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

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) 2 PbI 4 , (Rac-4-Br-MBA) 2 PbI 4 , and (R-4-Br-MBA) 2 PbI 4 (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA) 2 PbI 4 does not exhibit X D emission. X D has a lower energy than X B , with energy difference Δ E . In monoclinic samples, Δ E ∼ 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).