Band-Gap Engineering of Lead-Free Iron-Based Halide Double-Perovskite Single Crystals and Nanocrystals by an Alloying or Doping Strategy

Lead-free halide double perovskites (DPs) have been considered to be an effective alternative for solving the problem of lead perovskite toxicity and instability. We report a series of lead-free halide DP single crystals (SCs) and nanocrystals (NCs) (Cs₂Ag_xNa_1–xFeCl₆ (0 ≤ x ≤ 1)). The alloying strategy not only effectively regulates the optical band gap but also significantly improves the air stability and thermal stability. The absorption edge of the NCs is slightly blue-shifted compared to that of the corresponding SCs. Excitingly, we find that Fe³⁺ ions can be successfully doped into chloride DPs Cs₂M′MCl₆ (M′ = Na or Ag; M = In, Sb or Bi) and cause the band gap to effectively decrease with universality. In addition, the effect of crystal size on stability is further studied. This work not only explore a series of lead-free transition-metal DPs but also brings about more possibilities for optoelectronic applications in lead-free perovskites.