Phonon-Assisted Ultrafast Charge Separation in a Realistic PCBM Aggregate

TL;DR

This study reveals that ultrafast charge separation in PCBM organic solar cell acceptors is driven by vibrationally enabled transitions among delocalized electronic bands, leading to rapid electron transport within 100 femtoseconds.

Contribution

It demonstrates that vibrational fluctuations facilitate ultrafast charge separation in PCBM by enabling transitions among delocalized electronic states, a process modeled with a realistic tight-binding Hamiltonian.

Findings

Electron transport occurs within 100 fs over ~3 nm.

Vibrational fluctuations enable rapid inter-band transitions.

Ultrafast charge separation is driven by electronic band structure.

Abstract

Organic solar cells must separate strongly bound electron-hole pairs into free charges. This is achieved at interfaces between electron donor and acceptor organic semiconductors. The most popular electron acceptor is the fullerene derivative PCBM. Electron-hole separation has been observed on femtosecond timescales, which is incompatible with conventional Marcus theories of organic transport. In this work we show that ultrafast charge transport in PCBM arises from its broad range of electronic eigenstates, provided by the presence of three closely spaced delocalised bands near the LUMO level. Vibrational fluctuations enable rapid transitions between these bands, which drives an electron transport of $\sim$3 nm within 100 fs. All this is demonstrated within a realistic tight binding Hamiltonian containing transfer integrals no larger than 8 meV.