Microcrystalline silicon films and solar cells deposited by PECVD and HWCVD

The application of microcrystalline silicon (μc-Si:H) in thin-film solar cells is addressed in the present paper. Results of different technologies for the preparation of μc-Si:H are presented, including plasma enhanced chemical vapour deposition (PECVD) using 13.56 MHz (radio frequency, rf) and 94.7 MHz (very high frequency, vhf) and hot-wire chemical vapour deposition (HWCVD). The influence of the silane concentration (SC) on the material and solar cell parameters is studied for the different techniques as the variation of SC allows to optimise the solar cell performance in each deposition regime. The best performance of μc-Si:H solar cells is always observed near the transition to amorphous growth. The highest efficiency obtained so far at a deposition rate of 5 Å/s is 9.4%, achieved with rf-PECVD in a deposition regime of using high pressure and high discharge power. High deposition rates and solar cell efficiencies could be also achieved by vhf-PECVD. An alternative approach represents the HWCVD which also demonstrated high deposition rates for μc-Si:H. However, good material quality and solar cell performance could only be achieved at low substrate temperatures and, consequently, low deposition rates. The μc-Si:H solar cells prepared by HWCVD exhibit comparably high efficiencies up to 9.4% and exceptionally high open circuit voltages up to 600 mV but at lower deposition rates (≈1 Å/s). The properties of PECVD and HWCVD solar cells are carefully compared.