Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency ( 50 cm −1 ) optical phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight on the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. Here, 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 counter-cation. We present a phenomenological, yet quantitative framework to simulate excitonic absorption lineshapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier 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 lineshape, 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. Our work highlights the complexity of excitonic properties in this class of nanostructured materials. * carlos.silva@gatech.edu † srinivasa.srimath@iit.it 1 arXiv:1803.02455v3 [cond-mat.mtrl-sci] 30 Apr 2018 I. INTRODUCTION Organic-inorganic hybrid perovskites (HOIPs) consist of metal-halide octahedral motifs that form multi-dimensional lattice planes structurally separated by coordinating organic counter cations [1]. While the frontier orbitals that give rise to the semiconductor electronic structure are contributed by the metal-halide network, the organic cation plays a key role in the structural configuration as well as the stability of the lattice [2]. When the organic moieties are long enough to isolate the lattice planes electronically, the latter form quantum-well-like structures with strong two-dimensional (2D) electronic confinement within the metal-halide layer [3]. A consequence of such confinement is the creation of strongly bound excitons, which have been reported as early as 1989 by Ishihara et al. [4], with binding energies of 200-300 meV. In a general context, a variational approach of electron-hole correlations predicts that excitons in strongly confined quantum wells experience a four-fold enhancement in binding energy with respect to the bulk semiconductor [5], assuming a smooth dielectric environment around the well. This enhancement is generally observed in systems such as GaAs, which is characterized by an exciton binding energy of 4 meV in the bulk and 16 meV in quantum wells [6]. Intriguingly, there is more than a ten-fold increase in the binding energies going from 3D lead-halide perovskites (10-20 meV [7]) to their 2D counterparts [4].Beyond quantum confinement, dielectric confinement arising from the intercalating organic layers increases the Coulomb correlations substantially, resulting in such a strong increase in the exciton binding energy [3,4,8,9].There is now an increasing consensus ...

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 multi-exciton correlations.Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA) 2 PbI 4 (PEA = phenylethylammonium). We determine the binding energy of biexcitons -correlated two-electron, two-hole quasiparticles -to be 44 ± 5 meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalchogenides. Importantly, we show that this binding energy increases by ∼ 25% upon cooling to 5 K. Our work highlights the importance of multi-exciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder. * FT and SN are to be considered first co-authors of this manuscript. †