Halide Mixing and Phase Segregation in Cs₂AgBiX₆ (X = Cl, Br, and I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy

Abstract

All-inorganic double perovskites (elpasolites) are a promising potential alternatives to lead halide perovskites in optoelectronic applications. Although halide mixing is a well-established strategy for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single- and mixed-halide Cs 2 AgBiX 6 (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using 133 Cs solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible within a narrow range of halide ratios (<3 mol % I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol %. Our study shows that solid-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure–activity relationships, halide mixing, and phase segregation phenomena in Cs 2 AgBiX 6 (X = Cl, Br, and I) double perovskites.

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping

“…It has previously been found that the (practical) detection limit for iodide substituting bromide in a ternary metal compound by disorder-induced broadening of 133 Cs NMR signals is around 2%. 58 A similar limit may pertain here for substitutional Sn detection, although two factors suggest that 133 Cs NMR could be less sensitive to doping in our case. Firstly, since the average Sn ionic radius is much more similar to that of Bi( iii ) than the iodide radius is to the bromide radius, it is reasonable that Sn substitution would lead to less structural distortion.…”

Influence of hidden halogen mobility on local structure of CsSn(Cl1−xBrx)3 mixed-halide perovskites by solid-state NMR

“… 54,55 A similar nonlinear change in 133 Cs NMR chemical shifts was recently reported for mixed-halide double perovskites Cs 2 AgBi(Cl 1− x Br x ) 6 due to a non-additive effect of nearest and next-nearest neighbours on the chemical shift for 133 Cs nuclei and the associated extended halide environment. 56 …”

Lead‐Free Double Perovskite Cs2AgBiBr6: Fundamentals, Applications, and Perspectives

“…[103] Interestingly, the mixing of chloride and bromide does not change the PL emission peak significantly, possibly because of the large electron-phonon coupling and broad STE emission which is not substantially dependent on halide composition. [104,105] Unfortunately iodide can only be incorporated into bromide or chloride lattice at low concentration (<3 mol%) because higher alloying level results in non-perovskite phases. [104] Additionally, mixing sulphide and bromide (Cs 2 AgBiBr 5.8 S 0.1 ) is demonstrated to slightly contract the bandgap by shifting up the valence band edge.…”

“…[104,105] Unfortunately iodide can only be incorporated into bromide or chloride lattice at low concentration (<3 mol%) because higher alloying level results in non-perovskite phases. [104] Additionally, mixing sulphide and bromide (Cs 2 AgBiBr 5.8 S 0.1 ) is demonstrated to slightly contract the bandgap by shifting up the valence band edge. [106] Recently, we have reported an effective way to enhance the light harvesting of Cs 2 AgBiBr 6 by using Cu doping.…”