Theoretical Study on the Structure and Energetics of Cd Insertion and Cu Depletion of CuIn5Se8

Due to its attractive optical, electrical, and chemical properties, the ternary CuInSe2 (CIS) chalcopyrite-type semiconductor is widely employed as an absorber layer in thin-film photovoltaic devices. Experimental studies have shown that close to the interface of CIS with the CdS buffer layer the former is Cu-depleted, corresponding most probably to an ordered vacancy compound (OVC), and hosts some Cd atoms by diffusion, which originate from the buffer layer. To gain new insights into the insertion of Cd into Cu-depleted CIS phases, we have performed density functional theory calculations, and we have investigated the atomic structure and energetics of neutral Cd impurities in the OVC CuIn5Se8. We found that Cd atoms prefer predominantly to sit on Cu sites or on Cu vacancy sites. Furthermore, our calculations show that the Cu vacancies and Cd atoms have high binding energy at least in some specific configurations. Hence, the insertion of Cd into the CIS materials might enhance the formation of Cu-poor phases.