Methylated diammonium spacer modulated three-dimensional lead bromide perovskitoid hybrids with distinct photoconductivity anisotropy

Abstract

The emerged 3D APb₂X₆ (A represents diammonium; X is Cl, Br and I) perovskitoids hold huge potential to inherit the desirable charge transport properties and functional tunability of 3D APbX₃ perovskites. Limited by the Goldschmidt tolerance factor rule, the reported 3D perovskitoid species remain sparse, hindering us from exploring their structural effect dictated photoelectronic behavior. Herein, we construct novel 3D (NMDAP)Pb₂Br₆ (NMDAP = N-methyl propane diammonium) composed of Pb₂Br₁₀ dimer unit, modulated by methylated diammonium spacer and the synthetically controlled stoichiometric ratio of reactants. Significantly, 3D (NMDAP)Pb₂Br₆ shows remarkable self-trapped exciton (STE) emission induced by high octahedra distortion associated with strong electron–phonon coupling. The STE emission features broadband photoluminescence (PL) spectra ranging from 525 to 750 nm with a large Stokes redshift of ∼0.91 eV. Photophysical investigation further reveals a large Huang–Rhys factor of ∼146 and a long STE emission lifetime of 17 ns. DFT calculations suggest that the band edge states are mainly contributed by the Pb 6s/Br 4p orbitals along the equatorial direction rather than the axial direction in 3D (NMDAP)Pb₂Br₆. Moreover, the asymmetric structure enables 3D (NMDAP)Pb₂Br₆ anisotropic charge transport behavior, as underpinned by the experimental and theoretical investigations. As a consequence, a photoconductive detector based on 3D (NMDAP)Pb₂Br₆ single crystal delivers anisotropic photoconductivity performance, with the [001] direction showing a responsivity and detectivity of 0.28 A W⁻¹ and 2.12 × 10¹² Jones, about 3-fold enhancement compared to those along the [100] and [010] directions. These observations offer extended functional tunability in 3D perovskitoids and reveal the potential of 3D perovskitoids in advanced optoelectronic devices.