Exciton annihilation in molecular aggregates suppressed through quantum interference.

Exciton-exciton annihilation (EEA), an important loss channel in optoelectronic devices and photosynthetic complexes, has conventionally been assumed to be an incoherent, diffusion-limited process. Here we challenge this assumption by experimentally demonstrating the ability to control EEA in molecular aggregates using the quantum phase relationships of excitons. We employed time-resolved photoluminescence microscopy to independently determine exciton diffusion constants and annihilation rates in two substituted perylene diimide aggregates featuring contrasting excitonic phase envelopes.

Origin of layered perovskite device efficiencies revealed by multidimensional time-of-flight spectroscopy.

Mixtures of layered perovskite quantum wells with different sizes form prototypical light-harvesting antenna structures in solution-processed films. Gradients in the bandgaps and energy levels are established by concentrating the smallest and largest quantum wells near opposing electrodes in photovoltaic devices. Whereas short-range energy and charge carrier funneling behaviors have been observed in layered perovskites, our recent work suggests that such light-harvesting processes do not assist long-range charge transport due to carrier trapping at interfaces between quantum wells and interstitial organic spacer molecules.

Multidimensional time-of-flight spectroscopy.

Experimental methods based on a wide range of physical principles are used to determine carrier mobilities for light-harvesting materials in photovoltaic cells. For example, in a time-of-flight experiment, a single laser pulse photoexcites the active layer of a device, and the transit time is determined by the arrival of carriers at an acceptor electrode. With inspiration from this conventional approach, we present a multidimensional time-of-flight technique in which carrier transport is tracked with a second intervening laser pulse.

Nonlinear fluorescence spectroscopy of layered perovskite quantum wells.

Interest in layered organohalide perovskites is motivated by their potential for use in optoelectronic devices. In these systems, the smallest and largest quantum wells are primarily concentrated near the glass and air interfaces of a film, thereby establishing a gradient in the average values of the bandgaps. It has been suggested that this layered architecture promotes the funneling of electronic excitations through space in a manner similar to light-harvesting processes in photosynthetic antennae.

Distinguishing Energy- and Charge-Transfer Processes in Layered Perovskite Quantum Wells with Two-Dimensional Action Spectroscopies.

Interest in photovoltaic devices based on layered perovskites is motivated by their tunable optoelectronic properties and stabilities in humid conditions. In these systems, quantum wells with different sizes are organized to direct energy and charge transport between electrodes; however, these relaxation mechanisms are difficult to distinguish based on conventional transient absorption techniques. Here, two-dimensional "action spectroscopies" are employed to separately target processes that lead to the production of photocurrent and energy loss due to fluorescence emission.

Discovery and characterization of an acridine radical photoreductant.

Photoinduced electron transfer (PET) is a phenomenon whereby the absorption of light by a chemical species provides an energetic driving force for an electron-transfer reaction. This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics and photosensitive materials. In recent years, research in the area of photoredox catalysis has enabled the use of PET for the catalytic generation of both neutral and charged organic free-radical species.

Imaging Excited State Dynamics in Layered 2D Perovskites with Transient Absorption Microscopy.

Two-dimensional (2D) hybrid perovskites are generating broad scientific interest because of their potential for use in photovoltaics and microcavity lasers. It has recently been demonstrated that mixtures of quantum wells with different thicknesses can be assembled in films with heterogeneous quantum well distributions. Large (small) quantum wells are concentrated at the air side (substrate side) of the films, thereby promoting directional energy and/or electron transfer.

Nonlinear Photocurrent Spectroscopy of Layered 2D Perovskite Quantum Wells.

Two-dimensional coherent photocurrent spectroscopies directly probe the electronic states and processes that are relevant to the performance of a photovoltaic device. In this Letter, we apply two-pulse nonlinear photocurrent spectroscopy to a photovoltaic device based on layered perovskite quantum wells. The method effectively decomposes the photovoltaic response into contributions from separate quantum wells and excited-state species (i.

Excitation energy-dependent photocurrent switching in a single-molecule photodiode.

The direction of electron flow in molecular optoelectronic devices is dictated by charge transfer between a molecular excited state and an underlying conductor or semiconductor. For those devices, controlling the direction and reversibility of electron flow is a major challenge. We describe here a single-molecule photodiode.

Synthetic control over orientational degeneracy of spacer cations enhances solar cell efficiency in two-dimensional perovskites.

Two-dimensional perovskites have emerged as more intrinsically stable materials for solar cells. Chemical tuning of spacer organic cations has attracted great interest due to their additional functionalities. However, how the chemical nature of the organic cations affects the properties of two-dimensional perovskites and devices is rarely reported.

Energy transfer mechanisms in layered 2D perovskites.

Two-dimensional (2D) perovskite quantum wells are generating broad scientific interest because of their potential for use in optoelectronic devices. Recently, it has been shown that layers of 2D perovskites can be grown in which the average thicknesses of the quantum wells increase from the back to the front of the film. This geometry carries implications for light harvesting applications because the bandgap of a quantum well decreases as its thickness increases.

Communication: Uncovering correlated vibrational cooling and electron transfer dynamics with multidimensional spectroscopy.

Analogues of 2D photon echo methods in which two population times are sampled have recently been used to expose heterogeneity in chemical kinetics. In this work, the two population times sampled for a transition metal complex are transformed into a 2D rate spectrum using the maximum entropy method. The 2D rate spectrum suggests heterogeneity in the vibrational cooling (VC) rate within the ensemble.

Ultrafast Spectroscopic Signatures of Coherent Electron-Transfer Mechanisms in a Transition Metal Complex.

The prevalence of ultrafast electron-transfer processes in light-harvesting materials has motivated a deeper understanding of coherent reaction mechanisms. Kinetic models based on the traditional (equilibrium) form of Fermi's Golden Rule are commonly employed to understand photoinduced electron-transfer dynamics. These models fail in two ways when the electron-transfer process is fast compared to solvation dynamics and vibrational dephasing.