Origin of space-separated charges in photoexcited organic heterojunctions on ultrafast time scales

Abstract

We present a detailed investigation of ultrafast (subpicosecond) exciton dynamics in the lattice model of a donor/acceptor heterojunction. Exciton generation by means of a photoexcitation, exciton dissociation, and further charge separation are treated on equal footing. The experimentally observed presence of space-separated charges at 100 fs after the photoexcitation is usually attributed to ultrafast transitions from excitons in the donor to charge transfer and charge separated states. Here, we show, however, that the space-separated charges appearing on 100-fs time scales are predominantly directly optically generated. Our theoretical insights into the ultrafast pump-probe spectroscopy challenge usual interpretations of pump-probe spectra in terms of ultrafast population transfer from donor excitons to space-separated charges.

Identification of Ultrafast Photophysical Pathways in Photoexcited Organic Heterojunctions

“…This conclusion is in line with our recent results regarding ultrafast photophysics in a model where the LUMO-LUMO offset is comparable to the effective bandwidth of the LUMO band of the acceptor. 27 Namely, we have recognized that the resonant mixing between single-electron states in the LUMO bands of the two materials is at the root of the ultrafast direct optical generation of space-separated charges. Here, the same mechanism is responsible for the observed direct generation of excitons in PACB states, which now acquire nonzero oscillator strengths due to the energy alignment between single-electron states stemming from the donor LUMO orbital and higher-than-LUMO acceptor orbitals.…”

“…23 We have recently investigated the exciton dynamics oc-curring on a subpicosecond time scale following the excitation of the model D/A heterojunction. 27 Our model explicitly takes into account the physical mechanisms regarded as highly relevant for the ultrafast heterojunction dynamics, such as the carrier delocalization and the carrier-phonon interaction. Moreover, the exciton generation, exciton dissociation, and further charge separation are treated on equal footing and on a fully quantum level, which is essential to correctly describe processes taking place on ultrafast time scales.…”

Multiscale in modelling and validation for solar photovoltaics

“…Subsequently, effective hopping rates, carrier lifetimes and mobilities can be extracted from KMC simulations and inserted into a macroscopic driftdiffusion-Poisson solver, thus linking from mesoscopic to macroscopic scales [76]. The electronic structure and polarization at organic interfaces can also be studied on ab initio level using the charge patching method within DFT [77] or DFT-based tight-binding [78], and exciton formation as well as ultra-fast separation of photogenerated charge carriers at such interfaces have been assessed based on a density matrix formalism [79,80].…”

Exact description of excitonic dynamics in molecular aggregates weakly driven by light

“…The DCT scheme classifies DMs according to the lowest power with which they scale in the exciting field and, therefore, provides a recipe to analyze the dynamics up to any given order in the exciting field in terms of a finite number of electronic DMs. A DCT-based approach has been recently applied by one of us to study exciton generation and subsequent charge separation in photoexcited OPVs [33,34]. However, the truncation of the environment-assisted branch of the hierarchy within the DCT scheme still has to be performed separately [31], and it is commonly done in a low order in the excitationenvironment coupling [29,33,35].…”

“…A DCT-based approach has been recently applied by one of us to study exciton generation and subsequent charge separation in photoexcited OPVs [33,34]. However, the truncation of the environment-assisted branch of the hierarchy within the DCT scheme still has to be performed separately [31], and it is commonly done in a low order in the excitationenvironment coupling [29,33,35].…”