Microcrystalline Silicon-Germanium Alloys for Absorption Layers in Thin Film Solar Cells
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ABSTRACT
Thin microcrystalline silicon-germanium films (pc-Sil.KGeX:H) prepared by PECVD at 95 MHz have been investigated. The optical absorption of these films increases in the infrared spectral region with increasing germanium content. In addition to the shift of the indirect gap an increase of the absorption coefficient above the band edge is observed. The material shows high crystallinity and exhibits good structural quality similar to pure pc-Si:H films. The films are homogeneous on a macroscopic to a microscopic scale as confirmed by Raman spectroscopy and Electron Microscopy methods. p-i-n solar cells with pc-SilxGex:H i-layers have been prepared for the first time. An efficiency of rl = 3.1 % under AM1.5 has been obtained for a cell with 150 nm thin i-layer. INTRODUCTION For the application as low gap material in stacked solar cells or color sensors a-Si:Ge:H alloys have been developed in the past years with considerable success [1, 2]. Still, due to the inherent instability of a-Si:Ge:H, alternative low gap materials were looked for and recently pc-Si:H was proposed and used as low gap material in thin film devices [3, 4]. However, pc-Si:H is an indirect semiconductor with low absorption in the energy range of interest, films of some pm thickness are required for solar cell applications. It is therefore of interest to investigate, in how far, pc-Si:Ge:H alloys can overcome these restrictions of the pc-Si:H. This is motivated by the fact that crystalline (or polycrystalline) SiGe alloys are well known materials with reduced band
gap and altered band structure leading to changes of the interband absorption with respect to crystalline silicon [5, 6]. From this changes a shift of the indirect gap and an increase of the absorption coefficient above the band edge is expected also in the microcrystalline alloys. While microcrystalline silicon-germanium alloys (pc-Si:Ge:H) have already been prepared during the development of amorphous SiGe alloys, e.g. [7] only very recently such films have been considered as candidate for low gap absorption layers [8]. In the present study very high frequency (VHF) glow discharge is used to prepare pc-Si:Ge:H films of different composition. The VHF glow discharge technique has been used to improve the material quality and the growth rate of pc-Si:H [9]. The growth rate is one of the most crucial issues with respect to commercialization. Optical absorption spectroscopy and structural investigations by Raman spectroscopy and Electron Microscopy methods are used to characterize the samples and the results are compared with literature data for (poly-) crystalline SiGe alloys. Furthermore, we present the first results on p-i-n solar cells with Vc-Sit_,Gex:H as absorption layers of different band gap. EXPERIMENT The pc-SilxGex:H films and the solar cells were prepared by PECVD in a 6-chamber deposition system with base pressure < 109 mbar at a plasma excitation frequency of 95 MHz at 200 'C substrate temperature [10]. Mixtures of germane (GeH 4) and silane (SiH 4) or disilane Si 2H6
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