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

TLDR

Sodium incorporation in CZTSe thin‑film cells raises hole density, lowers the Fermi level, and reduces deep recombination centers, thereby increasing built‑in voltage, VOC, and conductivity (improving fill factor) while shrinking depletion width and slightly reducing short‑circuit current. Sodium incorporation boosts device efficiency by raising VOC and fill factor, though it slightly shortens minority‑carrier lifetime on Mo‑coated soda‑lime glass, an effect partially mitigated by adding NaF.

Abstract

Sodium incorporation into Cu2ZnSnSe4 (CZTSe) substantially improves the device efficiency by enhancing the open-circuit voltage (VOC) 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 VOC. Sodium reduces the concentration of certain deep recombination centers, which further benefits VOC. 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.