Ultrafast Spectroscopic Signature of Charge Transfer between Single-Walled Carbon Nanotubes and C60

The time scales for interfacial charge separation and recombination play crucial roles in determining efficiencies of excitonic photovoltaics. Near-infrared photons are harvested efficiently by semiconducting single-walled carbon nanotubes (SWCNTs) paired with appropriate electron acceptors, such as fullerenes (e.g., C-60). However, little is known about crucial photochemical events that occur on femtosecond to nanosecond time scales at such heterojunctions. Here, we present transient absorbance measurements that utilize a distinct spectroscopic signature of charges within SWCNTs, the absorbance of a trion quasiparticle, to measure both the ultrafast photoinduced electron transfer time (tau(pet)) and yield (phi(pet)) in photoexcited SWCNT-C-60 bilayer films. The rise time of the trion-induced absorbance enables the determination of the photoinduced electron transfer (PET) time of tau(pet) <= 120 fs, while an experimentally determined trion absorbance cross section reveals the yield of charge transfer (phi(pet) approximate to 38 +/- 3%). The extremely fast electron transfer times observed here are on par with some of the best donor: acceptor pairs in excitonic photovoltaics and underscore the potential for efficient energy harvesting in SWCNT-based devices.