Polaronic effects in excitonic correlations in two-dimensional metal-halide perovskites (Conference Presentation)

Abstract

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors—chemical, electronic, and structural—that govern strong multiexciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA)2PbI4 (PEA= phenylethylam- monium). Using coherent two-dimensional excitation spectroscopy, we determine the binding energy of biexcitons—correlated two-electron, two-hole quasiparticles—to be 44 ± 5meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalcogenides. Importantly, we show that this binding energy increases by ∼25% upon cooling to 5 K. Our work highlights the importance of multiexciton correlations in this class of technologically promising, solution-processablematerials, in spite ofthe strong effects of lattice fluctuations and dynamic disorder. We demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic countercation. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensionalWannier formalism. We include four distinct excitonic states separated by 35 ± 5 meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. We demonstrate, using high-resolution impulsive stimulated Raman spectroscopy, that the coupling of distinct excitonic resonanes to the lattice is unique, with some excitonic transition displaying lattice reorganization akin to photocarriers. Our work highlights the complexity of excitonic properties in this class of nanostructured materials.