| Summary: | This study evaluated the electronic structure and optical properties of Fe-doped, Ni-doped, and (Fe,Ni)-co-doped Cu<sub>2</sub>ZnSnS<sub>4</sub> through the first-principles pseudopotential plane-wave method based on density functional theory. The results indicated that Fe single-doping and Ni single-doping Cu<sub>2</sub>ZnSnS<sub>4</sub> can reduce the charge transfer number of adjacent S atoms, enhancing covalent bonding in Fe–S and Ni–S bonds and reducing the bond length, lattice constants <i>a</i> and <i>c</i>, and unit cell volume <i>v</i>. The formation energies for Fe-doping, Ni-doping, and (Fe,Ni)-co-doping were 1.0 eV, 0.58 eV, and 0.78 eV, respectively. Both Fe and Ni-doping introduced 3d electrons near the Fermi level, resulting in new impurity levels and a gradual decrease in the band gap of Cu<sub>2</sub>ZnSnS<sub>4</sub> from 0.16 eV. The conduction band density of Cu<sub>2</sub>ZnSnS<sub>4</sub> was primarilycontributed by Sn 5s, Sn 5p, and a portion of S 3p orbital electrons, whereas the valence band density mainly stemmed from Cu 3d, Sn 5p, and S 3p orbital electrons. Fe and Ni-doping also partly contributed to the 3d layer electrons. In the case of (Fe,Ni)-co-doping, the maximum static dielectric constant was 100.49, and the dielectric peak shifted toward the low-energy direction in the presence of both Fe and Ni. Within the visible light range, Fe-doping, Ni-doping, and (Fe,Ni)-co-doping in Cu<sub>2</sub>ZnSnS<sub>4</sub> exhibited absorption coefficients greater than 10<sup>4</sup> cm<sup>−1</sup>, with the maximum absorption coefficient being 1.6 × 10<sup>5</sup> cm<sup>−1</sup> in the case of (Fe,Ni)-co-doping. In the energy range from 1.5 to 6.3 eV, the reflectivity of Cu<sub>2</sub>ZnSnS<sub>4</sub> doped with Fe, Ni, or both was lower than 30%. Notably, a high conductivity peak at 1.9 eV indicated that Cu<sub>2</sub>ZnSnS<sub>4</sub> possesses good photoconductivity in the visible range. Fe-doping and Ni-doping resulted in a slight shift of the conductance peak position towardthe low-energy direction, accompanied by an increase in the peak value.
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