Abstract
We present a multiscale model based on many-body Green’s functions theory in the GW approximation and the Bethe–Salpeter equation (GW-BSE) for the simulation of singlet and triplet exciton transport in molecular materials. Dynamics of coupled electron–hole pairs are modeled as a sequence of incoherent tunneling and decay events in a disordered morphology obtained at room temperature from molecular dynamics. The ingredients of the rates associated to the events, i.e. reorganization energies, site energies, lifetimes, and coupling elements, are determined from a combination of GW-BSE and classical polarizable force field techniques. Kinetic Monte Carlo simulations are then employed to evaluate dynamical properties such as the excitonic diffusion tensor and diffusion lengths. Using DCV5T-Me(3,3), a crystalline organic semiconductor, we demonstrate how this multiscale approach provides insight into the fundamental factors driving the transport processes. Comparing the results obtained via different calculation models, we investigate in particular the effects of charge-transfer mediated high exciton coupling and the influence of internal site energy disorder due to conformational variations. We show that a small number of high coupling elements indicative of delocalized exciton states does not impact the overall dynamics perceptively. Molecules with energies in the tail of the excitonic density of states dominate singlet decay, independent of the level of disorder taken into account in the simulation. Overall, our approach yields singlet diffusion lengths on the order of 10 nm as expected for energetically disordered molecular materials.
Highlights
Processes involving electronic excitations play a vital role in many functional molecular systems, both natural [1,2,3] and synthetic [4,5,6]
Diffusion tensors and diffusion length are obtained from simulating exciton dynamics explicitly by means of a kinetic Monte Carlo algorithm and analyzing its trajectories
3 For typical donor molecules used in organic solar cells (OSC), we showed that the use of the Tamm–Dancoff approximation [46]3 (TDA) overestimates π–π transition energies by 0.2 eV but yields correct character of the excitations [32]
Summary
Processes involving electronic excitations play a vital role in many functional molecular systems, both natural [1,2,3] and synthetic [4,5,6]. For instance for technological applications in organic electronic devices, like organic light-emitting diodes (OLEDs) or organic solar cells (OSC), an in-depth understanding about the fundamental aspects of this interplay and how it gives rise to dynamical properties and eventually device performance is essential [7,8,9]. Computational models of these dynamics aiming at predictions of material characteristics need to take the multiscale nature of the problem explicitly into account.
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