Optimization of Transparent Passivating Contact for Crystalline Silicon Solar Cells

A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO ₂ /µc-SiC:H( n )] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the µc-SiC:H( n ) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of µc-SiC:H( n ) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO ₂ /µc-SiC:H( n ) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm ² . All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization.