Increase in conversion efficiency of above 14% in Cu(In,Ga)(3)Se-5 (beta-CIGS) solar cells by Na2S incorporation through the surface of beta -CIGS film

Abstract

Little work has been reported on the performance improvement of the beta-CIGS solar cell itself even though the beta-CIGS phase can have an ideal band gap for high-conversion efficiency solar cells. We incorporated Na2S to beta-CIGS film by supplying Na2S to three different stages: on the (In,Ga)(2)Se-3 layer, on the alpha-CIGS layer, and on the beta-CIGS layer in the three-stage co-evaporation process. The purpose of Na2S incorporation was to control the carrier concentration and passivate grain boundaries in beta-CIGS film. With Na2S incorporation on the beta-CIGS surface, both the Cu and Se concentrations at the beta-CIGS surface were greatly reduced and the Na-depleted subsurface area that existed in the referenced beta-CIGS film without Na2S was eliminated. The carrier concentration determined at 100 kHz was lowest with Na2S incorporation on the beta-CIGS surface, while that determined at 1 MHz was similar with various Na2S supply stages. The open-circuit voltage and fill factor greatly increased in the beta-CIGS solar cell with the Na2S incorporation. The cell conversion efficiency with Na2S incorporation on the beta-CIGS layer improved from 10.3% to 14.2% without AR coating, which is a record efficiency in beta-CIGS solar cells at this time.