# Vibronically assisted sub-cycle charge transfer at a non-fullerene acceptor heterojunction

## TL;DR

Researchers discovered that charge transfer in organic materials can happen extremely fast, under 20 femtoseconds, by using molecular vibrations instead of relying on large energy differences.

## Contribution

The study reveals that ultrafast charge transfer can occur without large energy offsets or strong coupling, enabled by vibrational assistance.

## Key findings

- Charge transfer occurs in ~18 fs with minimal energy offset (<100 meV) between donor and acceptor.
- Coherent wavepackets on vibrational modes facilitate ultrafast charge transfer.
- Vibrational modes mix Frenkel exciton and charge-transfer states to enable rapid charge separation.

## Abstract

Excited-state charge transfer underpins organic photovoltaics, photocatalysis and photodetection, but is traditionally thought to require large energy offsets and strong donor–acceptor coupling that can limit device performance. Here, we investigate through-space polymer non-fullerene-acceptor based model heterojunctions in which a perylene diimide acceptor is covalently tethered to a low-bandgap polymer donor. These systems feature an exceptionally small energy offset (< 100 meV) between frontier orbitals, with weak donor–acceptor coupling in the Franck–Condon region. We nevertheless achieve a charge-transfer timescale of ~18 fs. This ultrafast charge-transfer is accompanied via the launch of coherent wavepackets along a high-frequency vibrational coordinate (26 fs period) on the non-fullerene acceptor’s potential energy surface. We uncover specific polymer-centered driving vibrational modes that enable such rapid charge-transfer rates, by mixing Frenkel exciton and charge-transfer states following photoexcitation. Our results demonstrate that ultrafast charge-transfer can be achieved—ultimately limited by high-frequency vibrational periods—even in the absence of large energy offsets or strong ground-state coupling.

Researchers show that charge transfer in organic materials can occur in under 20 femtoseconds without large energy losses. Using model donor–acceptor systems, they reveal how molecular vibrations can drive ultrafast, efficient charge separation.