Using density functional theory, we calculated the electronic structure, the lattice dynamics, and the Raman spectra of Cu 2ZnSn(S 1-xSe x) 4, (CZTSSe) an emerging photovoltaic material for thin-film solar cells. In particular, we investigated the effects of the local arrangement of S and Se within the unit cell on the electronic properties of these materials. We find that the S-to-Se ratio (e.g., x) and the spatial distribution of the anions in the unit cell can significantly alter the band structure. In particular, the S-to-Se ratio and anion distribution determine the energy splitting between the electronic states at the top of the valence band and the hole mobility in CZTSSe alloys and solar cells. Moreover, we find that x-ray diffraction patterns and phonon dispersion curves are sensitive to the local anion ordering. The predicted Raman scattering frequencies and their variation with x agree with experimentally determined values and trends.
Bibliographical noteFunding Information:
This work was supported primarily by the MRSEC Program of the US National Science Foundation (US-NSF) under Award Number DMR-0819885. A.K. gratefully acknowledges financial support from a doctoral dissertation fellowship from the University of Minnesota (UMN). M.C. acknowledges partial support from the NSF Grant No. EAR-0810272. The authors acknowledge Andrea Floris, Paolo Umari, and Luigi Giacomazzi for useful discussions and advice on the computations. The authors are also grateful to the Minnesota Supercomputing Institute for providing the computational resources and technical support for this study.