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

Typically, bright excitons (X-B) emit lightin two-dimensional(2D) layered hybrid perovskites. There are also dark excitons (X-D), for which radiative recombination is spin-forbidden. Applicationof a magnetic field can somewhat relax the spin-rule, yielding X-D emission. Can we obtain X-D light emission in theabsence of a magnetic field? Indeed, we observe unusually intenseX(D) emission at & SIM;7 K for (Rac-MBA)(2)PbI4, (Rac-4-Br-MBA)(2)PbI4, and (R-4-Br-MBA)(2)PbI4 (Rac-MBA: racemic methylbenzylammonium), whichcrystallize in a lower symmetry monoclinic phase. For comparison,orthorhombic (R-MBA)(2)PbI4 does not exhibit X-D emission. X-D has a lower energy than X-B, with energy difference & UDelta;E. In monoclinicsamples, & UDelta;E & SIM; 20 meV is large enoughto suppress the thermal excitation of X-D to X-B, at temperatures <30 K. Consequently, X-D recombinesby emitting light with a long lifetime (& SIM;205 ns). At highertemperatures, the emission switches to the spin-allowed X-B (lifetime < 1 ns)