Current transport with and without grain-boundary recombination for polycrystalline copper indium selenium(2) solar cells

The relatively low efficiency of thin-film polycrystalline solar cells compared to the crystalline cells results in part from grain-boundary recombination. This recombination can enhance the forward current of solar cells and can severely limit the photovoltaic parameters. A model of minority-carrier transport mechanism with grain-boundary recombination has been developed and compared with the non-grain boundary situation for polycrystalline thin-film CuInSe{dollar}sb2{dollar} solar cells. The model is based on the self-consistent determination of barrier height, effective grain-boundary recombination velocity, and recombination rate. To get a physically reasonable effective grain-boundary recombination velocity, the quasi-Fermi level of the electrons must be allowed to vary with distance in the grain-boundary space-charge region. For typical CuInSe{dollar}sb2{dollar} cells, grain-boundary effects are small and can be neglected when grain-boundary trap density is below 5 {dollar}times{dollar} 10{dollar}sp{lcub}11{rcub}{dollar} cm{dollar}sp{lcub}-2{rcub}{dollar}. When trap density is above 10{dollar}sp{lcub}12{rcub}{dollar} cm{dollar}sp{lcub}-2{rcub}{dollar}, however, the grain-boundary recombination is comparable or even larger than the p-n junction space-charge region recombination.; The calculated current-voltage characteristics both with and without grain-boundary recombination are compared with temperature-dependent light and dark experimental results for three CuInSe{dollar}sb2{dollar} cells which were fabricated by different groups using different deposition methods. The results show that the calculations without grain-boundary recombination can adequately fit experimental data for cells with relatively small forward-current density. For cells with larger forward-current density, however, inclusion of grain-boundary effects is necessary to match the experimental results. When light forward-current density is extremely high, the calculations both with and without grain-boundary effects fail to fit the experimental I-V curves. In this situation, the grain-boundary effects on current generation may have to be considered.