Analysis of defects in coevaporated high-efficiency Cu(In,Ga)Se2 solar cells

Several Cu(In,Ga)Se₂ solar cells with varying Cu/In+Ga ratio were made by co-evaporation using a three-stage process. Device characterization showed an increased open-circuit voltage for cells closer to stoichiometry, however with a decreased fill factor. This phenomenon was investigated by defect spectroscopy by applying photoluminescence and capacitance profiling methods. We find that the defect and doping densities are highly non-uniform for these solar cells, with higher doping levels for cells with higher Cu/In+Ga content. The luminescence efficiency at room temperature is almost constant for the different cells, indicating that the bulk properties are comparable while the enhanced open-circuit voltage is related to a higher doping level close to the heterointerface