Polarized local excitons assist charge dissociation in Y6-based nonfullerene organic solar cells: a nonadiabatic molecular dynamics study

08 December 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Y6-based nonfullerene organic solar cells (OSCs) have achieved an outstanding power conversion efficiency (PCE) of over 19% due to the low energy loss and high exciton dissociation efficiency with a small energy offset. However, the exciton dissociation mechanism is still under debate. It is unclear why a small energy offset can lead to efficient exciton dissociation in nonfullerene systems, but causes significant charge recombination in fullerene ones. Most of the previous theoretical studies have focused on the static properties of donor–acceptor heterojunctions, while neglecting excited electron dynamics and nonadiabatic effects. Here, we applied nonadiabatic molecular dynamics simulations to study the charge transfer dynamics in both donor:Y6 and donor:C60 crystalline systems. We found that thermal effects do not significantly influence the exciton dissociation in the NT-4T-FF:Y6 system, which aligns with experimental observations. Based on our simulations, we identified a five-step charge transfer process in nonfullerene systems. While previous studies suggested electrostatic interfacial fields from non-fullerene small molecule acceptors, our research reveals that strong donor-acceptor interactions primarily affect the local exciton states rather than the ground state. Consequently, the polarized local excitons play a key role in reducing the Coulomb attraction between electrons and holes, thus facilitating exciton dissociation with a small energy offset. In contrast, this mechanism is not observed in fullerene OSC systems. Our findings provide a fundamental basis for the further development of novel OSC materials with the potential for achieving even higher PCE.

Keywords

Y6 molecule
organic solar cell
exciton dissociation
density functional theory
Ehrenfest molecular dynamics

Supplementary materials

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Title
Supporting Information: Polarized local excitons assist charge dissociation in Y6-based nonfullerene organic solar cells: a nonadiabatic molecular dynamics study
Description
Five configurations of the NT-4T-FF:Y6 blend, optimized crystalline NT-4T-FF:Y6 structures, one crystalline NT-4T-FF:Y6 blend structure after thermal relaxation, isosurfaces of molecular orbitals of the crystalline NT-4T-FF:Y6 blend, Energy diagram for the LE and CT states in a crystalline NT-4T-FF:Y6 blend
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