Electronic, optical and mechanical properties of lead-free halide double perovskites using first-principles density functional theory

First-principles density functional theory calculations were employed to study the structural, electronic, elastic and optical properties of lead free perovskites Cs2AgBiX6 (X = Cl, Br and I) in the cubic phase. Calculations were performed with spin-orbit coupling (SOC) for predicting the accurate band gap and comparison is made with calculations excluding SOC. The calculated band gaps for Cs2AgBiCl6, Cs2AgBiBr6 and Cs2AgBiI6 are 1.91 eV, 1.42 eV and 0.89 eV respectively and are in good agreement with reported theoretical and experimental results. It is evident from the partial density of state diagrams that the anti-bonding Ag/5s and Bi/6p states to the halogen 3p/4p states are involved in the transition from conduction to valence band and this interaction leads to an indirect band gap. Variations in the structural, mechanical, optical and electronic properties of the lead free halide double perovskites Cs2AgBiX6 under various exchange correlations are fully investigated. Our studies provide theoretical support for their potential optoelectronic application in solar energy conversion.