Review of Grain Interior, Grain Boundary, and Interface Effects of K in CIGS Solar Cells: Mechanisms for Performance Enhancement

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 KInSe₂ (or KIn_1−yGa_ySe₂) by co-evaporation or KF post-deposition treatment onto CIGS reduces buffer interface recombination in the final solar cells. KInSe₂ 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.