Coherent ultrafast charge transfer in an organic photovoltaic blend

TLDR

Organic photovoltaic blends of conjugated polymers and fullerene derivatives generate charge via ultrafast electron transfer from the donor polymer to the acceptor fullerene upon light absorption. This work investigates the initial quantum dynamics of that ultrafast charge transfer. Coherent electron transfer with a 25‑fs oscillation period, matching vibrational modes of the fullerene, was observed experimentally and reproduced by first‑principles simulations, showing that vibronic coupling drives charge delocalization and transfer in the noncovalently bound system.

Abstract

Blends of conjugated polymers and fullerene derivatives are prototype systems for organic photovoltaic devices. The primary charge-generation mechanism involves a light-induced ultrafast electron transfer from the light-absorbing and electron-donating polymer to the fullerene electron acceptor. Here, we elucidate the initial quantum dynamics of this process. Experimentally, we observed coherent vibrational motion of the fullerene moiety after impulsive optical excitation of the polymer donor. Comparison with first-principle theoretical simulations evidences coherent electron transfer between donor and acceptor and oscillations of the transferred charge with a 25-femtosecond period matching that of the observed vibrational modes. Our results show that coherent vibronic coupling between electronic and nuclear degrees of freedom is of key importance in triggering charge delocalization and transfer in a noncovalently bound reference system.