  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.
