A model of the dependence of photovoltaic properties on effective diffusion length in polycrystalline silicon

Abstract

In polycrystalline silicon, an effective diffusion length is often defined in order to take into account grain boundary recombination. The definition given here is associated with the surface photovoltage method which is generally used to determine the diffusion length in the base of a solar cell. The effective diffusion length and photovoltaic properties are calculated from the minority carrier distribution within a grain, which is determined by means of a three-dimensional model. The relation between effective diffusion length and photovoltaic properties has been calculated for various values of the material parameters (grain size, bulk diffusion length, effective grain boundary recombination velocity and doping level). It has been found that photovoltaic properties depend not only on the effective diffusion length but also on all the material parameters. It is demonstrated that measurement of the effective diffusion length can provide a sufficiently accurate estimate of the efficiency of a polycrystalline silicon solar cell, using an equivalent monocrystalline cell, but on the condition that the grain size is greater than ten times the average diffusion length within the grains. If this condition is not met, a three-dimensional model is required to obtain satisfactory accuracy.