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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. 

Spin–orbit coupling splits the exciton resonances of two-dimensional organic–inorganic hybrid perovskites (2D-OIHPs) into an optically active fine structure. Although circularly polarized light can induce macroscopic spin polarizations in ensembles of quantum wells, the orientations of the angular momentum vectors associated with individual excitons generally randomize on sub-picosecond timescales in 2D-OIHPs with single lead-iodide layers. In the present work, we investigate the nonlinear optical signatures of spin depolarization in 2D-OIHP materials with various organic layer thicknesses and polaron binding energies. Transient absorption experiments conducted using circularly polarized laser pulses establish time constants for spin equilibration ranging from 65 to 110 fs in the targeted systems. In addition, with inspiration from time-resolved Faraday rotation spectroscopies, we introduce a transient grating method in which spin relaxation promotes an elliptical-to-linear transformation of the signal field polarization. Spectroscopic signatures for all experiments are simulated with a common third-order perturbative model that incorporates orientationally averaged transition dipoles and the polarizations of the laser pulses. Spectroscopic line broadening parameters obtained for the 2D-OIHP systems are considered in the context of a rate formula for spin relaxation, wherein the spin–orbit coupling is combined with a cumulant expansion for fluctuations of the energy levels. Our analysis suggests that the insensitivity of the measured spin relaxation rates to the polaron binding energies of 2D-OIHPs reflects the suppression of an activation energy barrier due to motional narrowing. Model calculations conducted with empirical parameters indicate that motional narrowing of the spin relaxation processes originates in correlated thermal fluctuations of the energy levels comprising the exciton fine structure. 

Abstract Electron–phonon interactions play an essential role in charge transport and transfer processes in semiconductors. For most structures, tailoring electron–phonon interactions for specific functionality remains elusive. Here, it is shown that, in hybrid perovskites, coherent phonon modes can be used to manipulate charge transfer. In the 2D double perovskite, (AE2T)₂AgBiI₈(AE2T: 5,5“‐diylbis(amino‐ethyl)‐(2,2”‐(2)thiophene)), the valence band maximum derived from the [Ag_0.5Bi_0.5I₄]^2–framework lies in close proximity to the AE2T‐derived HOMO level, thereby forming a type‐II heterostructure. During transient absorption spectroscopy, pulsed excitation creates sustained coherent phonon modes, which periodically modulate the associated electronic levels. Thus, the energy offset at the organic–inorganic interface also oscillates periodically, providing a unique opportunity for modulation of interfacial charge transfer. Density‐functional theory corroborates the mechanism and identifies specific phonon modes as likely drivers of the coherent charge transfer. These observations are a striking example of how electron–phonon interactions can be used to manipulate fundamentally important charge and energy transfer processes in hybrid perovskites.