Effects of Fe and Ni Doping on the Electronic Structure and Optical Properties of Cu2ZnSnS4

This study evaluated the electronic structure and optical properties of Fe-doped, Ni-doped, and (Fe,Ni)-co-doped Cu2ZnSnS4 through the first-principles pseudopotential plane-wave method based on density functional theory. The results indicated that Fe single-doping and Ni single-doping Cu2ZnSnS4 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 a and c, and unit cell volume v. 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 Cu2ZnSnS4 from 0.16 eV. The conduction band density of Cu2ZnSnS4 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 Cu2ZnSnS4 exhibited absorption coefficients greater than 104 cm−1, with the maximum absorption coefficient being 1.6 × 105 cm−1 in the case of (Fe,Ni)-co-doping. In the energy range from 1.5 to 6.3 eV, the reflectivity of Cu2ZnSnS4 doped with Fe, Ni, or both was lower than 30%. Notably, a high conductivity peak at 1.9 eV indicated that Cu2ZnSnS4 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.