Ab initio study of M2SnBr6 (M = K, Rb, Cs): Electronic and optical properties

In this work, the ground-state properties of the solution processable semiconductor M2 SnBr6 (M = K, Rb, Cs) have been computed using density functional theory. Similarities in the band structures are observed among these three materials and are shown to result from minimal contributions of the cation to electronic states near the Fermi level. A fundamental bandgap of 1.2 eV is predicted for the materials, which is close to the ideal bandgap for single-junction photovoltaic applications. However, in reality, a larger bandgap is expected because DFT calculations with the PBE functional underestimate the gap. Material optical properties including dielectric constants, reflective indices, reflectance and absorption coefficients are shown to be competitive for solar-energy harvesting. The ionization energies are 6 eV below the vacuum level, while effective masses are relatively small around 0.3, with light hole masses comparable to those of electrons.