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%.