Increase in conversion efficiency of above 14% in Cu(In,Ga)3Se5 (β-CIGS) solar cells by Na2S incorporation through the surface of β –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 Na2S to β-CIGS film by supplying Na2S to three different stages: on the (In,Ga)2Se3 layer, on the α-CIGS layer, and on the β -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 β–CIGS film. With Na2S 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 Na2S was eliminated. The carrier concentration determined at 100kHz was lowest with Na2S incorporation on the β–CIGS surface, while that determined at 1MHz was similar with various Na2S supply stages. The open-circuit voltage and fill factor greatly increased in the β–CIGS solar cell with the Na2S incorporation. The cell conversion efficiency with Na2S 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.