Marinus Fischer

A scattering model for nano-textured interfaces and its application in opto-electrical simulations of thin-film silicon solar cells

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.

Micro-textures for efficient light trapping and improved electrical performance in thin-film nanocrystalline silicon solar cells

Micro-textures with large opening angles and smooth U-shape are applied to nanocrystalline silicon (nc-Si:H) solar cells. The micro-textured substrates result in higher open-circuit-voltage (Voc) and fill-factor (FF) than nano-textured substrates. For thick solar cells, high Voc and FF are maintained. Particularly, the Voc only drops from 564 to 541 mV as solar cell thickness increases from 1 to 5 μm. The improvement in electrical performance of solar cells is ascribed to the growth of dense nc-Si:H layers free from defective filaments on micro-textured substrates. Thereby, micromorph tandem solar cells with an initial efficiency of 13.3%, Voc = 1.464 V, and FF = 0.759 are obtained.

Designing optimized nano textures for thin-film silicon solar cells.

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.

The relation between the band gap and the anisotropic nature of hydrogenated amorphous silicon

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.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.

High pressure processing of hydrogenated amorphous silicon solar cells: Relation between nanostructure and high open-circuit voltage

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.

Influence of deposition temperature on amorphous structure of PECVD deposited a-Si:H thin films

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.

Raman spectroscopy on thin film silicon on non-transparent substrates and in solar cell devices

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<400 nm). For larger thicknesses we demonstrate that absorption is no longer negligible and the transmission needs to be calculated using the transfer-matrix method. The Si-H stretching modes (∼1900–2150 cm−1) provide detailed information on the microstructure of thin films and are generally studied by infrared absorption, demanding the film being deposited on an infrared transparent substrate.[2] We demonstrate an approach in which the Raman spectrum of the substrate is removed from the measured spectrum, resulting in a technique capable of comparing crystalline fraction or microstructure of absorber layers, even in p-i-n solar cell devices.

Structure and optical properties of the hydrogen diluted a-Si:H thin films prepared by PECVD with different deposition temperatures

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.