Microstructural Defects of Device Quality Hot-Wire Cvd Poly-Silicon Films

Abstract

AbstractTwo types of poly-Si:H thin films made by Hot Wire CVD have been evaluated with respect to utilisation in solar cells. Poly-Si:H films made at high hydrogen dilution are highly porous and have large interconnected voids. The void density is 25000/μm-3 as determined by XTEM. On the other hand, poly-Si:H layers made at low hydrogen dilution have a compact structure and a much smaller density of voids. In these films, two types of voids exist: globular voids smaller than 15 nm, and elongated voids, often located between columns of large crystals of 150-250 nm wide at the top. The density for the 5 - 15 nm spherical voids is usually -50/μm3, but larger concentrations often occur locally, up to 1000/pm3, i.e., 0.05% volume fraction. High oxygen content in the poly-Si films made at high hydrogen dilution is largely due to post deposition intrusion of water vapour through the interconnected voids. Profiled layers are made by depositing device quality poly-Si:H layers (low hydrogen dilution) on top of a seed layer (high hydrogen dilution) of high nucleation density. Cells incorporating profiled poly-Si:H films as i-layers at a deposition rate of 0.5 nm/s were made on stainless steel substrates in the configuration SS/n-μc-Si:H(PECVD)/i-poly-Si:H(HWCVD)/p-μc-Si:H(PECVD)/ITO. For our n-i-p solar cell with poly-Si i-layer we obtained an efficiency of 4.41% and a FF of 0.607. Due to native surface texture a current density of 19.95 mA/cm2 is generated in only ~1.22 μm thick i-layer without back reflector.