Theoretical insights into monovalent-metal-cation transmutation effects on lead-free halide double perovskites for optoelectronic applications

Alternative halide perovskites with characteristics similar to $\mathrm{CsPb}{X}_{3}$ perovskites have sparked interest over the past decade due to lead-induced toxicity and material instability. Here we utilize the technique of cation transmutation to create inorganic Pb-free halide double perovskites ${\mathrm{Cs}}_{2}M\mathrm{Sb}{X}_{6}$ ($M$ = Cu, Ag, Na, K, Rb, Cs, and $X=\mathrm{Cl}$, Br) and comprehensively study them from the perspective of their potential use in optoelectronic devices. Our first-principles density-functional-theory--based calculations reveal that all of these materials exhibit a cubic lattice with excellent phase stability against decomposition. Further, these systems are found to be mechanically stable and ductile in nature, with some amount of elastic anisotropy, which makes them an excellent choice for flexible materials. These materials are also found to possess interesting electronic and optical properties. While upon having $M$ = Cu, Ag, Na in ${\mathrm{Cs}}_{2}M\mathrm{Sb}{X}_{6}$, the system possesses an indirect band gap with a value ranging between $\ensuremath{\sim}1.19\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}4.82$ eV, a transition to direct band gap with values between $\ensuremath{\sim}4.50\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}5.22$ eV occurs on having heavier metals with $M$ = K, Rb, Cs. In accordance with the band gap, most of the systems are also found to exhibit excellent absorption capabilities from visible to near-ultraviolet light. Furthermore, investigations of transport and excitonic properties indicate low effective mass, excellent charge-carrier mobility ($\ensuremath{\sim}10--{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}{\phantom{\rule{0.16em}{0ex}}\mathrm{V}}^{\ensuremath{-}1}{\phantom{\rule{0.16em}{0ex}}\mathrm{s}}^{\ensuremath{-}1}$), low to moderate exciton binding energy, and longer to shorter exciton lifetimes for most of the examined materials, suggesting higher quantum yield and conversion efficiency of the examined materials. Overall, our study suggests that with atomic transmutation in ${\mathrm{Cs}}_{2}M\mathrm{Sb}{X}_{6}$, stable and flexible lead-free halide double perovskites with superior and tunable optoelectronic properties can be achieved, which makes these materials excellent candidates for flexible optoelectronic devices, in general, and photovoltaics in particular.