Effects of sodium incorporation in Co-evaporated Cu2ZnSnSe4 thin-film solar cells

Abstract

Sodium incorporation into Cu₂ZnSnSe₄ (CZTSe) substantially improves the device efficiency by enhancing the open-circuit voltage (V_OC) and fill factor. Sodium increases hole density, makes the acceptor shallower, shifts the Fermi level lower, and leads to higher built-in voltage and, consequently, higher V_OC. Sodium reduces the concentration of certain deep recombination centers, which further benefits V_OC. The increase of hole density and mobility enhances the CZTSe conductivity leading to higher fill factor. Sodium causes smaller depletion width, hence, lower short-circuit current. The minority-carrier lifetime decreases slightly after sodium is incorporated via the Mo-coated soda-lime glass, although adding NaF provides some amelioration.