Axial–Equatorial Halide Ordering in Layered Hybrid Perovskites from Isotropic–Anisotropic <sup>207</sup>Pb NMR
Michael A. Hope, Manuel Cordova, Aditya Mishra, Ummugulsum Gunes, Alessandro Caiazzo, Kunal Datta, René A. J. Janssen, Lyndon Emsley
Abstract
Abstract Bandgap‐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. 207 Pb NMR can be used to probe the atomic‐scale structure of lead‐halide materials, but although the isotropic 207 Pb shift is sensitive to halide stoichiometry, it cannot distinguish configurational isomers. Here, we use 2D isotropic–anisotropic correlation 207 Pb NMR and relativistic DFT calculations to distinguish the [PbX 6 ] configurations in mixed iodide–bromide 3D FAPb(Br 1− x I x ) 3 perovskites and 2D BA 2 Pb(Br 1− x I x ) 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.