VHF a-Si:H solar cells: A systematic material and cell study

A systematic study of material quality has been performed for intrinsic a–Si:H layers deposited by plasma enhanced chemical vapor deposition at excitation frequencies between 30 and 80 MHz (VHF). The process conditions were optimized not only for “device quality” opto-electronic properties but also for a uniformity in layer thickness better than 5% over the 10 cm × 10 cm substrate area. We found optimized homogeneities at different pressures depending on the excitation frequency. The effect of frequency at these optimum conditions on the material quality is small. VHF-intrinsic layers have been used in a–Si:H p+-i-n+ solar cells, in which both the p+ and n+ layer were made using 13.56 MHz. There is a clear correlation between material quality and solar cell parameters. Material deposited at low power densities is of so-called “device quality,” which is confirmed by demonstrating an initial efficiency of 10% for cells deposited at 65 MHz using a low power density. The deposition rate still is 2–3 times higher than the one at 13.56 MHz. Light-soaking of the cell leads to stabilization at 6% for the best cells, which compares well to conventional 13.56 MHz cells.