Lead-halide perovskites CsPbX3 (X = Cl, Br) have attracted broad interest in photovoltaic applications over the past decade due to their suitable bandgap and high optical absorption ability. But, lead-induced toxicity and materials instability has sparked interest in alternative materials with properties like CsPbX3 perovskites. Recently, lead-free inorganic double halide perovskites Cs2M(I)M(III)X6 have been proposed as promising nontoxic materials with enhanced chemical stability for optoelectronic applications. However, not much has been explored in the context of their mechanical and transport properties, which play a crucial role in defining an efficient and flexible optoelectronic device. We have, therefore, systematically conducted a series of first-principles density functional theory calculations to investigate the mechanical and transport properties of Cs2MSbX6 (M = Ag, Cu, Na, K, Rb, and Cs; X = Cl, Br) along with their structural, electronic, and optical properties. Our results indicate that all the systems maintain the standard cubic double perovskite lattice and show high phase stability against decomposition (decomposition enthalpy &gt; 20 meV/atom). Band structure calculations reflect that the compounds have a wide range of tunable bandgaps (1.6 eV - 4.2 eV), and the bandgap decreases on going from Cl to Br. These materials are also found to have excellent absorption capabilities from visible to ultraviolet light regions with high absorption coefficients (~ 105 cm -1). Investigation of transport properties elucidates that most of the Cs2MSbX6 materials exhibit small charge carrier effective masses (higher mobility) and low exciton binding energy, which make them suitable for a solar cell application, in particular. Further, on calculating the mechanical properties these compounds are found to exhibit low elastic moduli (bulk modulus, shear modulus, and Young’s modulus) that decrease from Cs2MSbCl6 to Cs2MSbBr6. Furthermore, except Cs2CuSbBr6, these compounds are predicted to be ductile in nature with a noticeable elastic anisotropy, revealing these materials to exhibit good flexibility. Overall, our study predicts Cs2MSbX6 to be a stable double perovskite with a wide range of tunable optoelectronic properties, and promising transport and mechanical properties, providing a helpful guide to developing next-generation photovoltaics and flexible optoelectronics.

Theoretical investigation of Cs2MSbX6 (M= Ag, Cu, Na, K, Rb, Cs and X = Cl, Br) double-perovskite materials for optoelectronic applications

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