Phonon-assisted ultrafast charge separation in the PCBM band structure

Organic solar cells separate strongly bound electron-hole pairs into free charges at interfaces between electron donor and acceptor organic semiconductors. Recently, electron-hole separation was observed on femtosecond time scales near crystallite phases of the fullerene derivative [6,6]-phenyl-C-71-butyric acid methyl ester (PCBM), which is incompatible with conventional Marcus theories of organic transport. Here we show that ultrafast charge transport in PCBM arises from its broad range of electronic eigenstates, provided by three closely spaced electronic bands near the lowest unoccupied molecular orbital. The highest band provides a charge transfer state resonant with delocalized states of the lower two bands away from the interface. This state acts as a bridge between the donor phase and the acceptor bulk, bypassing the trapped charge-transfer (CT) states below. Vibrational fluctuations enable rapid electronic transitions across this bridge, which can drive the electron more than 4 nm away from the interface within 100 fs. All this is demonstrated within a simple tight-binding Hamiltonian containing transfer integrals no larger than 8 meV.