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Two-dimensional organic–inorganic hybrid perovskite (2D-OIHP) quantum wells exhibit a triplet of bright exciton fine structure states near the band edge, enabling the generation of transient macroscopic spin alignments with circularly polarized light. Here, we investigate the microscopic origin of photoinduced spin relaxation in 2D-OIHPs using multidimensional coherent spectroscopy together with a theoretical framework that combines time-dependent perturbation theory with the Fokker–Planck equation. Analysis of the spectral line shapes reveals highly correlated exciton fluctuations within the fine structure manifolds of a pair of 2D-OIHPs featuring different organic layer thicknesses and polaron binding energies. In particular, the Gaussian correlation coefficients determined for the two lead-iodide-based systems range from 0.67 to 0.80, while their polaron binding energies span 11.8–18.9 meV. Incorporating time-coincident solvation dynamics into a stochastic model shows that these energy level correlations reduce the exciton–bath couplings and extend dephasing times for spin-flip transitions, even in spectral broadening regimes governed by Marcus-like kinetics (which are typically considered incompatible with motional narrowing). Since photoexcitation occurs on the seam of intersection between the excited-state free energy surfaces, spin relaxation can proceed without an activation barrier, provided it outpaces energy dissipation into the environment. Overall, these results demonstrate that correlated exciton fluctuations play a central role in accelerating spin depolarization in 2D-OIHPs through motional narrowing of coherences between exciton states. 

Conventional time-of-flight methods can be used to determine carrier mobilities for photovoltaic cells in which the transit time between electrodes is greater than the RC time constant of the device. To measure carrier drift on sub-ns timescales, we have recently developed a two-pulse time-of-flight technique capable of detecting drift velocities with 100-ps time resolution in perovskite materials. In this method, the rates of carrier transit across the active layer of a device are determined by varying the delay time between laser pulses and measuring the magnitude of the recombination-induced nonlinearity in the photocurrent. Here, we present a related experimental approach in which diffractive optic-based transient grating spectroscopy is combined with our two-pulse time-of-flight technique to simultaneously probe drift and diffusion in orthogonal directions within the active layer of a photovoltaic cell. Carrier density gratings are generated using two time-coincident pulse-pairs with passively stabilized phases. Relaxation of the grating amplitude associated with the first pulse-pair is detected by varying the delay and phase of the density grating corresponding to the second pulse-pair. The ability of the technique to reveal carrier diffusion is demonstrated with model calculations and experiments conducted using MAPbI3 photovoltaic cells. 

Low-dimensional organic/inorganic hybrid perovskites (OIHPs) are a promising class of materials with a wide range of potential applications in optoelectronics and other fields since these materials can synergistically combine individual features of organic molecules and inorganics into unique properties. Non-covalent interactions are commonly observed in OIHPs, in particular, π-effect interactions between the organic cations. Such non-covalent interactions can significantly influence important properties of the low-dimensional OIHPs, including dielectric confinement, bandgap, photoluminescence, quantum efficiency, charge mobility, trap density, stability, and chirality. This perspective reviews recent studies of non-covalent interactions involving the π systems of organic cations in low-dimensional OIHPs. The analysis of crystal structures of low-dimensional OIHPs offers significant insight into understanding such non-covalent interactions and their impacts on specific properties of these OIHPs. The developed structure–property relationships can be used to engineer non-covalent interactions in low-dimensional OIHPs for applications.