Optimization of intrinsic hydrogenated amorphous silicon deposited by very high-frequency plasma-enhanced chemical vapor deposition using the relationship between Urbach energy and silane depletion fraction for solar cell application

Abstract

Intrinsic hydrogenated amorphous silicon (i-a-Si:H) films were deposited by very high frequency (VHF) plasma enhanced chemical vapor deposition (PECVD) technique. It was found that there were three distinct deposition rate regions, when the deposition pressure and power were varied according to Paschen's law. The silane depletion fraction (SDF) is related to the reaction rates and the sticking probability of the radicals, which is smaller in the 1st region, where the SiH3 radical is the dominant deposition precursor giving rise to higher film density and a low Urbach energy (~68meV). The third region has higher SDF, where more SiH2 radicals are generated resulting in a reduction in film density and increased structural disorder due to polyhydride formation. The good quality films obtained with the condition of the 1st region, showed low SDF at lower pressure and power, following Paschen's law. Some of these i-a-Si:H films were used to fabricate p–i–n type solar cells. The measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (Voc)=800mV, short circuit current density (Jsc) of 16.3mA/cm2, fill-factor (FF) of 72%, and photovoltaic conversion efficiency (η) of 9.4%, which may be due to improved intrinsic layer. Jsc, FF and Voc of the cell can be improved further with optimized cell structure and with i-a-Si:H having a lower number of defects.