Phonon-Driven Exciton Dissociation at Donor−Acceptor Polymer Heterojunctions: Direct versus Bridge-Mediated Vibronic Coupling Pathways

Pulse-Induced Dynamics of a Charge-Transfer Complex from First Principles

The ultrafast dynamics of charge carriers in organic donor-acceptor interfaces are of primary importance to understanding the fundamental properties of these systems. In this work, we focus on a charge-transfer complex formed by quaterthiophene p-doped by tetrafluoro-tetracyanoquinodimethane and investigate electron dynamics and vibronic interactions also at finite temperatures by applying a femtosecond pulse in resonance with the two lowest energy excitations of the system with perpendicular and parallel polarization with respect to the interface. The adopted ab initio formalism based on real-time time-dependent density-functional theory coupled to Ehrenfest dynamics enables monitoring the dynamical charge transfer across the interface and assessing the role played by the nuclear motion. Our results show that the strong intermolecular interactions binding the complex already in the ground state influence the dynamics, too. The analysis of the nuclear motion involved in these processes reveals the participation of different vibrational modes depending on the electronic states stimulated by the resonant pulse. Coupled donor-acceptor modes mostly influence the excited state polarized across the interface, while intramolecular vibrations in the donor molecule dominate the excitation in the orthogonal direction. The results obtained at finite temperatures are overall consistent with this picture, although thermal disorder contributes to slightly decreasing interfacial charge transfer.

Influence of the electron-vibration coupling on the interfacial charge transfer in organic solar cells: a simple quantum model

In this paper, we analyze the influence of the electron-vibration interaction on the charge transfer process at the donor-acceptor interface in an organic solar cell. We present an essentially exact numerical analysis for a minimal model with only one vibration mode which is coupled to the charge transfer state. We show that the charge transfer state can be hot or cold depending on the parameters and in particular on the value of the energy of the lowest unoccupied molecular orbital on the donor side. We analyze also different regimes where electron-hole attraction or hybridization effects at the interface can modify the quantum yield of the transfer. We discuss also briefly the possible effects of the other vibration modes that are coupled to the charge either on the donor side or on the acceptor side.

Hot-carriers in organic photovoltaics

Photogenerated charge carriers in organic photovoltaics (OPVs) suffer relaxation and recombination losses. However, extracting these carriers at higher energy (‘Hot-carriers’) has been found to be effective to overcome such loss pathways and improve efficiency of OPVs. Excess energy and long delocalization length promotes hot-carrier escape from Coulombic attraction and dissociation into free charges. Here, I have reviewed the ways to generate hot-carriers and their extraction in organic backbones. In-depth understanding of their energetics and dynamics will help designing hot-carrier photovoltaics.

Photoexcitation-induced local phonon spectra and local hot excitons in polymer solar cells

In this article, based on nonadiabatic molecular dynamics with electronic transitions, the elaborate ultrafast process of hot excitons in conjugated polymer solar cells is revealed. When an external optical beam/pulse with the intensity of 30 µJ/cm^-2 is utilized to excite a conjugated polymer, just within only 50 fs, the electronic transition not only redistributes the electron population in the original molecular orbital, but also starts to localize the electron cloud of excited states and to distort the alternating bonds in the polymer chain. Up to 300 fs, the lattice distortion has been stabilized. During the formation of hot excitons, the prominent self-trapping effect of conjugated polymer triggers the occurrence of local infrared active phonon modes, with five peaks in the phonon spectrum as the hot excitons relax. The characteristic phonon spectrum and infrared modes hence form the fingerprint of the hot excitons of a conjugated polymer, which are readily distinguished from other excitation states in the polymer.

From Fundamental Theories to Quantum Coherences in Electron Transfer

Photoinduced electron transfer (ET) is a cornerstone of energy transduction from light to chemistry. The past decade has seen tremendous advances in the possible role of quantum coherent effects in the light-initiated energy and ET processes in chemical, biological, and materials systems. The prevalence of such coherence effects holds a promise to increase the efficiency and robustness of transport even in the face of energetic or structural disorder. A primary motive of this Perspective is to work out how to think about "coherence" in ET reactions. We will discuss how the interplay of basic parameters governing ET reactions-like electronic coupling, interactions with the environment, and intramolecular high-frequency quantum vibrations-impact coherences. This includes revisiting the insights from the seminal work on the theory of ET and time-resolved measurements on coherent dynamics to explore the role of coherences in ET reactions. We conclude by suggesting that in addition to optical spectroscopies, validating the functional role of coherences would require simultaneous mapping of correlated electron motion and atomically resolved nuclear structure.

Vibronic coupling in organic semiconductors for photovoltaics

Ultrafast two-dimensional electronic spectroscopy reveals vibronically-assisted coherent charge transport and separation in organic materials and opens up new perspectives for artificial light-to-current conversion.

Forbidden singlet exciton transitions induced by localization in polymer light-emitting diodes in a strong electric field

Through combining the electron transition process and dipole moment evolution as well as electron-phonon coupling, molecular dynamics calculations show that the radiative decay of singlet excitons in a conjugated polymer, such as a polymer light-emitting diode (PLED), is largely determined by the evolution of the dipole moment. Without an electric field, the decay life of a singlet exciton is about 1 ns. Once an electric field is applied and exceeds a critical value, with electron-phonon coupling, the original lattice structure evolves into two new localized lattice distortions, consistent with the experimental results. Owing to the new lattice structure and self-trapping, the dipole moment rapidly decreases to zero within 5 fs, eliminating the radiative decay of the singlet exciton.

Effective-mode representation of non-Markovian dynamics: a hierarchical approximation of the spectral density. II. Application to environment-induced nonadiabatic dynamics

The non-Markovian approach developed in the companion paper [Hughes et al., J. Chem. Phys. 131, 024109 (2009)], which employs a hierarchical series of approximate spectral densities, is extended to the treatment of nonadiabatic dynamics of coupled electronic states. We focus on a spin-boson-type Hamiltonian including a subset of system vibrational modes which are treated without any approximation, while a set of bath modes is transformed to a chain of effective modes and treated in a reduced-dimensional space. Only the first member of the chain is coupled to the electronic subsystem. The chain construction can be truncated at successive orders and is terminated by a Markovian closure acting on the end of the chain. From this Mori-type construction, a hierarchy of approximate spectral densities is obtained which approach the true bath spectral density with increasing accuracy. Applications are presented for the dynamics of a vibronic subsystem comprising a high-frequency mode and interacting with a low-frequency bath. The bath is shown to have a striking effect on the nonadiabatic dynamics, which can be rationalized in the effective-mode picture. A reduced two-dimensional subspace is constructed which accounts for the essential features of the nonadiabatic process induced by the effective environmental mode. Electronic coherence is found to be preserved on the shortest time scale determined by the effective mode, while decoherence sets in on a longer time scale. Numerical simulations are carried out using either an explicit wave function representation of the system and overall bath or else an explicit representation of the system and effective-mode part in conjunction with a Caldeira-Leggett master equation.

Effective-mode representation of non-Markovian dynamics: a hierarchical approximation of the spectral density. I. Application to single surface dynamics

An approach to non-Markovian system-environment dynamics is described which is based on the construction of a hierarchy of coupled effective environmental modes that is terminated by coupling the final member of the hierarchy to a Markovian bath. For an arbitrary environment, which is linearly coupled to the subsystem, the discretized spectral density is replaced by a series of approximate spectral densities involving an increasing number of effective modes. This series of approximants, which are constructed analytically in this paper, guarantees the accurate representation of the overall system-plus-bath dynamics up to increasing times. The hierarchical structure is manifested in the approximate spectral densities in the form of the imaginary part of a continued fraction similar to Mori theory. The results are described for cases where the hierarchy is truncated at the first-, second-, and third-order level. It is demonstrated that the results generated from a reduced density matrix equation of motion and large dimensional system-plus-bath wavepacket calculations are in excellent agreement. For the reduced density matrix calculations, the system and hierarchy of effective modes are treated explicitly and the effects of the bath on the final member of the hierarchy are described by the Caldeira-Leggett equation and its generalization to zero temperature.