INFLUENCE OF PRESSURE AND PLASMA POTENTIAL ON HIGH GROWTH RATE MICROCRYSTALLINE SILICON GROWN BY VHF PECVD

摘要

Microcrystalline silicon (μc-Si) based single junction solar cells are deposited by VHF PECVD using a showerhead cathode at high pressures in depletion conditions. 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 μc-Si i-layer of 1 μm. The i-layer is made near the transition from amorphous to crystalline. 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. It is observed that the performance of solar cells deposited at high rate improves under light soaking conditions at 50℃, which we attribute to post deposition equilibration of a fast deposited transition material. The performance is lower at higher rates due to poorer i-layer quality (higher defect density), which may be attributed to smaller relaxation times for growth precursors at the growth surface and the higher energy ion bombardment at higher plasma power. High process pressures can be used to reduce the ion energy by decreasing the mean free path. 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 quality of the μc-Si layers and the performance of the solar cells is further improved.