Characterization of intrinsic amorphous silicon layers for solar cells prepared at extremely high rates by atmospheric pressure plasma chemical vapor deposition

Using the pressure plasma chemical vapor deposition (CVD) technique, extremely high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films for photovoltaic layers of thin film solar cells was investigated. Amorphous Si:H films were deposited on glass substrates at atmospheric pressure in a 150 MHz very high frequency (VHF) plasma of gas mixtures containing He, H2 and SiH4. Electrical and optical properties of the films were investigated by measuring photo- and dark conductivity and optical absorption, and the relationship between each deposition parameter and the film properties was studied. It was found that an optimum VHF power existed for each SiH4 concentration to simultaneously improve the deposition rate and the film quality. It also became clear that photoconductivity of the film was almost constant and greater than 10−5 Ω−1 cm−1 in the range of deposition rate from 24 to 336 nm/s. Based on these results, the a-Si:H films were applied to the i-layers of p-i-n single junction solar cells, and it was demonstrated that the conversion efficiency was independent of the deposition rate for the proper value of hydrogen dilution ratio ([H2 concentration]/[SiH4 concentration]). As a result, an initial efficiency of 8.25% was obtained for the a-Si:H solar cell with an i-layer prepared at the very high deposition rate of 128.1 nm/s.