Direct calculation of two-dimensional collection probability in pn junction solar cells, and study of grain-boundary recombination in polycrystalline silicon cells

Abstract

A new method for the direct calculation of the two-dimensional collection probability in pn junction solar cells is presented. Based on its reciprocity properties, the inhomogeneous continuity equation for excess carriers is transformed to a homogeneous partial differential equation (PDE) for the probability of carriers being collected in the external circuit. The new PDE is easier to solve and directly gives the short-circuit current. The method is applied to study the impact of grain-boundary recombination on the performance of polycrystalline silicon solar cells. A critical grain width of four times the carrier diffusion length in the base is found to be the limiting boundary between polycrystalline behavior and monocrystalline behavior of the cell. The sensitivity of short-circuit AM1.5 collection efficiency to the grain width Wg, the grain-boundary recombination velocity Sg, minority-carrier diffusion lengths, and surface recombination velocities, is quantified for a variety of cell types and recombination parameters. The sensitivity analysis indicates that AM1.5 collection efficiency is most sensitive to grain width in narrow grain cells, and to base diffusion length in wide grain cells.