Influence of cationic substitution on the electronic-energy structure and optical properties of Cu<sub>2</sub>B<sup>IV</sup>S<sub>3</sub>(B<sup>IV</sup>=Si, Ge, Sn) compounds

Abstract

Purpose. Ternary Cu2B IVS3 (BIV = Si, Ge, Sn) compounds are promising materials that can be used as absorbing layers of solar cells. Methods. This paper presents the results of calculations of the band structure, the state density distribution of the electron density and such optical functions as real e1 and imaginary e2 part of the dielectric permittivity, reflectivity R, absorption coefficient α and refractive index n of Cu2B IVS3 compounds. Results. According to the results of the band structure calculations in the LDA+U approximation, the monoclinic phases Cu2SiS3, Cu2GeS3 and Cu2SnS3 are direct-gap semiconductors with the calculated band gaps of 2.46 eV, 1.5 eV and 0.93 eV, respectively, which agrees well with the experimental values obtained from the analysis of the intrinsic absorption edge. Valence band with for monoclinic phases Cu2B IVS3 consists of four subbands separated by forbidding intervals. The analysis of partial contributions into the density of electronic states allowed to identify the genetic origin of different subbands of the valence band, and also to obtain the formation of a chemical bond in the crystals under investigation. For optimized structures of Cu2SiS3, Cu2GeS3 and Cu2SnS3,the spectra of optical functions in the wide range energy of fundamental absorption were calculated for the first time using a unified technique. The spectra of the optical functions of these crystals are characterized by significant polarization anisotropy and are qualitatively similar to each other. The electron density was calculated and maps of the distribution of charge of valency electrons in the planes along the Cu-S and BIV–S bond lines in various structural units of Cu2B IVS3 crystals were constructed, which made it possible to describe the features of the formation of a chemical bond between atoms forming the corresponding crystal. Conclusions. : As a result of quantum-chemical calculations, it was established that the structure of the energy bands of monoclinic Cu2B IVS3 crystals is qualitatively similar to each other, contain the same number of filled bands (44) and have a characteristic topology in which traced four allowed energy region. According to the calculations in the LDA+U approximation, all three crystals are direct-gap semiconductors, the bandgap of which decreases monotonically with increasing atomic number of the cation BIV (Si→Ge→Sn; 2.46 еV → 1.5 еV → 0.93 еV). It has been established that the monoclinic phases Cu2B IVS3 are semiconductors with a complex system of covalent-ionic interatomic bonds