Structural, electronic, and optical properties of Cu${}_2$XSnS${}_4$ (X = Be, Mg, Ca, Cd, Hg) semiconductors. First-principles calculations

The structural, electronic, and optical properties of Cu${}_2$XSnS${}_4$ (X = Be, Mg, Ca, Cd, Hg) were investigated through first-principles calculations based on density-functional theory combined with the Hubbard U parameter. Cu${}_2$XSnS${}_4$ is a class of p-type direct band-gap semiconductors with band gaps ranging from 1.05 to 2.11 eV, isotropic effective electron masses, and anisotropic effective hole masses. Notably, Cu${}_2$HgSnS${}_4$ exhibits the average electron and hole effective masses of 0.096 and 0.216 m${}_0$, respectively, which are lower than those of Cu${}_2$ZnSnS${}_4$. According to the absorption spectra, the absorption coefficient of Cu${}_2$XSnS${}_4$ exceeds 10${}^4$ cm${}^{-1}$, with Cu${}_2$HgSnS${}_4$ exhibiting a higher average absorption coefficient compared to Cu${}_2$ZnSnS${}_4$. These findings indicate that Cu${}_2$HgSnS${}_4$ is the promising material for solar cell applications.