On the origin of improved conversion efficiency of solar cells based on SiGe with compositional distribution

We report on the theoretical investigation of photovoltaic power conversion of SiGe with a compositional distribution by a simple approach. The absorption coefficient and the intrinsic carrier concentration, which greatly affect photovoltaic properties, are treated as macroscopically uniform parameters and expressed using the compositional distribution. By solving a steady-state continuity equation under solar light illumination, the performance of the solar cell based on SiGe with the compositional distribution is predicted. As a result, the existence of a compositional window where the conversion efficiency of the solar cell based on SiGe can surpass that of the solar cell based on Si is suggested. Such a window appears due to the increased absorption coefficient when the diffusion length of minority carriers is finite. This prediction agrees with the experimental results of the performance of SiGe solar cells with a systematically varied average Ge composition.