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A profound comprehension of the nanostructure of hydrogenated amorphous silicon (a-Si:H) and the defect states in this material is currently still lacking despite several decades of research on this topic. Investigating the nature of defects in a-Si:H devices is especially important in view of the poorly understood Staebler–Wronski effect (SWE). Therefore we present the latest insights into the gap states in the a-Si:H bandgap by manipulating these sub gap states in three different ways: via the application of a voltage bias, via in situ light soaking, and via in situ annealing. To monitor the changes in the sub gap absorption we employ an advanced application of Fourier Transform Photocurrent Spectroscopy (FTPS). A recently published optical model that removes interference fringes from the photocurrent spectrum is presented to obtain the absorption coefficient. For FTPS measurements on solar cells the photocurrent spectrum is scaled to the external quantum efficiency (EQE) of the solar cell. Further, a mathematical data fitting routine is employed to accurately quantify the sub gap absorption and the changes therein during light soaking and annealing. The high sensitivity of FTPS is particularly helpful in identifying the sub gap states that play a role in the SWE in a-Si:H devices.

The bandgap of hydrogenated amorphous silicon (a-Si:H) is studied using a unique set of a-Si:H films deposited by means of three different processing techniques. Using this large collection of a-Si:H films with a wide variety of nanostructures, it is demonstrated that the bandgap has a clear scaling with the density of both hydrogenated divacancies (DVs) and nanosized voids (NVs). The presence of DVs in a dense a-Si:H network results in an anisotropy in the silicon bond-length distribution of the disordered silicon matrix. This anisotropy induces zones of volumetric compressed disordered silicon (larger fraction of shorter than longer bonds in reference to the crystalline lattice) with typical sizes of ~0.8 up to ~2 nm. The extent of the volumetric compression in these anisotropic disordered silicon zones determines the bandgap of the a-Si:H network. As a consequence, the bandgap is determined by the density of DVs and NVs in the a-Si:H network.

In consequence of previous investigation of individual transparent conductive oxide (TCO) and absorber layers a study was carried out on hydrogenated amorphous silicon (a-Si:H) solar cells with diluted intrinsic a-Si:H absorber layers deposited on glass substrates covered with different TCO films. The TCO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of using different TCO’s as a front contact in solar cells with structure as follows: Corning glass substrate/TCO (800, 950 nm)/ p -type μc-Si:H (∼5 nm)/ p -type a-Si:H (10 nm)/a-SiC:H buffer layer (∼5 nm)/intrinsic a-Si:H absorber layer with dilution R = [H 2 ]/[SiH 4 ] = 20 (300 nm)/ n -type a-Si:H layer (20 nm)/Ag + Al back contact (100 + 200 nm). Diode sputtered ZnO:Ga, textured and non-textured ZnO:Al [3] and commercially fabricated ASAHI (SnO 2 :F) U-type TCO’s have been used. The morphology and structure of ZnO films were altered by reactive ion etching (RIE) and post-deposition annealing. It can be concluded that the single junction a-Si:H solar cells with ZnO:Al films achieved comparable parameters as those prepared with commercially fabricated ASAHI U-type TCO’s.

We recently developed a scattering model based on the scalar scattering theory. In this contribution we present how we used the scattering model to investigate interface textures with optimized scattering properties. We used the simulated annealing algorithm to find optimized surface textures and applied the ASA device simulator to evaluate the influence of these optimized textures on the performance of thin film silicon solar cells. We found that the lateral feature size of the textures is crucial for efficient scattering of the incident light.

The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed dependence of the paramagnetic defect density on the Doppler S parameter indicates that porous, nanosized void-rich materials exhibit higher spin densities, while dense, divacancy-dominated materials show smaller spin densities. However, after light soaking more similar spin densities are observed, indicating a long-term defect creation process in the Staebler-Wronski effect that does not depend on the a-Si:H nanostructure. From ESE decays it appears that there are fast and slowly relaxing defect types, which are linked to various defect configurations in small and large open volume deficiencies. A nanoscopic model for the creation of light-induced defects in the a-Si:H nanostructure is proposed.

In this study it is revealed that the light induced defects (LIDs) responsible for the fast degradation of hydrogenated amorphous silicon (a-Si:H) solar cells under light soaking are located at nanosized voids. This important breakthrough in identifying the local environment of LIDs has been achieved by detailed study of the relation between the nanostructure of a-Si:H and metastability of corresponding solar cell devices under light soaking. We propose that a useful tool to define the nanostructure of a-Si:H is to determine the size distribution of the volume deficiencies, which range from small hydrogenated vacancies up to nanosized voids. The processing window used to vary and control the nanostructure in dense a-Si:H is based on a hydrogen rich plasma at unconventional high processing pressures (∼10 mbar). The dense absorber layers with different distributions of volume deficiencies are subsequently incorporated in solar cell devices. For the first time a clear relation between nanostructures of the a-Si:H absorber layer and the fast kinetics of the metastable LIDs of the solar cell during light soaking and thermal annealing is observed. The ‘fast’ degradation (first 10 hours of light soaking) strongly correlates to the density of largest volume deficiencies in the a-Si:H matrix. The “slow” regime (10 tot 1000 hours of light soaking) appears to be independent on the nanostructure of the absorber layer. In addition, the fast metastable defect states are the first ones to be annealed out at relative low annealing temperatures (120–130 C). Although solar cells processed at higher pressure have the same long term degradation kinetics, their FF recovers much faster by thermal annealing when compared to the cells processed at standard low pressure and low hydrogen dilution conditions.

We present a scattering model based on the scalar scattering theory that allows estimating far field scattering properties in both transmission and reflection for nano-textured interfaces. We first discuss the theoretical formulation of the scattering model and validate it for nano-textures with different morphologies. Second, we combine the scattering model with the opto-electric asa simulation software and evaluate this combination by simulating and measuring the external parameters and the external quantum efficiency of solar cells with different interface morphologies. This validation shows that the scattering model is able to predict the influence of nano-textured interfaces on the solar cell performance. The scattering model presented in this manuscript can support designing nano-textured interfaces with optimized morphologies.

The network and nature of hydrogenated amorphous silicon (a-Si:H) are conventionally interpreted in terms of a continuous random network (CRN) of Si-Si bonds, weak Si-Si, Si-H bond and dangling bonds. A CRN requires that the smallest anisotropic features like dangling bonds and bonded hydrogen are randomly distributed and reside as isolated configurations in the network. However, in recent years more and more theoretical and experimental evidence have been found that both the isolated dangling bond and the isolated hydrogen are not present in the a-Si:H network. To the contrary, all studies come to the conclusion that the real nature of the a-Si:H is to contain more local structural order than expected from a CRN. These insights offer new opportunities to revisit the origin of several properties of a-Si:H, which are up to now explained within the framework of the CRN model. In this contribution we will discuss that many diagnostics like nuclear magnetic resonance, positron annihilation, small angle x-ray spectroscopy, density analysis and infrared spectroscopy on a-Si:H consistently demonstrate that a-Si:H exhibits an anisotropic network. In dense disordered networks the hydrogen predominantly resides in hydrogenated divacancies, whereas for less dense networks the hydrogen predominantly resides in poly-vacancies up to nanosized voids. We will discuss that hydrogenated divacancies in a disordered network contribute to the amorphous nature of a-Si:H and its electronic structure like the band gap, gap tails and the defect gap states.

The properties of thin-film silicon strongly depend on the deposition method, conditions and the substrate material. The analysis of the microstructure of thin silicon films requires diagnostic tools which are independent on substrate or device concept. In this contribution Raman spectroscopy as a powerful tool for analyzing the microstructure and material phase of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μSi:H) is discussed. To improve the sensitivity for the bulk properties, a HeNe laser (λ = 633 nm) source is used, due to its longer absorption path length at this wavelength. Partial transmission of the light through the film onto the substrate results in a measured Raman spectrum consisting of the superposition of the spectra of film and substrate. Two methods using different approaches in thin film optics for distinguishing the substrate- and film spectrum from the measurement are discussed. One method to model transmission of the Raman spectrum originating from the substrate is based on thin film interference, adapted to a partial coherent quasi monochromatic light source.[1] This method is applicable in the limiting case of thin a-Si:H films (film thickness: d

The effect of deposition temperature on the structural and optical properties of amorphous hydrogenated silicon (a-Si:H) thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen was under study. The series of thin films deposited at the deposition temperatures of 50–200°C were inspected by XRD, Raman spectroscopy and UV Vis spectrophotometry. All samples were found to be amorphous with no presence of the crystalline phase. Ordered silicon hydride regions were proved by XRD. Raman measurement analysis substantiated the results received from XRD showing that with increasing deposition temperature silicon-silicon bond-angle fluctuation decreases. The optical characterization based on transmittance spectra in the visible region presented that the refractive index exhibits upward trend with increasing deposition temperature, which can be caused by the densification of the amorphous network. We found out that the scale factor of the Tauc plot increases with the deposition temperature. This behaviour can be attributed to the increasing ordering of silicon hydride regions. The Tauc band gap energy, the iso-absorption value their difference were not particularly influenced by the deposition temperature. Improvements of the microstructure of the Si amorphous network have been deduced from the analysis.

The paper deals with the hydrogenated amorphous silicon (a-Si:H) films about 300 nm in thickness prepared by using rf-PECVD with hydrogen dilution R = 10 of the silane source gas in the amorphous growth regime onto clean Corning Eagle 2000 glass substrates at different deposition temperatures ranging from 50 to 200 °C. Structural and optical properties of the films were obtained from X-ray diffraction and UV-Vis spectrophotometry. The full width at half maximum of the first scattering peak decreases with increasing of the deposition temperature up to 150 °C and then remains constant. Optical band-gaps are from 1.65 to 1.76 eV, which slightly decrease with increasing deposition temperature, whereas the refractive index increases with increasing deposition temperature. This indicates that the density of the films at higher temperature has increased.

We report on a giant missing row reconstruction emerging upon the adsorption and subsequent annealing of (sub)monolayer amounts of Au on Ge(001). The emerging microfacets are of (111) type and reminiscent of those in the well-known (2×1) missing row reconstruction of the clean (110) surfaces of the fcc transition metals Au, Ir, and Pt. The (111) microfacets are aligned along the [110] directions and decorated with Au atoms. The periodicity perpendicular to the ridges amounts to 1.6 nm. Surprisingly the corrugation of these Au-induced facets is found to be not less than 0.6 nm. The facets exhibit a (√3×√3)R30° reconstruction similar to the Au-induced reconstruction of Ge(111). Scanning tunneling microscopy and spectroscopy measurements reveal that the top ridges consist of buckled Ge-dimer rows.

Recently, it was found that atomic Pt chains can be grown on Ge(001) surfaces. These kink-free wires only grow on specific Pt-modified, so-called beta terraces, which are one atom wide and up to a micron long. Their self-assembly includes a crucial, high-temperature annealing step, following low-temperature (sub-) monolayer deposition. So far, details of the formation process remained unclarified. Here, we report on the formation of the Pt atomic chains on the Ge(001) surface after a high-temperature annealing at 1100 K. The results suggest that at high temperature, the platinum, previously submerged during and after deposition, emerges and forms Pt dimers located in the troughs between the surface dimer rows, which locally widen in this process. These Pt dimers are initially oriented parallel to the substrate dimers. However, upon surpassing a critical local coverage, the Pt dimers rotate by 90°, resulting in a zipperlike formation of the Pt chains.

This study gives a guideline on developing high bandgap, high quality hydrogenated amorphous silicon (a-Si:H) through a carefully engineered nanostructure. Single-junction a-Si:H solar cells with open-circuit voltages (Voc) above 950 mV and conversion efficiencies above 9% are realized by processing the absorber layers at high pressures of 7–10 mbar. The high Voc is a result of an increased bandgap, which is attributed to an increase in the average size of the open volume deficiencies in the absorber layer without a significant increase in the nanosized void density.

Thin-film silicon solar cells (TFSSC), which can be manufactured from abundant materials solely, contain nano-textured interfaces that scatter the incident light. We present an approximate very fast algorithm that allows optimizing the surface morphology of two-dimensional nano-textured interfaces. Optimized nano-textures scatter the light incident on the solar cell stronger leading to a higher short-circuit current density and thus efficiency. Our algorithm combines a recently developed scattering model based on the scalar scattering theory, the Perlin-noise algorithm to generate the nano textures and the simulated annealing algorithm as optimization tool. The results presented in this letter allow to push the efficiency of TFSSC towards their theoretical limit.