Time-resolved Photoconductivity as a Probe of Carrier Transport in Microcrystalline Silicon
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0910-A01-01
Time-resolved Photoconductivity as a Probe of Carrier Transport in Microcrystalline Silicon Steve Reynolds Institute for Photovoltaics, Forschungszentrum Juelich, Leo Brandt Str., Juelich, D-52425, Germany
ABSTRACT The use of transient photoconductivity techniques in the investigation of carrier transport in microcrystalline silicon is described. Results are presented which highlight variations in transport parameters such as carrier mobility and density of states with structure composition. Hole mobility is significantly enhanced by crystalline content in the film of 10% or less. The density of states inferred from transport measurements parallel to and at right angles to the direction of film growth differ somewhat, suggesting that transport may be anisotropic. INTRODUCTION Microcrystalline silicon (µc-Si:H) is an attractive thin-film semiconductor for large-area application in solar cells. Its increased optical absorption over amorphous silicon (a-Si:H) in the long wavelength region of the solar spectrum and improved resistance to degradation make it particularly suited as a bottom-cell material in tandem solar cells [1,2]. In addition, increased carrier mobilities have led to improvements in thin-film transistor properties [3]. It follows that there is a pressing need both to understand the carrier transport mechanisms prevailing and to correlate improvements in transport properties and device operation, mechanistically as well as empirically, with structure composition and deposition parameters. Figure 1 shows a cartoon of the variation in structure composition of thin-film silicon as the crystalline volume fraction is modified by changing the deposition conditions. There are several ways in which this can be done but most commonly the proportion of silane in the process gas, r = [silane] : [silane + hydrogen] is varied, with lower percentages of silane giving more crystalline material [4]. In general, films are composed of crystallites, voids and amorphous regions, and in the more crystalline material there is a tendency for small crystalline grains to be aggregated into larger columnar structures disposed in the direction of film growth. Most commonly the Raman integrated intensity ratio ICRS [4] is used to enable comparisons of crystalline content over a series of samples, using procedures that are reproducible between laboratories. The degree of porosity and hydrogen content also varies with deposition conditions and with the choice of substrate [5]. Porosity has been shown to have a profound effect on film and solar cell properties when exposed to oxygen and/or water vapour [6-8]. While it was originally thought that µc-Si:H was impervious to light-induced metastable degradation (Stäbler-Wronski effect) it is now known that such effects may occur and that they are related to film structure composition [9,10].
Figure 1. Schematic representation of silicon thin film, after Vetterl et al [2].
There is currently no agreed universal model of carrier transport in µc-Si:H. However it is probably
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