Quantitative Regulation of Cu_Zn Antisite Defects in Cu₂ZnSnS₄ via Cd²⁺ 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 Cu₂ZnSnS₄ (CZTS) solar cells. Herein, we systematically investigate the effects of Cd²⁺ substitution at Zn²⁺ 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 × 10¹⁶ to 4.21 × 10¹⁵ 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.