Effects of various hydrogen dilution ratios on the performance of thin film nanocrystalline/crystalline silicon solar cells

Low temperature growth of hydrogenated nanocrystalline silicon film (nc-Si:H) by microwave electron cyclotron resonance chemical vapor deposition has been performed employing a double dilution of silane, using a He carrier for SiH₄ and its subsequent dilution by H₂. A series of Raman spectra and AFM pictures has shown that a very thin (<100 angstrom) nc-Si:H layer initially grown with high H₂ dilution on a glass substrate can serve as a seed layer for the subsequent growth of the film with lower H₂ dilution, which results in a higher crystallinity of the whole film. The role of this thin layer in low temperature junction formation has been examined by the insertion of the layer between the interface of both nc-Si:H (deposited with lower H₂ dilution)/c-Si and a-Si:H/c-Si heterojunction type photovoltaic cells. This is to address the knowledge that the device's performance is strongly influenced by the quality of the thin film silicon/crystalline silicon interface. Various thicknesses and H₂ dilution ratios have been used to find the optimized condition providing the best performance of the cells. The maximum efficiency of 10.5% (J_sc=35.1mA/cm², V_oc=0.51V and FF=0.59) has been obtained, without an AR coating, by the successive deposition of nc-Si:H film with four different H₂ dilution ratios on a crystalline silicon substrate. This is potentially a low-temperature, low-cost solar cell fabrication process.