Review of grain interior, grain boundary, and interface effects of K in CIGS solar cells: Mechanisms for performance enhancement

Abstract Introducing K into Cu(In,Ga)(Se,S) 2 (CIGS) absorbers has led to recent world record power conversion efficiencies for thin film polycrystalline solar cells. In this work, the diverse phenomena associated with K in CIGS were reviewed, and overarching mechanisms were identified. The effects of K depend on its distribution among grain interiors (GIs), grain boundaries (GBs), and interfaces. High substrate Na and low temperature favor GI K incorporation, while low Na and high temperature favor segregation of K at GBs. Depositing KInSe 2 (or KIn 1−y Ga y Se 2 ) by co-evaporation or KF post-deposition treatment onto CIGS reduces buffer interface recombination in the final solar cells. KInSe 2 decomposes in air, which makes characterization difficult and may affect performance. The mechanism for reduced interface recombination could be direct passivation, beneficial compound precursor, oxidation barrier, or favorable diffusion alteration.