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 Cs2AgBiX6 (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using 133Cs 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 Cs2AgBiX6 (X = Cl, Br, and I) double perovskites.
Highlights
Since the first report of a lead halide perovskite solar cell (PSC) by Kojima et al in 2009,1 the field has quickly developed, leading to efficiencies above 25%.2,3 Halide perovskites can be represented as ABX3, where A is a small cation such as methylammonium (CH3NH3+, MA), dimethylammonium ((CH3)2NH+, DMA), formamidinium c(aoCs nHPsbi3s(2t+Ns, oHwf2h[)iB2le+X, 6X]F4Ai−s)oa,cthoaarhleiddceer:asi,Iuw−m,h.BerrTe−,hBeoirs inorganic a divalent Cl−
We have previously developed mechanosynthesis of hybrid[68−70] and all-inorganic[32] halide perovskites and have shown using solid-state magic angle spinning (MAS) NMR and X-ray diffraction (XRD) that it leads to materials indistinguishable from those prepared using solution-processed thin films.[32,34]
We prepared a series of single- and mixed-halide Cs2AgBiX6 (X = Cl, Br, and I) double perovskite materials using mechanosynthesis, in analogy to the protocols previously developed for lead halide perovskites.[67,72]
Summary
Since the first report of a lead halide perovskite solar cell (PSC) by Kojima et al in 2009,1 the field has quickly developed, leading to efficiencies above 25%.2,3 Halide perovskites can be represented as ABX3, where A is a small cation such as methylammonium (CH3NH3+, MA), dimethylammonium ((CH3)2NH+, DMA), formamidinium c(aoCs nHPsbi3s(2t+Ns, oHwf2h[)iB2le+X, 6X]F4Ai−s)oa,cthoaarhleiddceer:asi,Iuw−m,h.BerrTe−,hBeoirs inorganic a divalent Cl−. This result is consistent with previous reports which identified that the double perovskite phase of Cs2AgBiI6 does not form,[77] the synthesis of its nanocrystalline form has been reported.[78] We note that the spectrum of Cs3Bi2I9 has two peaks that correspond to two inequivalent cesium sites in the asymmetric unit cell, consistent with a previous report.[79] The XRD data corroborate these assignments and display the same phase segregation phenomena, their diffraction fingerprint is considerably more complex (Figure S2) Taken together, these results show that small amounts of iodide can replace Br in the Cs2AgBiBr6 structure, but phase segregation occurs for I/Br ratios higher than 0.20:5.80 (3 mol % I). While Cs2AgBiBr6 still exhibits an appreciable signal up to around 2 μs, the iodidedoped sample, Cs2AgBiBr5.90I0.10, decays to negligible values within less than 0.2 μs (Figure 7b)
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