First Principle Study of Cesium-based Lead-free Halide Double Perovskites

Inorganic halide double perovskites A(2)B'B"X-6 have gained significant interests for their diverse composition, stable physicochemical properties, and potential for photoelectric applications. The influences of trivalent and monovalent cations on the formation energy, decomposition energy, electronic structure and optical properties of cesium-based lead-free Cs2B'B"Br-6 (B' = Na+, In+ Cu+, or Ag+; B"= Bi3+, Sb3+, In3+) are systematically studied. In view of the analysis and results of the selected double perovskites, for the double perovskites with different B- site trivalent cation, the band gap increases in the order of Cs2AgInBr6, Cs2AgSbBr6 and Cs2AgBiBr6, with Cs2AgBiBr6 possessing the highest thermodynamic stability. Therefore, the Bi-based perovskites are further studied to elucidate the effect of monovalent cation on their stability and electronics. Results show that the thermodynamic stability rises in the sequence of Cs2NaBiBr6, Cs2InBiBr6, Cs2AgBiBr6 and Cs2CuBiBr6. Notably, Cs2CuBiBr6 exhibits a relatively narrow and appropriate band gap of 1.463 4 eV, together with the highest absorption coefficient than other compounds, suggesting that Cs2CuBiBr6 is a promising light absorbing material that can be further explored experimentally and be applied to optoelectronic devices. Our research offers theoretical backing for the potential optoelectronic application of cesium-based lead-free halide double perovskites in solar energy conversion.