Bandgap engineering of lead-free double perovskite Cs₂NaBiCl₆ through indium and silver alloying

In recent years, double perovskites (DPs) have received much attention due to their low toxicity and intrinsic thermodynamic stability. However, most pure phase DPs crystals have the problem of weak photoluminescence (PL) intensity. It has been proved that alloying through metal ion is an effective method to improve the PL intensity of DPs. We have successfully prepared a series of Cs2NaBi1-xInxCl6(0, 0.25, 0.5, 0.75, 1) crystals by a solvothermal method. By alloying Cs2NaBiCl6 crystals with In3+ to adjust the band gap, the optical band gap was increased from 3.06 eV (x = 0) to 3.66 eV (x = 1), and the PL emission spectra showed a broad emission peak. After further Ag+ alloying, the PL intensity was further improved, and the photoluminescence quantum yield (PLQY) was increased to 57.63%. It was confirmed by electronic structure calculation under the framework of density functional theory (DFT) that the composition of Bi-6p orbital near the conduction band minimum (CBM) and the bandgap can be efficiently tuned by introduction of Ag+. Both Bi3+ and Ag+ can locate at the edge of the band, effectively enhancing PL intensity through self-trapping excitons (STEs). The above research showed that the alloying of lead-free bismuth sodium halide double perovskites with other metals can provide a direction for non-toxic and high-performance light-emitting devices.