Exciton-polaron spectral structures in two-dimensional hybrid lead-halide perovskites

Abstract

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 ...

Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification

“…The PL spectra are nearly invariant in width but have an increased Stokes shift as the cation becomes larger, 6 indicating that excitons find energetic minima before recombining, consistent with disorder and/or polaron formation. 5,9,13,[17][18][19][20][21] As the temperature is decreased from 300 K to 10 K, the excitonic absorption and PL resonances for 2-F, 2-Cl, and 2-Br linearly red-shift ( Figure S5 reports by us and others for Pb-based hybrid perovskites 6,22,23 as well as lead chalcogenides. [24][25][26] α is much smaller than that we observed for (PEA)2PbI4 and its derivatives which have longer cations with unchanged cross-sectional area, where α ranged from -0.14 to -0.16 meV/K, 27 indicative of a weaker temperature dependence of the electronic properties of 2DHPs as the Pb-I-Pb bond angle decreases and the band gap increases.…”

“…Several other hypotheses have been proposed for the origin of excitonic structure in this class of materials. [8][9][10][11] One of these hypotheses is that the excitonic structure is caused by distinct excitonic states likely caused by polaronic effects. 9 Our data here indicate that both vibronic progressions and such distinct excitonic transitions may occur.…”

“…Our hypothesis that the excitonic structure is caused by a vibronic progression has been controversial, though all proposed explanations we are aware of involve some form of excitonphonon coupling. [8][9][10][11] Coupling of excitons to phonons on the organic and inorganic frameworks has been optically observed in 2DHPs 6,12,13 and in all-inorganic 3D halide perovskite nanocrystals. 14 Here, we demonstrate that exciton dynamics in 2DHPs can be modified by tuning the structure of the organic cation, even though the band edges in 2DHPs are formed by Pb and I orbitals.…”

Phase-Transition-Induced Carrier Mass Enhancement in 2D Ruddlesden–Popper Perovskites

“…To date their structural, dielectric, optical, and excitonic properties remain largely unexplored.Many of 2D perovskite family members exhibit complex absorption and emission spectra with many sidebands. 31,[35][36][37][38] Today their origin is generally attributed to phonon replicas, 31,32,37,38 however, other interpretations invoking bound excitons or biexcitons can be found in literature. 29,39,40 Moreover, early magneto-optical studies report different diamagnetic coefficients for the various sidebands, which is not consistent with the phonon replica picture.…”

“…46 The equal spacing of first three transition is characteristic for phonon-assisted transitions (phonon replicas). 31,37,38,47 The energy of the involved LO phonon 15 meV can be attributed to the vibration of the inorganic part of the 2D perovskite (Pb-I character). 31,37 In-3 Figure 2: (a) Comparison of transmittance spectra of (C 6 H 13 NH 3 ) 2 PbI 4 sample at 0 T (black) and 67 T for σ + (blue) and σ − (red) polarization.…”