Effect of Cu/(In+Ga) ratios on the secondary phases and the performance of Cu2InGa(S,Se)4 thin film solar cells

Abstract

This study investigated the influence of Cu/(In+Ga) ratios on the formation of secondary phases, leakage current, and their effects on the performance in Cu2InGa(S,Se)4 (CIGSSe) thin film solar cells. An increase in Cu content in the CIGSSe absorber layer was shown to contribute to the growth of grains in that layer as well as the migration of Ga to the surface of the absorber, which increased the energy gap. Excess Cu content resulted in the formation of Cu2−xSe hexagonal nanoplates on the surface of the absorber layer. Conductive atomic force microscopy identified grain boundaries in the CIGSSe absorber layer as the main path involved in the transfer of current through the device. The formation of Cu2−XSe or InSe phases on the surface of the absorber layer expanded the region acting as a current pathway, leading to a breakdown in the P-N junction following the application of negative bias. Measurements of external quantum efficiency under negative bias revealed that secondary phases in the absorber layer act as a recombination center, resulting in a reduction in the Voc and Jsc of the device. CIGSSe thin film solar cells fabricated by sputtering in this study achieved a maximum conversion efficiency of 13.06%.