Voronoi network modelling of multicrystalline silicon solar cells

A model of a multicrystalline silicon solar cell with columnar grains having different carrier recombination properties is constructed from a two-dimensional Voronoi network. The prismatic grains are approximated by cylinders embedded in an effective medium with diffusion length L-e, and the charge collection probability phi(r) within each grain is calculated by solving a three-dimensional diffusion equation. The value of L-e is calculated self-consistently through the integral of phi over the cell volume. The function phi also allows the calculation of the contribution of the base to the reverse saturation current of the cell. Numerical examples illustrate the influence of areal inhomogeneities and grain boundary recombination on the value of L-e, dark current and open-circuit voltage. It is found that small-area grains with low lifetime mainly reduce the open-circuit voltage without significantly lowering L-e and the photocurrent. Histograms obtained for multicrystalline cells by the light beam induced current technique are simulated and compared with experiment.