Grain Boundary Evaluation of Cu(In1-xGax)Se2 Solar Cells

Abstract

The grain boundary (GB) properties of polycrystalline Cu(In_1-x ,Ga _x )Se₂ (CIGS) have been characterized using electron beam-induced current (EBIC) measurements, electron backscatter diffraction (EBSD) patterns, and scanning spreading resistance microscopy (SSRM) measurements. The polished cross section of CIGS solar cells was evaluated by these three methods, and the surface EBIC image was obtained by scanning electron microscopy (SEM). A combination of the EBIC and EBSD techniques makes it possible to investigate the effect of the GBs on the minority carrier collection. Furthermore, the SSRM mapping enables the analysis of grain-by-grain carrier profiling by measuring the spreading resistance of CIGS solar cells. It was found from these results that the twin boundaries of CIGS grains do not contribute to carrier recombination. Furthermore, the brighter EBIC signals were observed at the GBs of CIGS, which showed that the produced electron–hole pairs easily separate from each other and that the minority carriers are repelled from the GBs. This remarkable property of the GBs is suitable for application of CIGS to solar cells.