Microcrystalline Si (µc-Si) based single junction solar cells were deposited by very high-frequency plasma-enhanced chemical vapour deposition (VHF-PECVD) using a showerhead cathode at high pressures in depletion conditions. The i-layers were made near the transition from amorphous to crystalline. An energy conversion efficiency of 9.9% was obtained with a single junction solar cell that was deposited on a texture-etched ZnO:Al front contact. The µc-Si i-layer was 1.5µm thick, deposited at a rate of 0.5nm/s. In order to control the material properties in the growth direction, the H dilution of silane in the gas phase was graded following different profiles with a parabolic shape. Materials with higher deposition rates were developed by increasing the RF power and the total gas flow such that the depletion condition was constant. At a deposition rate of 4.5nm/s, a stabilized conversion efficiency of 6.7% was obtained for a single junction solar cell with a µc-Si i-layer of 1µm. It was found that the defect density increases one order of magnitude upon the increase in deposition rate from 0.45 to 4.5nm/s. This increase in defect density was partially attributed to the increased energy of the ion bombardment during the plasma deposition. An additional method was introduced in order to limit the ion energy by controlling the DC self bias voltage using an external power source. In this way, the defect density in the µc-Si layers was decreased and the performance of the solar cells was further improved. It was observed that the performance of solar cells deposited at high rate improves under light soaking conditions at 50C, which was attributed to post-deposition equilibration of a fast deposited transition material.

Influence of Pressure and Plasma Potential on High Growth Rate Microcrystalline Silicon Grown by Very High Frequency Plasma Enhanced Chemical Vapour Deposition. A.Gordijn, M.Vanecek, W.J.Goedheer, J.K.Rath, R.E.I.Schropp: Japanese Journal of Applied Physics, 2006, 45[8A], 6166-72