Grain boundary compositions in Cu(InGa)Se2

Abstract

A microchemical analysis study is reported comparing the grain and grain boundary chemistries of Cu(In,Ga)Se2 (CIGS) films deposited by three different laboratories by different processes. An analysis of a GaAs wafer is described to provide a calibration of the accuracy and precision of the energy dispersive spectroscopy nanoprobe analysis as applied in the current instruments. When averaged over many measurements the precision of the instrument is ±0.1at.% and the accuracy is ±1.0at.% for individual points. The analysis of the CIGS shows less than 0.5at.% composition difference for each constituent element between grain and grain boundary compositions when averaged over hundreds of data points for most samples. One sample deposited at 400°C as a bilayer shows different grain and grain boundary compositions with grain boundaries being In deficient and Se rich. Both grain and grain boundary data in this sample scatter along a line between the Cu(In,Ga)Se2 and CuSe2 phases, which is a nonequilibrium behavior. The scatter in individual analyses is significantly greater than the noise in the analysis technique indicating real composition differences at different points within both grains and grain boundaries. No evidence was found for other substitutional impurities such as O or Na in the grain boundaries. The results suggest that the grain boundaries are self-passivating without a chemistry change.