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Unexpectedly Fast Phonon-Assisted Exciton Hopping between Carbon Nanotubes

Davoody, A. H., Karimi, F., Arnold, M. S., Knezevic, I.
The Journal of Physical Chemistry C 2017 v.121 no.24 pp. 13084-13091
carbon nanotubes, energy transfer, momentum, physical chemistry, solar energy
Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics. The hopping rate of excitons between CNTs directly affects the efficiency of these devices. 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. As dark excitons are difficult to probe in experiment, predictive theory is critical for understanding exciton dynamics in CNT composites.