Theoretical Insights into Cs₂MSbX₆ (M = Cu, Ag, Na, K, Rb, Cs and X = Cl, Br) Halide Double-Perovskite Materials for Optoelectronic Applications

Lead-halide perovskites APbX₃ (A = Cs, CH₃NH₃ and X = Cl, Br) have attracted broad interest in photovoltaic applications over the past decade due to their suitable bandgap and high optical absorption ability [1]. But lead-induced toxicity and materials instability has sparked interest in alternative materials with properties like APbX₃ perovskites. Recently, lead-free inorganic double halide perovskites Cs₂M(I)M(III)X₆have been proposed as promising nontoxic materials with enhanced chemical stability for optoelectronic applications [2]. However, not much has been explored in the context of their mechanical, transport, and excitonic properties, which play a crucial role in defining an efficient and flexible optoelectronic device. We have, therefore, systematically conducted a series of state-of-the-art first-principles density functional theory, hybrid density functional theory, and many-body perturbation theory (namely GW and BSE) calculations to investigate the mechanical, transport, and excitonic properties of Cs₂MSbX₆ (M = Cu, Ag, 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 lattice and show high phase stability against decomposition (decomposition enthalpy &gt; 20 meV/atom). Band structure calculations (HSE06 and G₀W₀@PBE) reflect that the compounds have a wide range of tunable bandgaps (1.2 eV - 5.2 eV), and the bandgap decreases on going from Cl to Br. BSE calculations reveal that these materials have excellent absorption capabilities from visible to ultraviolet light region. Investigation of transport properties and excitonic parameters elucidates that most of the Cs₂MSbX₆materials exhibit small charge carrier effective masses (higher mobility) and low to moderate exciton binding energy as well as longer to shorter exciton lifetime, which make them suitable for a solar cell application. Further, on calculating the mechanical properties these compounds are found to exhibit low elastic moduli 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. Further, on considering the mixed halide double perovskites Cs₂MSbBr_6-xCl_x(x = 0-6, M=Ag) it is found that the optoelectronic properties can be widely tuned, and thus, such materials may a suitable component for the future generation of optoelectronic devices. Overall, our study predicts Cs₂MSbX₆ to be a stable double perovskite with a wide range of tunable optoelectronic properties, and promising transport and excitonic properties, providing a helpful guide to developing next-generation photovoltaics and flexible optoelectronics.<br/> <br/><b>References:</b><br/> <br/><i>1. Linear Relationship between the Dielectric Constant and Band Gap in Low-Dimensional Mixed-Halide Perovskites, Yujing Dong, Rui Zhu*, and Yu Jia* J. Phys. Chem. C 125, 14883- 14890 (2021). </i><br/> <br/><i>2. Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells? Christopher N. Savory, Aron Walsh, and David O. Scanlon* ACS Energy Lett. 1, 949- 955 (2016).</i>