Quantitative Regulation of Cu _Zn Antisite Defects in Cu ₂ ZnSnS ₄ via Cd ²⁺ Substitution

Cation disorder-induced defects, particularly Cu Zn antisite defects, are widely recognized as a major origin of band tailing and open-circuit voltage deficit in Cu 2 ZnSnS 4 (CZTS) solar cells. Herein, we systematically investigate the effects of Cd 2+ substitution at Zn 2+ sites on the microstructure, defect properties, and photovoltaic performance of CZTS absorbers. Moderate Cd incorporation induces lattice expansion without secondary-phase formation and promotes grain growth, resulting in a compact columnar microstructure. Bandgap narrowing and enhanced long-wavelength photoresponse led to a pronounced increase in short-circuit current density. Defect-sensitive analyses reveal that Cd substitution effectively mitigates energetic disorder. The Urbach energy extracted from external quantum efficiency spectra decreases from 29.2 to 24.4 meV, indicating suppressed band tailing. Capacitance–voltage and drive-level capacitance profiling measurements further demonstrate a substantial reduction in bulk and interface defect densities, accompanied by an expanded depletion width. Temperature-dependent admittance spectroscopy identifies Cu Zn antisite defects as the dominant electrically active states and quantitatively confirms that Cd substitution reduces their concentration from 1.16 × 10 16 to 4.21 × 10 15 cm –3 . Consequently, Fermi-level pinning is alleviated, carrier transport is enhanced, and a maximum efficiency of 8.53% is achieved for the Cd-3 device. This work provides quantitative insight into defect regulation in kesterite absorbers and highlights Cd incorporation as an effective defect engineering strategy.