Defect Passivation for Kesterite CZTSSe Solar Cells via In Situ Al 2 O 3 Incorporation into the Bulk CZTSSe Absorber

A method for in situ Al 2 O 3 incorporation into a Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorber and its effect on solar cell performance is reported. Al 2 O 3 ‐incorporated CZTSSe films can be prepared by spraying a precursor solution containing Al metal salt dissolved together with Cu, Zn, and Sn metal salts and thiourea. X‐ray photoemission spectroscopy (XPS) and field‐emission transmission electron microscopy (FESEM) reveal the existence of an Al 2 O 3 phase in the CZTSSe film. X‐ray diffraction (XRD) and Raman spectroscopy indicate that Al 2 O 3 incorporation (Al: 0%–2%) does not have a noticeable effect on the crystallization process through postselenization and does not change the lattice parameters of the selenized CZTSSe. However, optimal Al 2 O 3 incorporation is found to reduce the defect level and defect density, as confirmed by various characterization methods, such as 442 nm Raman spectroscopy, photoluminescence, admittance spectroscopy, and temperature‐dependent current density measurements. In addition, Al 2 O 3 incorporation reduces the Urbach energy and back‐contact barrier height. Al 2 O 3 incorporation is found to induce enhancement of Na and O concentrations in the absorber, which facilitates defect passivation effects. Owing to the observed beneficial effects of Al 2 O 3 incorporation, the device performance is significantly enhanced, achieving a maximum power conversion efficiency of 11.76%.