Coupling Between Organic and Inorganic Sublattices of 2D Organic–Inorganic Hybrid Perovskites Regulates Electron‐Phonon Interaction and Charge Carrier Mobility

Chemically tuning the interactions between inorganic sublattices and organic cations in organic–inorganic hybrid perovskites (OIHPs) spreads various optoelectronic applications. However, the coupling between inorganic sublattices and organic cations (denoted as electron‐vibrational coupling, EVC) as well as its impact on performance are poorly understood due to the challenges in probing this coupling. Here, it is demonstrated for the first time that femtosecond time‐resolved sum frequency generation vibrational spectroscopy enables the determination of EVC dynamics from the perspective of specific vibrational coordinates of organic cations. Through a systematic study on the ultrafast vibrational dynamics of spacer cations in mixed‐halide 2D OIHPs (PEA) 2 PbI 4‐x Br x (PEA: phenylethylamine, x = 0–4), it is found that the vibrational state‐trapping time of PEA cations and their subsequent vibrational energy relaxation are associated with the strength of EVC. By combining these results with photoluminescence and optical pump terahertz probe spectroscopy measurements, a dynamics‐property relationship is established among EVC, electron‐phonon coupling, and charge carrier mobility. Specifically, weaker EVC enhances charge carrier mobility by reducing electron‐phonon interactions. These findings demonstrate a significant interplay between spacer cations and inorganic sublattices in 2D OIHPs, providing useful guidelines for boosting optoelectronic performance through modulating vibrational motion of the organic cations.