Defects and interfaces in Cu(In,Ga)Se2–based thin-film solar cells with and without Na diffusion barrier

Abstract

AbstractTo characterize the influence of sodium diffusion on the Cu(In,Ga)Se2 (CIGS) layer, samples with and without an A12O3 diffusion barrier were investigated by transmission electron microscopy. The soda lime glass acts as a sodium source for samples without diffusion barrier while a post deposition treatment with NaF adds sodium to the CIGS layer for samples with diffusion barrier. For low deposition temperatures in the three-stage growth process for CIGS slightly columnar CIGS grains near the Mo back contact are found and larger grains near the top surface. Samples subject to high-temperature post deposition treatments show slightly larger grain sizes in the CIGS layer than samples prepared at the same temperature but with sodium from the soda lime glass. The surface smoothness of the CIGS layer and therefore the continuity of the following layers is improved for samples with higher deposition temperature. Occasional defects are found on the surface of the ZnO layer which can locally alter reflectivity and absorption properties of these solar cells. Despite the high CIGS surface roughness, the CdS layer with an average thickness of only 50 nm appears continuous and even fills trenches between CIGS grains near the interface.