The Influence of Grain-Boundary Recombination and Grain Size on the I(V)-Characteristics of Polycrystalline Silicon Solar Cells
The grain-boundary recombination velocity and the grain size determine the I(V)-characteristics and the energy-conversion efficiency of a polycrystalline cell in addition to those parameters known from the monocrystalline cell. The influence of these two additional parameters is analysed quantitatively by means of a model calculation. The calculation is based on an idealized model structure with grain boundaries perpendicular to the pn-junction. The analysis of the carrier-flow problem requires the solution of the 2-dimensional diffusion equation which becomes inhomogeneous under optical illumination. It is shown by means of the balance of charge and currents that the physical properties of the grain boundaries may be expressed by one single parameter, the grain-boundary recombination velocity. Thus the boundary conditions of the carrier-flow problem consider the grain boundaries as recombination planes. The theoretical results for the I(V)-characteristics and the spectral sensitivity are compared to experimental results obtained at commercial polysilicon cells. Calculations and experiments show that the maximum value of the spectral sensitivity shifts to shorter wavelengths for increased recombination and reduced grain size. Furthermore we find the reciprocal slope factor of the I(V)-characteristics to be 1 for mono and polysilicon cells except for small current densities.
KeywordsSpectral Sensitivity Photovoltaic Solar Excess Carrier Recombination Current Optical Illumination
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