3D-Modelling of polycrystalline silicon solar cells

Abstract

This work is concerned with the effect of grain boundaries (gb) on the main parameters which characterize polycrystalline silicon solar cells. The variation with illumination of the grain boundary effective recombination velocity has been calculated by means of a self-consistent procedure which takes into account the bending of the minority carrier quasi-Fermi level in the space-charge region and in the gb quasi-neutral region. Abaci have been plotted which allow to predict the photovoltaic properties as a function of the characteristic quantities of a polycrystalline material: grain size g, diffusion length Ln and doping concentration in the grains and interfacial recombination velocity S at the grain boundaries. It is shown by means of a 3D-model that the conversion efficiency can be enhanced by optimizing the doping concentration of the base of the cells ; the optimum doping level and the related efficiency are given as a function of grain size for different interface state densities. Finally, an effective diffusion length taking into account the gb recombination is introduced. This effective diffusion length can be measured by the Surface Photo Voltage (SPV) method. Its variation as a function of S, Ln and g have been determined. This quantity allows to forecast approximate values of the photovoltaic properties