Tuning to the band gap by complex defects engineering: Insights from hybrid functional calculations in CuInS₂

Tuning band gaps of semiconductors in terms of defect control is essential for the optical and electronic properties of photon emission or photon harvesting devices. By using first-principles calculations, we study the stability condition of bulk CuInS₂ and formation energies of point and complex defects in CuInS₂ with hybrid exchange-correlation functionals. We find that at Cu-rich and In-poor conditions, 2Cu_i + Cu_In is the main complex defect, while In_Cu + 2V_Cu is the main complex defect at In-rich and Cu-poor conditions. Such stable complex defects provide the feasibility of tuning band gaps by varying the [Cu]/[In] molar ratios. These results present how the off-stoichiometry CuInS₂ crystal structures, and electronic and optical properties can be optimized by tuning the [Cu]/[In] ratio and Fermi level, and highlight the importance of complex defects in achieving better photoelectric performance in CuInS₂. Such band gap tuning in terms of complex defect engineering is a general approach and thus applicable to other photo-harvest or light-emission semiconductors.