Thermal Suppression of Secondary Reactions in PECVD of Amorphous Silicon Solar Cells For Reduction of Staebler Wronski Degradation |
Gautam Ganguly GG Consulting, Prescott Valley, AZ, United States |
Amorphous silicon (a-Si) is a material used in a variety of commercial devices including large area monolithic solar panels. One common factor that causes performance issues in these devices is metastability. While hydrogen dilution of the source gas, silane, in a parallel-plate plasma-chemical-vapor-deposition system, reduces the formation of metastable defects in a-Si based solar cells, they are still a limiting factor in the performance of these devices (10% modules). Disilane and higher silanes have been associated with formation of metastable centers in a-Si. Here we use the fact that the formation of disilane, by reaction of silylene with silane, is exothermic, to further reduce the rate of formation of disilane. Thus we demonstrate a reduction of metastability, relative to state-of-the-art hydrogen-dilution prepared solar cell devices, by increasing the gas temperature near the cathode, through heating of the earth shield surrounding the cathode, while keeping the substrate, on which the material is deposited, at the same temperature. We suggest that taking this process to its logical extension – cooling the anode to keep the substrate temperature constant while further increasing the gas temperature near the cathode, would practically eliminate metastability in the material. This would enable a-Si solar panels with efficiency over 20%, making them the low cost, short energy recovery time and superior hot climate performance option. |