Compositionally induced valence-band offset at the grain boundary of polycrystalline chalcopyrites creates a hole barrier

First-principles calculations of model grain boundaries (GBs) in CuInSe2 and CaGaSe2 show that cation-terminated GBs have a valence-band offset with respect to the grain interior (GI). This offset repels holes from the GBs, thus depriving electrons there from recombination at the GB defects. Anion-terminated GBs have no such valence offset. CuGaSe2 has, in addition, a conduction-band offset at the GB/GI interface, attracting electrons to the GBs. These features explain how polycrystalline chalcopyrite solar cells could outperform their crystalline counterparts.