LAUSR

Low-dimensional perovskites have—in view of their high radiative recombination rates—shown great promise in achieving high luminescence brightness and colour saturation. Here we investigate the effect of electron–phonon interactions on the luminescence of single crystals of two-dimensional perovskites, showing that reducing these interactions can lead to bright blue emission in two-dimensional perovskites. Resonance Raman spectra and deformation potential analysis show that strong electron–phonon interactions result in fast non-radiative decay, and that this lowers the photoluminescence quantum yield (PLQY). Neutron scattering, solid-state NMR measurements of spin–lattice relaxation, density functional theory simulations and experimental atomic displacement measurements reveal that molecular motion is slowest, and rigidity greatest, in the brightest emitter. By varying the molecular configuration of the ligands, we show that a PLQY up to 79% and linewidth of 20 nm can be reached by controlling crystal rigidity and electron–phonon interactions. Designing crystal structures with electron–phonon interactions in mind offers a previously underexplored avenue to improve optoelectronic materials' performance.Films of exfoliated crystals of two-dimensional hybrid metal halide perovskites with phenyl groups as organic cations show increased molecular rigidity, reduced electron–phonon interactions and blue emission with photoluminescence quantum yield approaching 80%.