Origin of Reduced Efficiency in Cu(In,Ga)Se$_2$ Solar Cells With High Ga Concentration: Alloy Solubility Versus Intrinsic Defects

It is well known that adding Ga to CuInSe ₂ forming CuIn _1-x Ga _x Se ₂ (CIGS) alloy can significantly improve the solar cell efficiency, but adding too much Ga will lead to a decline of the solar cell efficiency. The exact origin of this puzzling phenomenon is currently still under debate. It is especially unclear whether it is caused by either structural or electronic issues. In this paper, we conclude that the defect issue, especially antisite defects M _Cu (M = In, Ga), rather than the alloy solubility is the key problem for the reduced efficiency in CIGS. The deep levels that are induced by M _Cu defects can pin the open-circuit voltage (V _oc ) of CIGS. Self-compensation in CIGS, which forms 2V _Cu + M Cu defect complexes, is found to be beneficial to quenching the deep-trap levels induced by M _Cu in CIGS. Unfortunately, the density of isolated M _Cu is quite high and cannot be largely converted into 2V _Cu + M _Cu complexes under thermal equilibrium condition. Thus, nonequilibrium growth conditions or low growth temperature that can suppress the formation of the deep-trap centers M _Cu will be necessary to improve the efficiency of CIGS solar cells, especially with high Ga concentrations.