Surface passivation for germanium and silicon back contact type photovoltaic cells

Back contact type cells have large area contacts at the back, and have no front grid contact preventing light absorption. The cells are able to simultaneously offer reduced resistance loss for high current densities and high light absorption. However, the structure requires low surface recombination loss to obtain high efficiency. We studied various structures to reduce the loss for Si and Ge back contact type cells. A floating emitter at the surface of the Si cell is fabricated to reduce recombination loss with a SiO2 passivation layer. The Ge cell for thermophotovoltaics has a high refractive index SiNx passivation layer at the front surface. The layer also serves as a bottom layer of the anti reflection coating. In a GaInP/GaAs/Ge triple-junction cell with a Ge back contact type bottom cell, the energy band at the front of the Ge cell is bent due to the diffusion layer, and is effective in reducing recombination loss.