Lead-Free Alloyed Double-Perovskite Nanocrystals of Cs2(NaxAg1–x)BiBr6 with Tunable Band Gap

Lead-free halide double perovskites (DPs) have emerged as a stable and greener alternative to very toxic lead-based perovskites owing to their outstanding photophysical properties. Here, a series of Cs2(NaxAg1–x)BiBr6 (x = 0, 0.25, 0.5, 0.75, and 1) double-perovskite nanocrystals (NCs) are synthesized, which have not been much explored yet. The formation of highly miscible Cs2(NaxAg1–x)BiBr6 solid solutions is favored because of very low lattice mismatch between Cs2NaBiBr6 and Cs2AgBiBr6. These DP NCs have tunable band gaps, which successively increase with increasing Na/Ag ratio, enabling us to tune their band gap by more than 0.5 eV. The band gap tuning is further confirmed by electronic structure calculations using density functional theory (DFT), which is in line with the band gap estimated from the absorption data. The Cs2(NaxAg1–x)BiBr6 (x = 0, 0.25, 0.5) DP NCs are found to be more stable in the ambient conditions than the Cs2(NaxAg1–x)BiBr6 (x = 0.75, 1.00) DP NCs.