Origin of off-stoichiometry and electrical benignity at the grain boundaries in CuInSe2: A first-principles study

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Grain boundaries (GBs) in polycrystalline CuInSe2 are of both fundamental interest and technological significance for photovoltaic applications. However, the atomic composition and the exact roles of the GBs in CuInSe2 are still unclear despite a large off-stoichiometry around the GBs being reported. In this work, based on first-principles calculations and using ∑3(114) GB as an example, we show that the GB acts as a sink of defects, leading to defect segregations and off-stoichiometry. Furthermore, depending on the chemical potential conditions, different point defects dominate the different segregations. Under common experimental conditions with In rich and Cu poor, we find that the most dominant defect at the GBs is the antisite defect InCu. Our studies show that the existence of InCu can eliminate defect states in the bandgap and thus suppress recombination of photo-generated electron–hole pairs, making the GB electrically benign. To enhance the formation of InCu, we propose an optimal region of chemical potential to realize In segregation and Cu depletion at the GB. Our work thus provides useful insights and understandings for further improvement of CISe polycrystalline solar cells.