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)Se-2 (CIGS) films were controlled by Se annealing of CIGS film. The photoluminescence intensities originating from such defects were reduced by Se annealing at 300 degrees 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 degrees 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. (C) 2017 Elsevier B.V. All rights reserved.