Insights into High-Efficiency Ag-Alloyed CZTSSe Solar Cells Fabricated through Aqueous Spray Deposition

Kesterite Cu₂ZnSnSe₄ (CZTSe), Cu₂ZnSn(S,Se)₄ (CZTSSe), and Cu₂ZnSnS₄ (CZTS) solar cells show considerably lower open-circuit voltages than their theoretical values. The large open-circuit voltage deficiency (V_oc^def) hinders the improvement of the power conversion efficiency (PCE) and the development of the pathway to mass production of kesterite solar cells. The main reason behind the V_oc^def is considered to be the low formation energy of Cu/Zn disorders and their highly distributed defect complexes. To diminish the Cu/Zn disorder, we substituted Ag with a relatively large atomic radius into the host CZTSSe as (AgCu_1-)₂ZnSn(S,Se)₄ (ACZTSSe) and investigated its beneficial effect in a systematic way. The ACZTSSe absorbers were all fabricated using aqueous spray pyrolysis in ambient air. The device performance was found to increase up to the optimum Ag substitution and decrease after the optimum Ag substitution. Admittance spectroscopy revealed that the optimal substitution of Ag reduced the Cu-/Zn-related defects, that is, charge recombination centers, which further mitigates the band tailing issue and enhances the PCE of the solar cell, and higher Ag substitution induced the generation of deeper defects, which decreases the PCE back. At the optimum Ag content of Ag/(Ag + Cu) = ∼9%, the ACZTSSe solar cell with the highest PCE of 11.83% was obtained, where both the interface recombination and bulk recombination were found to be minimized.