Axial–Equatorial Halide Ordering in Layered Hybrid Perovskites from Isotropic–Anisotropic 207Pb NMR

Abstract

AbstractBandgap‐tuneable mixed‐halide 3D perovskites are of interest for multi‐junction solar cells, but suffer from photoinduced spatial halide segregation. Mixed‐halide 2D perovskites are more resistant to halide segregation and are promising coatings for 3D perovskite solar cells. The properties of mixed‐halide compositions depend on the local halide distribution, which is challenging to study at the level of single octahedra. In particular, it has been suggested that there is a preference for occupation of the distinct axial and equatorial halide sites in mixed‐halide 2D perovskites. 207Pb NMR can be used to probe the atomic‐scale structure of lead‐halide materials, but although the isotropic 207Pb shift is sensitive to halide stoichiometry, it cannot distinguish configurational isomers. Here, we use 2D isotropic–anisotropic correlation 207Pb NMR and relativistic DFT calculations to distinguish the [PbX6] configurations in mixed iodide–bromide 3D FAPb(Br1−xIx)3 perovskites and 2D BA2Pb(Br1−xIx)4 perovskites based on formamidinium (FA+) and butylammonium (BA+), respectively. We find that iodide preferentially occupies the axial site in BA‐based 2D perovskites, which may explain the suppressed halide mobility.