Reduction of point defects and Cu surface composition in Cu(In,Ga)Se2 film by Se annealing with a NaF overlayer at intermediate temperatures

Donor-type point defects such as a Se vacancy or cation antisite in Cu(In,Ga)Se2 (CIGS) films were controlled by Se annealing of CIGS film. The photoluminescence intensities originating from such defects were reduced by Se annealing at 300 °C. The short-circuit current of the CIGS solar cell with the Se annealing increased but the fill factor and open-circuit voltage were degraded due to the out-diffusion of Cu from the bulk to the CIGS surface. With a NaF overlayer on the CIGS film the Cu concentration at the CIGS surface was decreased by Se annealing at 300 °C. The literature has demonstrated that the Cu concentration is reduced by applying both NaF and KF together on the CIGS film. However, we found that the application of a NaF overlayer also greatly reduced the Cu concentration at the CIGS surface. In addition, the Na concentration increased greatly at the CIGS surface, forming a desirable surface layer with a lower valence band maximum. As a result, in addition to the increase of short-circuit current, the fill factor and open-circuit voltage increased significantly. The origin of the improvement in cell performance is described by analyzing the point defects from low-temperature photoluminescence, the valence band maximum from x-ray photoelectron spectroscopy, the reverse saturation current from diode curves, and the carrier lifetimes from time-resolved photoluminescence.