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Kesterite Cu 2 ZnSnSe 4 (CZTSe), Cu 2 ZnSn(S,Se) 4 (CZTSSe), and Cu 2 ZnSnS 4 (CZTS) solar cells show considerably lower open-circuit voltages than their theoretical values. The large opencircuit 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 ocdef 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 (Ag x Cu 1−x ) 2 ZnSn(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.

Due to their inherent nature, kesterite solar cells exhibit complex secondary phases and intrinsic lattice defects. As a result, the power conversion efficiencies (PCEs) of kesterites are considerably lower than the theoretical limit. One of the main culprits of limitation is the open-circuit voltage deficiency (V oc def) due to CuZn antisite-related deep defects that originate from the similar ionic radii of constituent elements of Cu and Zn. Partial replacement of Zn with Cd is known to mitigate the defect states from CuZn antisite-related defects. In this regard, we studied the defect physics of Cd-alloying effects on CZTSSe solar cells by varying the Cd/(Cd + Zn) ratio from 0.04 to 0.3. We further investigated Cd-alloyed CZTSSe (CZCTSSe) devices using temperature-dependent admittance spectroscopy, current–voltage characteristics, and time-resolved photoluminescence measurements. Spectroscopic characterization and analysis revealed that the trap energy level, Urbach energy, and band tailing decreased with low Cd alloying (Cd/(Cd + Zn) < 0.08) and increased again with further Cd alloying (Cd/(Cd + Zn) > 0.08) in CZCTSSe absorbers. Experimental results indicate that the alloying amount of Cd has a huge impact on the defect states of absorber films, which dominates the PCE of CZCTSSe solar cells. With fine tuning of the Cd/(Cd + Zn) ratio = 0.06, a CZCTSSe device efficiency of 11.73% was achieved without an antireflection coating using aqueous spray pyrolysis.

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