Anomalous Grain Boundary Physics in PolycrystallineCuInSe2: The Existence of a Hole Barrier

First-principles modeling of grain boundaries (GB) in ${\mathrm{C}\mathrm{u}\mathrm{I}\mathrm{n}\mathrm{S}\mathrm{e}}_2$ semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB. Consequently, the absence of holes inside the GB prevents GB electrons from recombining. At the same time, the GI is purer in polymaterials than in single crystals, since impurities segregated to the GBs. This explains the puzzle of the superiority of polycrystalline ${\mathrm{C}\mathrm{u}\mathrm{I}\mathrm{n}\mathrm{S}\mathrm{e}}_2$ solar cells over their crystalline counterpart. We identify a simple and universal mechanism for the barrier, arising from reduced $p\mathrm{\text{−}}d$ repulsion due to Cu-vacancy surface reconstruction. This discovery opens up possibilities for the future design of superior polycrystalline devices.