The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (<1 picosecond) via infrared (IR) optical excitation of electron-hole pairs bound at the heterojunction. Atomistic modeling showed that the IR photons promote bound charge pairs to delocalized band states, similar to those formed just after singlet exciton dissociation, which indicates that such states act as the gateway for charge separation. Our results suggest that charge separation in efficient organic photoconversion systems occurs through hot-state charge delocalization rather than energy-gradient–driven intermolecular hopping.

Ultrafast Spectroscopy

Bakulin, A., Rao, A., Pavelyev, V. G., van Loosdrecht, P. H. M., Pshenichnikov, M. S., Niedzialek, D., … Friend, R. (2012). The Role of Driving Energy and Delocalised States for Charge Separation in Organic Semiconductors. Science, 335(6074), 1340–1344. doi:10.1126/science.1217745