Redox conditions correlated with vibronic coupling modulate quantum beats in photosynthetic pigment-protein complexes

Measuring carrier diffusion in MAPbI3 solar cells with photocurrent-detected transient grating spectroscopy

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.

Probing drift velocity dispersion in MAPbI3 photovoltaic cells with nonlinear photocurrent spectroscopy

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.

Leveraging Dynamical Symmetries in Two-Dimensional Electronic Spectra to Extract Population Transfer Pathways

We present a method to deterministically isolate population transfer kinetics from two-dimensional electronic spectroscopic signals. Central to this analysis is the characterization of how all possible subensembles of excited state systems evolve through the population time. When these dynamics are diagrammatically mapped by using double-sided Feynman pathways where population time dynamics are included, a useful symmetry emerges between excited state absorption and ground state bleach recovery dynamics of diagonal and below diagonal cross-peak signals. This symmetry allows removal of pathways from the spectra to isolate signals that evolve according to energy transfer kinetics. We describe a regression procedure to fit to energy transfer time constants and characterize the accuracy of the method in a variety of complex excited state systems using simulated two-dimensional spectra. Our results show that the method is robust for extracting ultrafast energy transfer in multistate excitonic systems, systems containing dark states that affect the signal kinetics, and systems with interfering vibrational relaxation pathways. This procedure can be used to accurately extract energy transfer kinetics from a wide variety of condensed phase systems.

Light Emission from Vibronic Polaritons in Coupled Metalloporphyrin-Multimode Cavity Systems

In this study, we explore how one can use cavity polariton formation and a non-Condon vibronic coupling mechanism to form a type of hybrid light-matter state we denote as Herzberg-Teller (HT) vibronic polaritons. We use simple models to define the basic characteristics of these hybrid light-matter excitations including their dispersive energies. Experimentally, we find evidence of HT polaritons in the light emission spectra from copper(II) tetraphenylporphyrin (CuTPP) molecules strongly coupled to both single and multimode Fabry-Perot resonator structures. For specific resonator designs, we find evidence of significant enhancement of light emission from a short-lived sing-doublet state of CuTPP, which couples to a higher energy singlet state via a non-Condon vibronic mechanism. The results of a two-state model support the conclusion that this enhancement and the temperature-dependent dispersion of the light emission peak energy stem from radiative relaxation into cavity photon states dressed by collective vibrations of the molecules participating in polariton formation. These results show how researchers can leverage the complex interplay of electronic and nuclear degrees of freedom in light absorbing molecules to form a vaster array of coherent light-matter states and potentially transform platforms in optoelectronic and photocatalytic technologies.