Photoexcitation Dynamics of Coupled Semiconducting\nCarbon Nanotube Thin Films

Abstract

Carbon nanotubes are a promising\nmeans of capturing photons for\nuse in solar cell devices. We time-resolved the photoexcitation dynamics\nof coupled, bandgap-selected, semiconducting carbon nanotubes in thin\nfilms tailored for photovoltaics. Using transient absorption spectroscopy\nand anisotropy measurements, we found that the photoexcitation evolves\nby two mechanisms with a fast and long-range component followed by\na slow and short-range component. Within 300 fs of optical excitation,\n20% of nanotubes transfer their photoexcitation over 5–10 nm\ninto nearby nanotube fibers. After 3 ps, 70% of the photoexcitation\nresides on the smallest bandgap nanotubes. After this ultrafast process,\nthe photoexcitation continues to transfer on a ∼10 ps time\nscale but to predominantly aligned tubes. Ultimately the photoexcitation\nhops twice on average between fibers. These results are important\nfor understanding the flow of energy and charge in coupled nanotube\nmaterials and light-harvesting devices.