Fine-Tuning Exciton Polaron Characteristics via Lattice Engineering in 2D Hybrid Perovskites

The layered structure of 2D metal halide perovskites (MHPs) consisting of an ionic metal halide octahedral layer electronically separated by an organic cation, exhibits strong coupling between high-binding-energy excitons and low-energy lattice phonons. Photoexcitations in these systems are believed to be exciton polarons, Coulombically bound electron-hole pairs dressed by lattice vibrations. Understanding and controlling the structural and chemical factors that govern this interaction is crucial for optimizing exciton recombination, transport, and many-body interactions. Our study examines the role of the organic cation in a prototypical 2D-MHP system, phenylethylammonium lead iodide, (PEA)2PbI4, and its halogenated derivatives, (F/Cl-PEA)2PbI4. These substitutions allow us to probe polaronic effects while maintaining the average lattice and electronic structure. Using resonant impulsive stimulated Raman scattering (RISRS), we analyze the metal-halide sub-lattice motion coupled to excitons. We apply formalism based on a perturbative expansion of the nonlinear response function on the experimental data to estimate the Huang-Rhys parameter, $S=1/2 \Delta^2$, to quantify the lattice displacement ($\Delta$) due to exciton-phonon coupling. A direct correlation emerges between lattice displacement and octahedral distortion, with F-PEA exhibiting the largest shift and Cl-PEA exhibiting the least, significantly influencing the fine structure features in absorption. Additionally, 2D electronic spectroscopy reveals that F-PEA, with the strongest polaronic coupling, exhibits the least thermal dephasing, supporting the polaronic protection hypothesis. Our findings suggest that systematic organic cation substitution serves as a tunable control for the fine structure in 2D-MHPs, and offers a pathway to mitigate many-body scattering effects by tailoring the polaronic coupling.