Correlative transmission Kikuchi diffraction and atom probe tomography study of Cu(In,Ga)Se-2 grain boundaries

We combined transmission Kikuchi diffraction and atom probe tomography techniques to investigate the relationship between the structure and chemistry of grain boundaries in Cu(In,Ga)Se-2 thin films. Kikuchi patterns with the tetragonal structure of Cu(In,Ga)Se-2 were simulated to emphasize the pseudosymmetry issue in this material system and, hence, the orientation determination ambiguity in case of indexing with a cubic zinc-blende structure. We compared these patterns with experimental data. We detect an elemental redistribution at random high-angle grain boundaries but no chemical fluctuations at Sigma 3 twin boundaries. The atom probe tomography analyses reveal Cu depletion as well as In and Se enrichment at random grain boundaries and, at some random grain boundaries, a slight Ga depletion. This In on Cu scenario is accompanied by cosegregation of Na and K originating from the soda-lime glass substrate. The amount of impurity segregation does vary not only from one grain boundary to another but also along an individual grain boundary. Hence, our results suggest that the degree of passivation of detrimental, nonradiative recombination centers does differ not only between Sigma 3 twin boundaries and random grain boundaries but also within the same random grain boundary.