Lead-Free Double Cs 2 Ag(Bi,Sb)(Br,I) 6 Perovskites: Going below 1.8 eV Bandgap by Anion Exchange and Solid-State Reactions

High Resolution Image Download MS PowerPoint Slide Anion exchange of microcrystalline Cs 2 AgBiBr 6 double perovskite (CAB-B) with NaI under ambient conditions yielded mixtures of Cs 3 Bi 2 (Br,I) 9 (CB-(B)I) and CsAg 2 I 3 (CA-I) double salts, which were transformed by annealing at 250–300 °C into a tetragonal Cs 2 AgBi(Br,I) 6 double perovskite (CAB-(B)I) with ca. 80 mol % iodide. The thermally activated solid-state reaction between CB-(B)I and CA-I was confirmed by annealing mechanically mixed CB-(B)I and CA-I, which yielded CAB-(B)I perovskites. Optimization of the anion exchange and the solid-state reaction (AE/T) resulted in phase-pure CAB-(B)I perovskite with ca. 90 mol% iodide and a band gap slightly below 1.9 eV. The general applicability of the proposed approach was demonstrated in a series of AE/T-driven transformations of more complex precursors, including the conversion of Cs 2 AgBi x Sb 1– x Br 6 into tetragonal Cs 2 AgBi x Sb 1– x (Br,I) 6 perovskites with variable x, the highest iodide content of ca. 90%, and the lowest band gap of 1.78 eV observed at a Bi/Sb ratio of 1:1, as well as solid-state reactions between ternary mixtures of CB-(B)I, CA-I, and Cs 3 Bi 2 Br 9 double salts, yielding tetragonal Cs 2 AgBi(Br y I 1– y ) perovskites with linear compositional variations of the lattice parameters and band gaps over a broad range of y = 0.08–0.76. The reported two-stage AE/T approach is highlighted as a general, flexible, and sustainable pathway for the combinatorial synthesis of stable tetragonal double perovskites with variable compositions and levels of complexity.