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