Unraveling Exciton-Carrier Correlations in Orthorhombic Lead Halide Perovskite

Exciton correlations to charge carriers in condensed matter systems represent a rich field of study with significant implications for both fundamental physics and technological applications. Hybrid organic-inorganic perovskites represent an exceptional material platform for such explorations. In contrast to the tetragonal phase at room temperature, the orthorhombic phase of methylammonium lead halide perovskites exhibits a pronounced excitonic absorption, enabling simultaneous generation and temporal tracking of excitons and charge carriers using broadband laser pulses. In this study, we employed two-dimensional electronic spectroscopy (2DES) to examine the temporal evolution and correlations of these photoinduced excitons and carriers at 15 K. The low-temperature conditions enhance spectral resolution and allow for the identification of spectral features associated with exciton-carrier interactions. Notably, the 2DES spectra exhibit prominent exciton-related signals and cross-peaks between excitons and carriers, while the free-carrier diagonal features are absent. This absence is interpreted as arising from many-body effects, specifically excitation-induced dephasing and excitation-induced shifts, mediated by interactions between excitons and a bath of incoherent free carriers. To get insight into exciton-carrier correlations, we utilized fully quantum mechanical dynamics simulations based on the hierarchy equation of motion method. Our simulations incorporated vibrational modes using a linear vibronic coupling model and charge carriers modeled as a Fermion bath. The results demonstrate that low- and high-frequency vibrational modes substantially influence population dynamics, while variations in vibronic coupling strength have minimal impact on coherence lifetimes. Overall, our findings reveal that many-body interactions strongly influence exciton-carrier dynamics in orthorhombic perovskites, contributing to a deeper understanding of exciton-carrier interactions.