Coherent Real-Space Charge Transport Across a Donor–Acceptor Interface Mediated by Vibronic Couplings

Abstract

There is growing experimental and theoretical evidence that vibronic couplings, couplings between electronic and nuclear degrees of freedom, play a fundamental role in ultrafast excited-state dynamics in organic donor–acceptor hybrids. Whereas vibronic coupling has been shown to support charge separation at donor–acceptor interfaces, so far, little is known about its role in the real-space transport of charges in such systems. Here we theoretically study charge transport in thiophene:fullerene stacks using time-dependent density functional tight-binding theory combined with Ehrenfest molecular dynamics for open systems. Our results reveal coherent oscillations of the charge density between neighboring donor sites, persisting for ∼200 fs and promoting charge transport within the polymer stacks. At the donor–acceptor interface, vibronic wave packets are launched, propagating coherently over distances of more than 3 nm into the acceptor region. This supports previous experimental observations of long-range ballistic charge-carrier motion in organic photovoltaic systems and highlights the importance of vibronic coupling engineering as a concept for tailoring the functionality of hybrid organic devices.

Revealing generation, migration, and dissociation of electron-hole pairs and current emergence in an organic photovoltaic cell

“…Since this also affects PCEs, it needs to be considered when developing solar cells with higher efficiency. The observed photophysical process is consistent with previous experimental and theoretical results (6)(7)(8)(11)(12)(13)(14), which reported that this light-induced process takes place on the scale of tens of femtoseconds.…”

Free Charge Carriers in Homo-Sorted π-Stacks of Donor–Acceptor Conjugates

“…The effective mass is calculated from the band structure of the system, which in turn can be determined using multiple methods, including the widely used tight-binding approximation method. 300 The relaxation time depends on the elastic scattering of the carriers with the defects as well as the lattice phonons.…”

“…According to the Boltzmann transport equation with relaxation-time approximation, the average mobility of the carrier in the band transport regime along an axis is given by where τ and m* are the components of second rank tensors of relaxation time and effective mass tensor along respective axes. The effective mass is calculated from the band structure of the system, which in turn can be determined using multiple methods, including the widely used tight-binding approximation method . The relaxation time depends on the elastic scattering of the carriers with the defects as well as the lattice phonons.…”

Ultrafast charge transfer and vibronic coupling in a laser-excited hybrid inorganic/organic interface

“…Open questions concern the dynamics of charge transfer and its transient regime in the ultrashort time window immediately following the laser perturbation. In studies performed on all-organic complexes, the coherent nuclear motion following the impulsive optical excitation was shown to be mainly responsible for the charge transfer at both covalently or non-covalently bound interfaces [65][66][67][68][69]. This scenario is intrinsically different from the charge-transfer excitons mentioned above, where the electron-hole separation results from the electronic structure of the material.…”