Emissive Dark Excitons in Monoclinic Two-Dimensional Hybrid Lead Iodide Perovskites.
basic_science · Level V
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- Record sourced from PubMed, PMID 37487113.
- Also identified by DOI 10.1021/acs.nanolett.3c01627.
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Abstract
Typically, bright excitons (X<sub>B</sub>) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (X<sub>D</sub>), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding X<sub>D</sub> emission. Can we obtain X<sub>D</sub> light emission in the absence of a magnetic field? Indeed, we observe unusually intense X<sub>D</sub> emission at ∼7 K for (Rac-MBA)<sub>2</sub>PbI<sub>4</sub>, (Rac-4-Br-MBA)<sub>2</sub>PbI<sub>4</sub>, and (R-4-Br-MBA)<sub>2</sub>PbI<sub>4</sub> (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA)<sub>2</sub>PbI<sub>4</sub> does not exhibit X<sub>D</sub> emission. X<sub>D</sub> has a lower energy than X<sub>B</sub>, with energy difference Δ<i>E</i>. In monoclinic samples, Δ<i>E</i> ∼ 20 meV is large enough to suppress the thermal excitation of X<sub>D</sub> to X<sub>B</sub>, at temperatures <30 K. Consequently, X<sub>D</sub> recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed X<sub>B</sub> (lifetime < 1 ns).