measurements performed earlier on a number of μc-SiC:H samples show intensive and complex spectra with multiple overlapped components. In order to resolve components of the μc-SiC:H spectra, we performed a series of pulsed EPR measurements in S-, X-, and Q-band completed with a Q-band ENDOR experiment. The experiment was successful to resolve hidden complexity of the EPR spectra of μc-SiC:H and determine the nucleus responsible for the hyperfine pattern observed in previous studies. [ABSTRACT FROM AUTHOR]

The unique microstructure of nanocrystalline silicon oxide (nc-SiO X :H) thin films results in excellent optoelectronic properties that can be tuned in a wide range to fulfill the requirements of the specific application. For photovoltaic applications, this material is used as doped layers in silicon heterojunction solar cells and intermediate reflectors in multijunction thin-film solar cell. In this paper, we present a microstructure model based on a large number of n- and p-doped nc-SiO X :H films that were deposited under various deposition pressures, plasma powers, plasma frequencies and gas mixtures. This model is meant to provide guidelines for the systematic classification of the complex material system nc-SiO X :H by establishing a link between the structure of the deposited films and the optoelectronic performance of nc-SiO X :H. Based on this model, the deposition of nc-SiO X :H films can be divided into four characteristic regions: (i) fully amorphous region, (ii) onset of nc-Si formation, (iii) oxygen and nc-Si enrichment region, and (iv) deterioration of nc-Si. According to our microstructure model, an optimal phase composition with respect to the optoelectronic performance can be achieved with a high amount of highly conductive nc-Si percolation paths embedded in an oxygen rich a-SiO X :H matrix. [ABSTRACT FROM AUTHOR]

A detailed investigation of the effects of prolonged postdeposition annealing on the performance of amorphous silicon (a‐Si:H) solar cells and the properties of individual a‐Si:H layers that are fabricated at low temperature of 120 °C is presented. A substantial improvement in all parameters of the current–voltage curves of these solar cells is observed upon annealing, consistent with an improvement in the collection voltage of the solar cells. Modifications of p‐type layers during deposition of the solar cells are found to make no significant contribution to the annealing behavior of solar cells, while variations in the properties of n‐type and intrinsic layers contribute substantially. The results indicate that the largest contribution to the annealing effect originates from changes in the electron μτ‐product in the intrinsic absorber layer upon annealing, while changes in hole μτ‐products have a minor contribution to the annealing effect in the solar cell. Besides a lack of significant changes in the number of recombination centers upon annealing, an improvement in the external quantum efficiency curves upon annealing may be accurately reproduced in computer simulations by assuming an increase in the band mobilities of both electrons and holes. [ABSTRACT FROM AUTHOR]

In less than 3 years' time, a vast progress in power conversion efficiencies of organometal halide perovskite solar cells has been achieved by optimization of the device architecture, charge transport layers, and interfaces. A further increase in these efficiencies is expected from an improvement in the optical properties via anti-reflection coatings and nanophotonic light management concepts. In this contribution, we report on the development and implementation of a nanophotonic front electrode for perovskite solar cells. The nanostructures were replicated via the versatile and large-area compatible UV-nanoimprint lithography. The shallow design of the used transparent and conductive nanostructures enabled easy integration into our solution-based baseline process. Prototype methylammonium lead iodide perovskite solar cells show an improvement of 5% in short-circuit current density and an improvement from 9.6% to 9.9% in power conversion efficiency compared to the flat reference device. [ABSTRACT FROM AUTHOR]

N-type microcrystalline silicon carbide (μc-SiC:H(n)) deposited by hot wire chemical vapor deposition provides advantageous opto-electronic properties for window layer material in siliconbased thin-film solar cells and silicon heterojunction solar cells. So far, it is known that the dark conductivity (σd) increases with the increase in the crystallinity of μc-SiC:H(n)films. However, due to the fact that no active doping source is used, the mechanism of electrical transport in these films is still under debate. It is suggested that unintentional doping by atmospheric oxygen (O) or nitrogen (N) contamination plays an important role in the electrical transport. To investigate the impact of O and N, we incorporated O and N in μc-SiC:H(n) films and compared the influence on the microstructural, electronic, and optical properties. We discovered that, in addition to increasing the crystallinity, it is also possible to increase the σd by several orders of magnitude by increasing the O-concentration or the N-concentration in the films. Combining a high concentration of O and N, along with a high crystallinity in the film, we optimized the σd to a maximum of 5 S/cm. [ABSTRACT FROM AUTHOR]

Abstract: Accurate evaluation of the defect density (ND) is of high relevance for the optimization of thin film silicon. The spin density (NS) measured in ESR experiments is often used as a measure for the density of deep defects in the material, assuming that all defects are in a paramagnetic charge state. However, exposure to air, water, or acid during ESR sample preparation can potentially change the NS in a sample and lead to misinterpretation of ND. We have investigated how the preparation procedures of a Si thin film ESR sample may affect the properties of its ESR spectrum. Samples of different structural composition from highly crystalline μc-Si:H to a-Si:H deposited by PECVD on Mo-foil, Al-foil and ZnO:Al were studied for different states of exposure to ambient conditions and annealing. NS measured directly after sample preparation and after air exposure was found to be higher than NS measured in the annealed state. Particularly in highly crystalline material this discrepancy may reach one order of magnitude. On the other hand in a-Si:H and medium crystalline μc-Si:H relevant for applications, the difference in NS between air-exposed and annealed conditions is smaller. ESR measurements performed at 40K suggest that atmospheric exposure leads to charging of the defect states, which in turn influences the evaluated spin density. [Copyright &y& Elsevier]

Abstract: Photovoltaic properties of 4 µm thick microcrystalline silicon p–i–n solar cells have been studied, over a range of crystallinity determined using Raman spectroscopy. Low-crystallinity material (below 10%) appears to absorb disproportionately strongly in the infrared, possibly due to increased light scattering or to relaxation of the crystal momentum selection rule. A minimum in solar cell efficiency is observed under AM1.5 illumination when V OC ≈580 mV, with blue response most strongly affected. This is consistent with a reduction in electron mobility to a value below that of amorphous silicon for low-crystallinity material, in agreement with time-of-flight measurements. [Copyright &y& Elsevier]

Abstract: Microcrystalline silicon carbide (μc-SiC) was prepared at substrate temperatures between 300 °C and 450 °C using hot-wire chemical vapour deposition (HWCVD). The SiC films were deposited from monomethylsilane (MMS) diluted in hydrogen on glass and crystalline silicon substrates. The influence of the hydrogen dilution, the filament temperature and the deposition pressure on the deposition rate, the structural and the optoelectronic properties was investigated. Infrared and Raman spectroscopy and transmission electron microscopy (TEM) were employed to study the structural properties. Optical absorption measurements by photothermal deflection spectroscopy (PDS) as well as dark- and photo-conductivity measurements were used to investigate the optoelectronic properties of the material. [Copyright &y& Elsevier]

The lateral resolution of photocurrent techniques such as light-addressable potentiometric sensors (LAPS) or scanning photo-induced impedance microscopy (SPIM) is limited by the properties of the semiconductor material used. We investigated metal-insulator-semiconductor (MIS) structures based on amorphous silicon (a-Si) prepared as a thin layer on transparent glass substrates. It was shown that a sub-micrometer resolution can be achieved for this material, which is much better than the results for single crystalline Si. Some limitations caused by light scattering in the structure were observed. [Copyright &y& Elsevier]

Microcrystalline silicon carbide (μc-SiC:H) deposited by hot wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD) provide advantageous optoelectronic properties, making it attractive as a window layer material in silicon thin-film and silicon heterojunction solar cells. However, it is still not clear which electrical transport mechanisms yield dark conductivities up to 10-3 S/cm without the active use of any doping gas and how the transport mechanisms are related to the morphology of μc-SiC:H. To investigate these open questions systematically, we investigated HWCVD and PECVD grown layers that provide a very extensive range of dark conductivity values from 10-12 S/cm to 10-3 S/cm. We found out by secondary ion mass spectrometry measurements that no direct correlation exists between oxygen or nitrogen concentrations and high dark conductivity σd, high charge carrier density n, and low activation energy Ea. Higher σd seems to rise from lower hydrogen concentrations or/and larger coherent domain sizes LSiC. On the one hand, the decrease of σd with increasing hydrogen concentration might be due to the inactivation of donors by hydrogen passivation that gives rise to decreased n. On the other hand, qualitatively consistent with the Seto model, the lower σd and lower n might be caused by smaller LSiC, since the fraction of depleted grain boundaries with higher Ea increases accordingly. [ABSTRACT FROM AUTHOR]

The thermal stability of thick (≈4 μm) plasma‐grown hydrogenated amorphous silicon (a‐Si:H) layers on glass upon application of a rather rapid annealing step is investigated. Such films are of interest as precursor layers for laser liquid‐phase crystallized silicon solar cells. However, at least half‐day annealing at T ≈550 °C is considered to be necessary so far to reduce the hydrogen (H) content and thus avoid blistering and peeling during the crystallization process due to H. By varying the deposition conditions of a‐Si:H, layers of rather different thermal stability are fabricated. Changes in the surface morphology of these a‐Si:H layers are investigated using scanning electron microscopy and profilometry measurements. Hydrogen effusion, secondary‐ion mass spectrometry (SIMS) depth profiling, and Raman spectroscopy measurements are also carried out. In summary, amorphous silicon precursor layers are fabricated that can be heated within 30 min to a temperature of 550 °C without peeling and major surface morphological changes. Successful laser liquid‐phase crystallization of such material is demonstrated. The physical nature of a‐Si:H material stability/instability upon application of rapid heating is studied. [ABSTRACT FROM AUTHOR]

Photovoltaic driven electrochemical (PV‐EC) water splitting technology is considered as one of the solutions for an environmental‐friendly hydrogen supply. In a PV‐EC system, efficient catalysts are required to increase the rate of both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, the development of a CoFeVOx bifunctional catalyst produced by a simple electrodeposition method is presented. It is found that after the water splitting reaction, vanadium is almost completely depleted in the mixture of elements for OER, while its concentration at the HER catalyst is similar or even higher after the reaction. For the OER catalyst, the depletion of vanadium might lead to the formation of pores, which could be correlated with the activity enhancement. The developed catalyst is integrated into PV‐EC devices, coupled with different types of silicon PV. An average solar to hydrogen efficiency of 13.3% (9.6 cm2 PV aperture area) is achieved with a shingled module consisting of three laterally series‐connected silicon heterojunction solar cells. [ABSTRACT FROM AUTHOR]

We report on the systematic optimization of the intrinsic amorphous silicon oxide buffer layer in interplay with doped microcrystalline silicon oxide contact layers for silicon heterojunction solar cells using all silicon oxide based functional layers on flat p-type float-zone wafers. While the surface passivation quality is comparably good within a wide range of low oxygen contents, the optical band gap increases and the dark conductivity decreases with increasing oxygen content, giving rise to an inevitable trade-off between optical transparency and electrical conductivity. On the cell level, fill factor FF and short circuit current density Jsc losses compete with the open circuit voltage Voc gains resulting from a thickness increase of the front buffer layers, whereas Jsc and Voc gains compete with FF losses resulting from increasing thickness of the rear buffer layers. We obtained the highest active area efficiency of ηact = 18.5% with Voc = 664 mV, Jsc = 35.7 mA/cm2, and FF = 78.0% using 4 nm front and 8 nm rear buffer layer with an oxygen content of 5%. [ABSTRACT FROM AUTHOR]

Abstract: The performance of multijunction amorphous silicon-based thin film solar cells has been reported using thin layers of TiO2 and SiO x acting as refractive index matching optical layers for different interfaces of the superstrate device structure. Improvement of short-circuit current from the sub-cells of a-Si/μc-Si cells is demonstrated with TiO2 as anti-reflection layer at TCO/Si interface and SiO x as intermediate-reflector layer between two sub-cells. An initial efficiency of 11.8% is achieved by applying both the TiO2 and SiO x optical layers in a-Si/μc-Si solar cell. [Copyright &y& Elsevier]

Abstract: Microcrystalline silicon carbide (μc-SiC) was prepared at low substrate temperatures using Hot Wire chemical vapor deposition (HWCVD). High crystalline volume fractions were achieved at high hydrogen dilution and high deposition pressure. Without intentional doping, such material shows high dark conductivity and high optical absorption below the band gap. The material prepared at low deposition pressure or low hydrogen dilution, on the other hand, shows much lower conductivity and sub-gap absorption, but high spin densities up to 5×1019 cm−3. This high absorption can be attributed to free carriers, different to μc-Si:H where a correlation between the sub-gap absorption and the spin density is observed. [Copyright &y& Elsevier]

Hot-wire chemical vapor deposition (HWCVD) was utilized to develop a fast and high quality a-Si:H thin film fabrication method for poly-Si/SiOx carrier selective passivating contacts targeting at n-type passivated emitter rear totally diffused crystalline silicon solar cells. The microstructure and hydrogen content of the a-Si:H thin films were analyzed by Fourier-transform infrared spectroscopy in order to understand the influence of film properties on passivation and conductivity. Dense layers were found to be beneficial for good passivation. On the other hand, blistering appeared as a-Si:H layers became more and more dense. However, by adjusting the SiH4 flow rate and the substrate heater temperature, blistering of a-Si:H could be avoided. A suitable process window was found and firing-stable implied open circuit voltage (iVoc) of up to 738 mV was achieved. In addition to high iVoc, a low contact resistivity of 0.034 Ω cm2 was also achieved. The deposition rate of the a-Si:H layers was 7 Å/s by using HWCVD, which is one order of magnitude higher than the deposition rate reported using other deposition methods. [ABSTRACT FROM AUTHOR]

Abstract Paper-based flexible thin film solar cells are promising power sources for the emerging portable and wearable electronics, for which high conversion efficiency and flexibility are demanded. Herein, flexible thin film silicon solar cells using textured aluminum-doped zinc oxide (AZO) for light scattering, and ultrathin silver electrodes combined with cellophane paper substrates, were demonstrated. A universal route to form textured AZO thin film by chemical etching with hydrochloric acid vapor was proposed. The AZO layers textured in this way exhibited similar light scattering properties as compared with state-of-the-art AZO thin films. Applying textured AZO to solar cells on cellophane paper substrates resulted in current density and conversion efficiency increase from 8.7 to 9.5 mA/cm2 and from 4.1% to 5.7%, respectively. Thin film solar cells on cellophane exhibited excellent mechanical flexibility, which maintained 90% of their initial efficiency after bending at a radius of 1 mm for 50 cycles. Graphical abstract fx1 Highlights • Textured AZO thin film on cellophane paper was formed by chemical etching with hydrochloric acid vapor. • Textured AZO thin film on cellophane exhibited high transmittance haze of 59% at 550 nm. • Applying textured AZO to a-Si:H solar cells on cellophane resulted in conversion efficiency increase from 4.1% to 5.7%. • Thin film solar cells on cellophane maintained 90% of initial efficiency after bending at a radius of 1 mm for 50 cycles. [ABSTRACT FROM AUTHOR]

TiO 2 is a common protection layer on semiconductor electrodes for photoelectrochemical water splitting. We investigate the interface formation of TiO 2 on amorphous silicon tandem solar cells by X-ray photoelectron spectroscopy. In order to optimize the contact properties, we prepare TiO x interface layers with various oxygen content by reactive magnetron sputter deposition. We observe, that a TiO x interface layer can reduce the silicon oxide growth during the film deposition on the amorphous silicon, but it forms a non-ohmic contact. The electrochemical investigation shows, that the benefit due to the reduction of the silicon oxide is counteracted by the unfavorable contact formation of TiO x interface layers prepared with low oxygen content. [ABSTRACT FROM AUTHOR]

In order to increase the efficiency of high performance silicon heterojunction solar cells even further, it is paramount to increase the photoelectric current by enhancing the amount of light being captured within the absorber. Therefore, to reduce the parasitic absorption in the other layers, optoelectronically favorable hydrogenated nanocrystalline silicon oxide films can substitute the commonly used hydrogenated amorphous silicon layers. In this work, we systematically investigate the combination of hydrogenated nanocrystalline silicon oxide and front side nano-imprint textures as anti-reflection layers in silicon heterojunction solar cells. Ultimately, we were able to tune the parasitic absorption via variation of the front surface field layer and enhance the short-circuit current of the planar solar cells by about 2 mA cm−2 due to a random silicon pyramid textured imprint layer. A maximum active area efficiency of 20.4% was achieved with a short-circuit current of 37.7 mA cm−2. [ABSTRACT FROM AUTHOR]

An integrated solar water splitting device based on thin film silicon multijunction photocathodes is presented. A graphical representation of the photovoltaic current-voltage data is introduced which allows for an estimation of the maximum achievable solar-to-hydrogen efficiency of the integrated device. Furthermore, a simple yet very useful series circuit model is used to predict the photoelectrochemical performance of the integrated device in a more elaborate way when the j- V characteristics of the individual components are known. Within the model, the j- V characteristics of each component can be either modeled with parameters from the literature or measured. The photocathode, the electrolyte concentration, and the hydrogen and oxygen evolving catalysts were varied exemplarily and the impact of each component on the integrated device performance was evaluated. A maximum solar-to-hydrogen efficiency of 9.5% was found using a triple junction solar cell functionalized with a Pt catalyst for the hydrogen evolution and a RuO2 catalyst for the oxygen evolution reaction in a 1 M KOH electrolyte. This result was confirmed experimentally and is compared to efficiencies reported in the literature. [ABSTRACT FROM AUTHOR]

Maintaining efficient light management is an essential part for obtaining high efficiency flexible thin-film solar cells. Transparent plastic films like polyethylene terephthalate (PET) are a cost efficient substrate alternative but also impose additional constraints on the light management approaches available. In this study, we investigate and compare two approaches to prepare substrates with light scattering characteristics. We have developed low temperature ZnO:Al layers, which are fully compatible with PET substrates, and are textured by wet-chemical methods. These low temperature textured ZnO:Al layers implemented in amorphous silicon solar cells on glass substrates result in similar efficiencies as highly optimized high temperature-etched ZnO:Al layers. Nanoimprint lithography was used as an alternative light management approach and we show that an improved solar cell performance can be achieved on flexible PET substrates with both methods. Besides the effect on solar cell performance, pros and cons of both approaches with respect to flexibility in choice of materials and textures, fabrication process, stress evolution, and reproducibility are discussed. [ABSTRACT FROM AUTHOR]

This work demonstrates that the combination of a wet-chemically grown SiO2 tunnel oxide with a highly-doped microcrystalline silicon carbide layer grown by hot-wire chemical vapor deposition yields an excellent surface passivation for phosphorous-doped crystalline silicon (c-Si) wafers. We find effective minority carrier lifetimes of well above 6 ms by introducing this stack. We investigated its c-Si surface passivation mechanism in a systematic study combined with the comparison to a phosphorous-doped polycrystalline-Si (pc-Si)/SiO2 stack. In both cases, field effect passivation by the n-doping of either the µc-SiC:H or the pc-Si is effective. Hydrogen passivation during µc-SiC:H growth plays an important role for the µc-SiC:H/SiO2 combination, whereas phosphorous in-diffusion into the SiO2 and the c-Si is operative for the surface passivation via the Pc-Si/SiO2 stack. The high transparency and conductivity of the µc-SiC:H layer, a low thermal budget and number of processes needed to form the stack, and the excellent c-Si surface passivation quality are advantageous features of µc-SiC:H/SiO2 that can be beneficial for c-Si solar cells. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The fabrication process of high performance quadruple junction thin film silicon solar cells is described and the application of the solar cells in an integrated photoelectrochemical water splitting device is demonstrated. It is shown that the performance of solar cells can be adjusted by varying the process parameters and the thickness of the absorber layers of the individual sub cells and by integrating microcrystalline silicon oxide as intermediate reflecting layers. Thereby current matching of the sub cells was improved and a high open-circuit voltage of 2.8 V was achieved. Furthermore, the solar cell stability against light-induced degradation was investigated. Efficiencies of 13.2% (initial) and 12.6% (after 1000 h of light-soaking) were achieved. Bias-free water splitting with a solar-to-hydrogen efficiency of 7.8% was demonstrated in an integrated photovoltaic–electrochemical device using the developed quadruple junction photocathode. Finally, it is shown that in the case of quadruple junction solar cells the light-induced degradation has a lower effect on the photovoltaic–electrochemical efficiency as on the photovoltaic efficiency. [ABSTRACT FROM AUTHOR]

This paper highlights the effect of the operation temperature on the performance of a photovoltaic-biased electrosynthetic cell (PV-EC) device for solar hydrogen production based on a triple junction thin film silicon solar cell. The influence of the temperature in the range from 25 °C to 60 °C was studied individually for all components of the device: the solar cell, the hydrogen evolving cathode, the oxygen evolving anode, and the electrolyte. Based on the experimental data, the overall temperature-dependent current–voltage characteristics of the complete PV-EC device was modeled by merging the current–voltage characteristics of the individual components in an empirical series circuit model. We found that a decrease in the photovoltage of the solar cells with increasing temperature can be compensated by an improved electrochemical kinetics with temperature. This lead to a slight improvement in the performance of the integrated PV-EC device. Under an assumption of 100% faradaic efficiency, a maximum solar-to-hydrogen efficiency of 9.5% was found in 1 M KOH at an operation temperature of 50 °C. [ABSTRACT FROM AUTHOR]

Direct solar-to-hydrogen conversion via water splitting was demonstrated in an integrated photovoltaic–electrochemical (PV–EC) device using a hydrogenated amorphous silicon thin film tandem junction (a-Si:H/a-Si:H) solar cell as photocathode. The solar cell was adapted to provide sufficient photovoltage to drive both the hydrogen and oxygen evolution reactions. The best results, in terms of photoelectrochemical stability and performance, were obtained with an Ag/Pt layer stack as H 2 evolving photocathode back contact and with a RuO 2 counter electrode for O 2 evolution. Under irradiation by simulated sunlight (AM 1.5 spectrum with 100 mW/cm 2 ), we achieved 6.8% solar-to-hydrogen efficiency at 0 V applied bias in a two-electrode set-up. This sets a fresh benchmark for integrated thin film silicon tandem based photoelectrochemical devices. In addition, the photovoltage at constant current (−3 mA/cm 2 ) was measured over a prolonged period of time and revealed an excellent chemical stability (operation over 50 h) of the photocathode. Furthermore, we present an empirical serial circuit model of the PV–EC device, in which the corresponding photovoltaic and electrochemical components are decoupled. This allows for a detailed comparison between the solar cell and the PV–EC cell characteristics, from which the relevant loss processes in the overall system could be identified. The model was further used to compare calculated and measured photocurrent–voltage characteristics of the investigated PV–EC device which showed excellent agreement. [ABSTRACT FROM AUTHOR]

We present an experimental study combined with computer simulations on the effects of wide band-gap absorber and window layers on the open-circuit voltage ( Voc) in single junction thin film silicon solar cells. The quantity Δ Ep, taking as the difference between the band gap and the activation energy in ⟨p⟩ layer, is treated as a measure of the p-layer properties and shows a linear relation with Voc over a range of 100 mV with a positive slope of around 430 mV/eV. Two limiting mechanisms of Voc are identified: the built-in potential at lower Δ Ep and the band gap of the absorber layer at higher Δ Ep. The results of the experimental findings are confirmed by computer simulations. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We present a nanoimprint based approach to achieve efficient light management for solar cells on low temperature transparent polymer films. These films are particularly low-priced, though sensitive to temperature, and therefore limiting the range of deposition temperatures of subsequent solar cell layers. By using nanoimprint technology, we successfully replicated optimized light trapping textures of etched high temperature ZnO:Al on a low temperature PET film without deterioration of optical properties of the substrate. The imprint-textured PET substrates show excellent light scattering properties and lead to significantly improved incoupling and trapping of light in the solar cell, resulting in a current density of 12.9 mA/cm2, similar to that on a glass substrate. An overall efficiency of 6.9% was achieved for a flexible thin-film silicon solar cell on low cost PET substrate. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A dual hot-wire arrangement has been designed and investigated for the deposition of various thin film materials by the hot-wire chemical vapor deposition (HWCVD) technique. Tantalum and rhenium wires were used for silicon and silicon carbide depositions with hydrogen diluted silane and monomethylsilane, respectively. It is shown that the both types of filaments are mechanically stable after alternate depositions of silicon and silicon carbide with a total deposition time of about 80 h. Good material quality of the deposited films is demonstrated. By taking advantage of this dual hot-wire arrangement, it is possible to deposit both kinds of thin film materials with the individual optimum deposition conditions in a single HWCVD chamber. [ABSTRACT FROM AUTHOR]

Thin film tandem solar cells based on amorphous and microcrystalline silicon (a-Si:H/μc-Si:H) are employed as the cathode in a photoelectrochemical converter for solar water splitting. It is setup in such a way that the silver back contact of the cell is directly connected to the electrolyte and the light enters the cell through the glass substrate. This arrangement offers a number of distinct advantages compared to the conventional designs. The cathode is further optimized by the deposition of platinum nanoparticles to achieve higher conversion efficiencies. The front contact of the photovoltaic cell is connected to a standard platinum counter electrode in a three-electrode arrangement. Photon to current conversion efficiencies can reach up to 3% for our design, which has not been optimized to the requirements of the water splitting reaction, yet. The optimization of such tandem devices made from abundant silicon in combination with nanoparticle catalysts offers an affordable pathway for direct solar-to-fuel conversion devices in form of an artificial inorganic leaf. [ABSTRACT FROM AUTHOR]

We investigated the evolution of the spectrally resolved absorption coefficients of SiC and SiOx materials as well as of their multilayer systems during thermal annealing and hydrogen passivation, with focus on the nature of optically active defects induced during annealing. We propose that both dangling bonds (paramagnetic defects) and strained bonds (non-paramagnetic defects) formed during annealing contribute to the sub-band gap absorption and that the associated defects can be partially removed by hydrogen reincorporation. The difference in the evolution of the absorption spectra for different sample types upon annealing and passivation are linked to the fundamental difference in their atomic structures. The much lower optical band gap and the significantly higher sub-band gap absorption of SiC single layers in the annealed state as compared to SiOx single layers can be traced back to the lower flexibility of the relatively dense 4-fold coordinated atomic structure of the SiC material. [ABSTRACT FROM AUTHOR]

Metastability effects because of atmospheric exposure, high purity gasses, and deionized water in hydrogenated microcrystalline silicon thin films with different crystalline volume fractions were studied using well accepted steady-state characterization methods of dark conductivity, steady-state photoconductivity, steady-state photocarrier grating (SSPG) and dual beam photoconductivity (DBP) methods. A standard measurement procedure has been established before using the steady state methods, in which a steady state condition of dark conductivity was established by monitoring the time dependence of dark conductivity. Samples deposited on smooth glass and rough glass substrates exhibit similar reversible and irreversible changes in the properties of microcrystalline silicon film. A reliable correlation of reversible and irreversible changes indicate that dark conductivity and photoconductivity values increase, sub-bandgap absorption spectrum obtained from DBP method decrease and correspondingly minority carrier diffusion lengths obtained from the SSPG method increase in the metastable state in various amount for microcrystalline films with crystalline volume fraction, > 0.30. Amorphous silicon and microcrystalline silicon films with < 0.30 do not show detectable metastable changes as samples exposed to atmospheric condition as well as high purity oxygen gas and deionized water. [ABSTRACT FROM AUTHOR]

Metastability and instability effects due to oxygen exposure in thick intrinsic hydrogenated microcrystalline silicon films deposited by very high frequency plasma enhanced chemical vapour deposition on smooth glass substrates were investigated using temperature-dependent dark conductivity, steady state photoconductivity, and sub-bandgap absorption measurements obtained using the dual beam photoconductivity (DBP) method. No significant changes in dark conductivity and photoconductivity were detected even after long-term air exposure of samples in room ambient as well as after oxygen exposure when samples were characterized in oxygen ambient. However, characterization of the oxygen-exposed state in high vacuum caused an increase in dark conductivity and photoconductivity as well as a significant decrease in the sub-bandgap absorption coefficient spectra in the low energy region in samples with . These changes are partially irreversible for samples and mostly reversible for compact materials with significant amorphous fraction. No detectable metastable changes occurred in microcrystalline silicon samples with as well as in pure amorphous silicon. [ABSTRACT FROM AUTHOR]

Metastability effects in hydrogenated microcrystalline silicon thin films due to air, high purity nitrogen, helium, argon, and oxygen were investigated using temperature-dependent dark conductivity, photoconductivity, and steady-state photocarrier grating methods. It was found that short-term air, nitrogen, and inert gases caused a small reversible increase of σDark and σphoto within a factor of two, but they did not affect the minority carrier μτ-products significantly. These changes are partially reduced by vacuum treatment and completely reduced after heat treatment at 430 K. However, oxygen gas treatment at 80 °C resulted in more than an order of magnitude increase in both σDark and σphoto and an increase in the diffusion length, LD, by 50% from that of the annealed-state value in highly crystalline samples, while no significant metastability is detected in amorphous and low crystalline silicon thin films. A following heat treatment partially recovers both σDark and σphoto to their annealed-state values, while LD decreases only slightly. Such increase in the LD values could be due to a decrease in the density of recombination centers for holes below the Fermi level, which may be related to passivation of defects by oxygen on the surface of crystalline grains. [ABSTRACT FROM AUTHOR]

We present the design and implementation of a scanning probe microscope, which combines electrically detected magnetic resonance (EDMR) and (photo-)conductive atomic force microscopy ((p)cAFM). The integration of a 3-loop 2-gap X-band microwave resonator into an AFM allows the use of conductive AFM tips as a movable contact for EDMR experiments. The optical readout of the AFM cantilever is based on an infrared laser to avoid disturbances of current measurements by absorption of straylight of the detection laser. Using amorphous silicon thin film samples with varying defect densities, the capability to detect a spatial EDMR contrast is demonstrated. Resonant current changes as low as 20 fA can be detected, allowing the method to realize a spin sensitivity of [formula] at room temperature. [ABSTRACT FROM AUTHOR]

SiC/SiO x hetero-superlattices as well as single layers of SiC and SiO x were fabricated and annealed at various annealing temperatures to investigate the evolution of annealing induced defects in these materials. We show that in SiC and SiO x materials and in SiC/SiO x multilayer system, crystallization, atomic diffusion, structural reconstruction, and hydrogen effusion take place upon annealing with different characteristic temperature dependences. The dependence of the spin densities on annealing temperature is explained as an overlap of the temperature dependence of the creation of additional dangling bonds due to hydrogen effusion and of the healing of dangling bonds due to element motion during the structural reconstruction. The stronger increase of spin density upon annealing in SiC as compared to SiO1.2 is ascribed to the effusion of a higher amount of hydrogen and the less efficient atomic diffusion up to the annealing temperature of 1050 °C. [ABSTRACT FROM AUTHOR]

Abstract: Improvement of the performance of solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon requires understanding of the role of the deep defects – dangling bonds – in the bulk of the intrinsic a-Si:H or μc-Si:H absorber layers. A straightforward way to understand how these defects may affect the performance of the cells is to investigate changes in the device performance upon variation in the defect density. In the present work solar cells with a-Si:H and μc-Si:H absorber layers were exposed to 2MeV electron bombardment. The performance of the cells after various bombardment doses and annealing steps was evaluated in view of the changes in the defect density of intrinsic layers, measured with ESR on nominally identical absorber layers irradiated in parallel with the cells. The defect density was varied over a range of 2 orders of magnitude. In the solar cells a strong degradation of performance is observed upon irradiation with the biggest effect on the short circuit current density JSC for both types of absorber layers. In most cases both VOC and JSC recover after the final annealing step (at 160°C) for both types of cells. [Copyright &y& Elsevier]

Abstract: The SiC/SiOx hetero-superlattice (HSL) consisting of alternating near-stoichiometric SiC barrier layers for the electrical transport and silicon rich SiOx matrix layers for the quantum dot formation is a promising approach to the realization of silicon quantum dot (Si–QD) absorbers for 3rd generation solar cells. However, additional defect states are generated during post deposition annealing needed for the Si–QD formation causing an increase in sub-band gap absorption and a decrease in PL intensity. Proper passivation of excess defects is of major importance for both the optical and electrical properties of the SiC/SiOx HSL Si–QD absorber. In this work, we investigate the effectiveness of the hydrogen reincorporation achieved with hydrogen plasma in a plasma-enhanced chemical vapor deposition (PECVD) reactor, hydrogen dissociation catalysis in hot-wire chemical vapor deposition (HWCVD) reactor and annealing in forming gas atmosphere (FGA). Both the HSL samples and single layer reference samples are tested. The passivation quality of the hydrogen reincorporation was examined by comparing electrical and optical properties measured after deposition, after annealing and after passivation. In addition, the formation of Si–QDs in SiC/SiOx HSL was evaluated using high resolution transmission electron microscopy. We demonstrated that hydrogen can be successfully reincorporated into the annealed HSL sample and its single layer reference samples. FGA passivation is most effective for SiO1.2 single layers and HSL samples. Passivation with PECVD appeared to be only effective for SiC single layers. [Copyright &y& Elsevier]

Abstract: Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σD, photoconductivity, σph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O2 gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied. [Copyright &y& Elsevier]

Abstract: Al-doped p-type μc-SiC:H is prepared in a wide range of HWCVD preparation parameters like Al-doping ratio, deposition pressure, substrate and filament temperatures. We investigate the structural and electrical properties, and focus on identification of paramagnetic defect states by electron spin resonance (ESR). Nominally undoped μc-SiC:H is of a high n-type conductivity (σD =10−6–10−1 S/cm) and shows a narrow central ESR line (g≈2.003, peak-to-peak linewidth ∆H pp ≈4G) with two pairs of satellites and a spin density NS =1019 cm−3. Al-doping results in the compensation of dark conductivity to as low as σD =10−11 S/cm and at higher doping concentrations to effective p-type material. Increase of Al-doping results in reduction of crystallinity (I C IR ), ESR line shifts to g≈2.01 and becomes as broad as ∆H pp ≈30G, not unlike to the resonance of singly occupied paramagnetic valence band tail states in a-Si:H. ESR spectrum of highly crystalline Al-doped μc-SiC:H however has a g-value very close to undoped μc-SiC:H. Electron spin density in compensated material decreases to 5×1017 cm−3 before it increases again for the highly doped material. [Copyright &y& Elsevier]

Abstract: Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E 04 is about 3.0–3.2eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5μm, the short circuit current density (j SC) increases from 23 to 26mA/cm2 with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j SC =29.6mA/cm2 and η =9.6% were achieved in a cell with a 2-μm-thick absorber layer. [Copyright &y& Elsevier]

Abstract: A study of the effects of microcrystalline silicon i-layer modification near p/i interface in tandem configuration silicon thin film solar cells is presented. The structural properties of the absorber layers were investigated by Raman spectroscopy at different stages of growth. The results indicate the possibility of improving both the nucleation process and the film homogeneity in the direction of growth, without specific re-optimization of the p-layer, transferred from a single-junction microcrystalline silicon cell. Structural modifications of the i-layer have been correlated with performance of tandem solar cells, leading to improvements in the bottom cell current J sc (up to 11.4mA/cm2) and initial tandem-cell conversion efficiency (up to 11.3%). [Copyright &y& Elsevier]