Stable Solar Cells Prepared from Dichlorosilane
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STABLE SOLAR CELLS PREPARED FROM DICHLOROSILANE Y.Yamamoto,W.Futako,K.Fukutani,M.Hagino,T.Sugawara,T.Kamiya,C.M.Fortmann and I.Shimizu, Dept. of Innovative and Engineering Materials,Tokyo Institute of Technology, 4259 Nagatsuta,Midori-ku,Yokohama,Japan 226-8502 ABSTRACT Amorphous silicon films and solar cell i-layers were prepared from dichlorosilane(DCS) by ECR- and VHF-CVD. The hydrogen content, the chlorine content and the band gap could be controlled by varying argon and hydrogen dilution. The interaction of energetic and reactive plasma species with substrates and other previously deposited layers was studied. DCS,ECR-CVD causes darkening of TCO substrates even when buffer layers and/or doped layers were previously deposited by RF-CVD. Therefore n-i-p solar cell structures were prepared on NiCr and subsequent p-i-n solar cells were prepared with VHF-CVD which did not causeTCO reduction or other reactions in previously deposited amorphous layers. Preliminary results indicate that the VHF-CVD solar cells are at least as stable as standard amorphous silicon solar cells. INTRODUCTION The amorphous silicon solar cell (e.g.,see Carlson[1]) is one of the most promising thin film photovoltaic devices due to its low manufacturing cost. However, further cost reduction and improved stability are necessary in order to achieve large scale power generation with these solar cells. In this work, we investigate the use of dichlorosilane(DCS) as an alternative to silane as DCS is both less expensive and less dangerous than silane. Previously Yokoi [2]and also Azuma[3,4] demonstrated that intrinsic amorphous silicon films prepared from DCS showed substantially increased stability as compared to amorphous silicon prepared from silane. Typically these films had large band gap (1.8-1.9eV), had defect densities of only 1-5x10 cmn3 in the annealed state, and defect densities of only 2-5x10 16cm3 in the degraded state. Furthermore, Azuma[4] showed that amorphous silicon prepared from DCS could deposited by ECR-CVD at very high growth rate with nearly 100% utilization of the DCS source material. It is important to note the short range order as measured by Raman spectroscopy indicated that these materials had similar bond length and bond angle distributions as standard device quality a-Si:H. In these previous studies high hydrogen dilution was needed to ensure that Cl was removed from the growth surface and subsurface regions. To avoid the hydrogen induced crystallization, low substrate temperatures (Ts
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