Search for: All records

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. 

Coexistence of excitons and free charge carriers can complicate conventional spectroscopic studies of transport mechanisms in layered perovskite solar cells. Because of their large concentrations and absorbance cross sections, excitons tend to dominate spectroscopic signals and obscure observations of free charges in this class of systems. To investigate the effects of interstitial organic molecules on charge transport in photovoltaic devices, we apply a newly developed four-pulse transient grating method with photocurrent detection to layered perovskites possessing a range of quantum well thicknesses. In this method, a phase-stabilized “pump” pulse-pair photoexcites a carrier density grating in the active layer of a photovoltaic cell, whereas transport is time-resolved using the carrier density grating generated by a subsequent “probe” pulse-pair. Carrier diffusion mechanisms are revealed by measuring the recombination-induced nonlinear response of the device while varying the delay between pulse-pairs and phase difference between density gratings. Like drift velocity dispersion, our data suggest that encounters with inorganic–organic interfaces broaden the range of diffusivities in addition to skewing the distributions toward slower transit times. Rather than tunneling through the potential energy barriers associated with the organic material, the experimental measurements support a physical picture in which the photoexcited carriers traverse circuitous paths through the active layer while occupying the phases of the thickest quantum wells. 

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. 

Conventional time-of-flight (TOF) measurements yield charge carrier mobilities in photovoltaic cells with time resolution limited by the RC time constant of the device, which is on the order of 0.1–1 µs for the systems targeted in the present work. We have recently developed an alternate TOF method, termed nonlinear photocurrent spectroscopy (NLPC), in which carrier drift velocities are determined with picosecond time resolution by applying a pair of laser pulses to a device with an experimentally controlled delay time. In this technique, carriers photoexcited by the first laser pulse are “probed” by way of recombination processes involving carriers associated with the second laser pulse. Here, we report NLPC measurements conducted with a simplified experimental apparatus in which synchronized 40 ps diode lasers enable delay times up to 100 µs at 5 kHz repetition rates. Carrier mobilities of ∼0.025 cm2/V/s are determined for MAPbI3 photovoltaic cells with active layer thicknesses of 240 and 460 nm using this instrument. Our experiments and model calculations suggest that the nonlinear response of the photocurrent weakens as the carrier densities photoexcited by the first laser pulse trap and broaden while traversing the active layer of a device. Based on this aspect of the signal generation mechanism, experiments conducted with co-propagating and counter-propagating laser beam geometries are leveraged to determine a 60 nm length scale of drift velocity dispersion in MAPbI3 films. Contributions from localized states induced by thermal fluctuations are consistent with drift velocity dispersion on this length scale.