Emissive Dark Excitons in Monoclinic Two-Dimensional Hybrid Lead Iodide Perovskites

Abstract

Typically, bright excitons (X_B) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (X_D), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding X_D emission. Can we obtain X_D light emission in the absence of a magnetic field? Indeed, we observe unusually intense X_D emission at ∼7 K for (Rac-MBA)₂PbI₄, (Rac-4-Br-MBA)₂PbI₄, and (R-4-Br-MBA)₂PbI₄ (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA)₂PbI₄ does not exhibit X_D emission. X_D has a lower energy than X_B, with energy difference ΔE. In monoclinic samples, ΔE ∼ 20 meV is large enough to suppress the thermal excitation of X_D to X_B, at temperatures <30 K. Consequently, X_D recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed X_B (lifetime < 1 ns)