Similarities and Critical Differences in Heavy Alkali-Metal Rubidium and Cesium Effects on Chalcopyrite Cu(In,Ga)Se₂ Thin-Film Solar Cells

Both elemental Rb and Cs are known as effective dopants in enhancing Cu­(In,Ga)­Se₂ (CIGS) thin-film photovoltaic device performance. It is, however, found that there are critical differences in the effects of these two alkali metals on CIGS thin-film properties. Namely, CIGS film surfaces show RbInSe₂ compound formation after RbF postdeposition treatment (PDT), whereas no such segregated compound formation is observed with CsF-PDT. On the other hand, both Rb and Cs have the effect of increasing effective hole carrier density, photoluminescence intensity, and carrier lifetime in CIGS similar to the effects of the lighter alkali-metals Na and K. The fundamental mechanism behind metastable acceptor formation, which is known as the light-soaking or bias-soaking effect leading to CIGS photovoltaic performance enhancement, is experimentally revealed to be independent of the presence of alkali metals in CIGS, and alkali-metals still have an effect of facilitating metastable acceptor formation.