Exploring the electronic and optical properties of Cu2Sn(1-x)Ge(x)S3 and Cu2Sn(1-x)Si(x)S3 (x = 0, 0.5, and 1)

Abstract

To accelerate environmental friendly thin‐film photovoltaic technologies, earth‐abundant, non‐toxic, and low‐cost materials are demanded. We study the compounds of Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3 (x = 0, 0.5, and 1) employing first‐principles method within the density functional theory. The compounds have comparable band dispersions. The band‐gap energies Eg can be tailored by cation alloying the Sn atoms with Ge or Si. The gap energies of Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3, with x = 0, 0.5, and 1, vary almost linearly from 0.83 to 1.43 eV and 2.60 eV, respectively. However, the gap energy of Cu2SiS3 does not follow the linear relation for x > 0.8. The effective electron masses at the Γ‐point of the lowest conduction band are almost isotropic for all materials, which are between 0.15m0 and 0.25m0. On the other hand, the effective hole masses of the topmost valence band show very strong anisotropy for all compounds. In the (010) direction, the hole masses are estimated to be between 1.01m0 and 1.85m0, while between 0.11m0 and 0.41m0 in the (001) direction. Calculations reveal that all compounds have high absorption coefficients that are comparable with that of Cu2ZnSnS4. The absorptions in the energy region from Eg + 0.5 eV to Eg + 1.0 eV are even higher for Ge‐ and Si‐alloying of Cu2SnS3, compared with Cu2ZnSnS4. The high‐frequency dielectric constants of the compounds are between 6.8 and 8.9. Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3 can be considered as potential candidates for absorber materials in thin‐film solar cells. This research has been accepted and published. © 2017 Wiley