Study on heterojunction structures for high efficiency solar cells

Abstract

In this thesis, the main inherent reasons behind the low conversion efficiency of p-n junction solar cells have been studied. The p-n junction solar cells are sensitive only to photons with quantum energy higher than the band gap of the material from which they are made. The conversion efficiency of such cells are limited by the fact that the ratio of output work to input photon energy becomes maximum only for the photons having energy close to the band gap of the material. The efficiency of p-n junction solar cells is also reduced due to reduction in junction potential barrier under illuminated condition, which causes an enhanced rate of internal recombination of the photo generated carriers. Prevention of internal recombination of electron hole pairs under illuminated condition seems to be a solution towards the enhancement of the cell efficiency. In this work, the mechanism of electron-hole recombination in illuminated p-n junction solar cells have been studied. It has been shown here that the value of density of states and mobility difference between electrons and holes have significant impact on the mechanism of internal recombination of photogenerated carriers and cell efficiency. Accordingly, some potential heterojunction structures have been proposed that can effectively prevent the high rate of such recombination. Results presented here indicate that significant improvement in cell efficiency can be achieved by using these proposed structure.