Exploring the electronic and optical properties of Cu 2 Sn 1-xGexS 3 and Cu 2 Sn 1-xSixS 3 ( x = 0 , 0 . 5 , and 1 )

In order to accelerate for environmental friendly thin-film photovoltaic industry, earth-abundant, non-toxic, and low-cost absorber 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. Comparable band dispersions for all compounds are found. Moreover, the band-gap energies Eg of those materials 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 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 of lowest conduction band at the Γ-point are relatively isotropic for all materials, which are between 0.15 and 0.25 m0 in (010) direction, and between 0.13 and 0.22 m0 in (001) direction. On the other hand, the effective hole masses of topmost valence band at the Γ-point show very strong anisotropy for all compounds. In the (010) direction, the hole masses are estimated to be between 1.01 and 1.85 m0, while between 0.11 and 0.41 m0 in the (001) direction. Calculations reveal that all compounds have relatively high absorption coefficients, comparable with that of Cu2ZnSnS4. However, the absorption coefficients in the energy region Eg + 0.5 to Eg + 1 eV are higher for Cu2GeS3, Cu2SiS3, and Cu2Sn0.5Si0.5S3 compared with Cu2ZnSnS4. Here, a dense k-mesh is required in order to observe the details of absorption spectra, especially near band-gap region. The high-frequency dielectric constants of all compounds are between 6.2 and 7.5, which are similar to the one of Cu2ZnSnS4 (~6.7). Therefore, Cu2Sn1-xGexS3 and Cu2Sn1-xSixS3 can be potential candidates as absorber materials in thin-film solar cells.