Role of light and heavy alkalis in Cu(In,Ga)Se ₂ absorbers

Abstract In this work, we investigate the distribution of single- and co-doped alkali elements—sodium (Na), potassium (K), and rubidium (Rb)—within the bulk and grain boundaries (GBs) of Cu(In,Ga)Se ₂ (CIGSe) thin-film solar cells, and examine their electronic impact on device efficiency. Atomically resolved analysis using atom probe tomography reveals pronounced compositional fluctuations of both matrix and dopant elements between GBs and the bulk. We propose mechanisms governing the copper and indium (In) distribution by classifying Na as a light alkali element, Rb as a heavy alkali, and K as an intermediate case. Notably, a correlation is observed between alkali concentration and In enrichment at the GBs, which is attributed to donor-like defect state formation. These variations effectively impact the local electronic structure, with Na and K segregation reducing the n-type character of GBs, while Rb accumulation enhances it. Our findings, supported by density functional theory calculations, provide fundamental insights into the role of alkali metal dopants in tuning GB characteristics, offering guidance for optimized alkali insertion in CIGSe solar cells.