Stable passivations for high-efficiency silicon solar cells

Initial designs of single-crystal silicon point-contact solar cells have shown a degradation in their efficiency after being exposed to concentrated sunlight. The main mechanism appears to be an increase in recombination centers at the Si/SiO₂ interface due to ultraviolet light photoinjecting electrons from the silicon conduction band into the silicon dioxide that passivates the cell's front surface. Trichloroethane, texturization, and aluminum during the forming gas anneal all contribute to the instability of the interface. A reasonably good resistance to UV light can be obtained by putting a phosphorus diffusion at the surface and can be improved further by stripping off the deposited oxide after the diffusion and regrowing a dry thermal oxide. A second technique, which utilizes ultrathin oxides and thin polysilicon films and can yield stable point-contact solar cells that are more efficient at higher concentrations, is also described.