Recombination mechanisms in Cu(In,Ga)(Se,S)2 solar cells

An improved understanding on the recombination mechanisms in Cu(In,Ga) (Se,S)2 solar cells is required to enhance the performance of these devices. This chapter first reviews the experimental work that has been undergone in the past to investigate the recombination mechanisms in Cu(In,Ga)(Se,S)2 of different compositions. Then, we discuss analytical and numerical modeling of ZnO/CdS/Cu(In,Ga)(Se,S)(2) heterojunction solar cells and discuss the differences between the wide-gap devices and their low-gap counterparts. It is shown that a major role in the suppression of interface recombination is played by the Cu-poor surface defect layer that; forms in Cu-poor chalcopyrites and leads to surface band gap widening. However, the beneficial effect of the surface defect layer is reduced when increasing the band gap without decreasing the valence band edge. The improvement due to the surface defect layer can be explained by either n-doping within the layer or by a Fermi-level pinning effect. The n-doping situation is somewhat beneficial in that it allows for higher excess defect concentrations. Based on realistic material properties a maximum efficiency of wide-gap Cu(In,Ga)Se-2 (CIGS) devices of 15.5% is expected due to limitations by interface recombination and the unfavorable band alignment.