Influence of pressure and plasma potential on high growth rate microcrystalline silicon grown by very high frequency plasma enhanced chemical vapour deposition

Microcrystalline silicon (mu c-Si) based single junction solar cells have been deposited by very high-frequency plasmaenhanced chemical vapor deposition (VHF-PECVD) using a showerhead cathode at high pressures in depletion conditions. The i-layers are made near the transition from amorphous to crystalline. An energy conversion efficiency of 9.9% is obtained with a single junction solar cell that is deposited on a texture-etched ZnO:AI front contact. The mu c-Si i-layer is 1.5 mu m thick, deposited at a rate of 0.5 nm/s. In order to control the material properties in the growth direction, the hydrogen dilution of silane in the gas phase is 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 is constant. At a deposition rate of 4.5 nm/s, a stabilized conversion efficiency of 6.7% is obtained for a single junction solar cell with a mu c-Si i-layer of 1 mu m. It is found that the defect density increases one order of magnitude upon the increase in deposition rate from 0.45 to 4.5 nm/s. This increase in defect density is partially attributed to the increased energy of the ion bombardment during the plasma deposition. We have introduced an additional method 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 ltc-Si layers is decreased and the performance of the solar cells is further improved. It is observed that the performance of solar cells deposited at high rate improves under light soaking conditions at 50 degrees C, which we attribute to post deposition equilibration of a fast deposited transition material.