Ultrafast Phonon-assisted Exciton Transfer in Carbon Nanotube Films*

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics due to their tunable optical bandgap and high optical. However, the hopping rate of excitons between CNTs directly related to the efficiency of these devices. Here, we theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (∼1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons with large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. These results are important as optically inactive excitons are invisible in the most experiments so one needs to complement measurements with theoretical calculations in order to get a complete picture of exciton dynamics in these types of nanostructures.