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

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. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4802972]