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

Abstract

Typically, bright excitons (XB) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (XD), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding XD emission. Can we obtain XD light emission in the absence of a magnetic field? Indeed, we observe unusually intense XD 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 XD emission. XD has a lower energy than XB, with energy difference ΔE. In monoclinic samples, ΔE ∼ 20 meV is large enough to suppress the thermal excitation of XD to XB, at temperatures <30 K. Consequently, XD recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed XB (lifetime < 1 ns)

Thermal Evolution of the Structure and Luminescence of the Hybrid-Cation-Stabilized [(4AMTP)PbBr2]2PbBr4 Layered Perovskite

“…Similar broad and red-shifted emission compared to excitonic emission has been observed for many other layered hybrid perovskites and has been previously described as STE emission. 38,30 The PL decay of the 2.126 eV emission is a few microseconds, as shown in Figure S11 of the SI, which also agrees with the typical STE emission.…”

“…A similar situation has been reported previously from other 2D layered hybrid perovskites with monoclinic structure, where the lower energy emission has been assigned to spin-forbidden dark exciton emission with a longer lifetime, and the higher energy emission has been assigned to spin-allowed bright exciton. 30 A similar excitonic fine structure is plausible for the case of our [(4AMTP)-PbBr 2 ] 2 PbBr 4 , which also possesses the lower-symmetry monoclinic structure. The energy difference between the two states (bright and dark excitons) is small, ∼15 meV (Figure 4b).…”

“…The reduced symmetry of the monoclinic structure has been previously shown to influence the exciton fine structure yielding dark exciton emission. 30 Clearly, the hybrid-A site cation can significantly influence the structure and properties of layered hybrid perovskites. The exploration of the hybrid-A site cation has just begun, with a huge scope of tailoring the structure and functionalities of halide perovskites.…”

“…A prior report suggested that layered perovskites with a monoclinic phase lead to the observation of a spin-forbidden "dark exciton" emission at lower temperatures. 30 Furthermore, at the lower temperatures, the nature of PL of layered perovskites often evolves, even with the appearance of a new lower energy peak attributed to STE emission. Also, any possible temperature-dependent phase transition might get reflected in the PL.…”

Influence of Organic Spacer Cation on Dark Excitons in 2D Perovskites

“…In reality, optical transitions between the ground and dark state can be observed at zero-field due to factors such as crystal disorder and symmetry, and in some cases can even dominate the observed optical spectra. [67] The expected intensity (I DX ) of the observed DX transition in thermal equilibrium is proportional to temperature (T) and magnetic field by the expression:…”