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Chirality Specific Triplet Exciton Dynamics in Highly Enriched (6,5) and (7,5) Carbon Nanotube Networks

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journal contribution
posted on 01.08.2016, 00:00 by Abasi Abudulimu, Florian Spaeth, Imge Namal, Tobias Hertel, Larry Lüer
Single-walled carbon nanotubes (SWNTs) show high aspect ratio, thermal and chemical stability as well as charge mobility, and therefore appear ideally suited for improving charge extraction in organic photovoltaic (OPV) devices. Since typical charge extraction times in OPV devices are in the microsecond range, the interplay of the desired charged states with long-lived neutral states in SWNTs such as triplet excitons becomes important. Triplet excitons have recently been investigated in (6,5) SWNTs with an optical yield close to 32%. Here, we present transient absorption (TA) dynamics of (6,5)- and (7,5)-rich SWNT networks from the femtosecond to microsecond time scale. Comparing our TA spectra to results from a recent spectroelectrochemical study allows us to distinguish between contributions from charged and neutral photoexcitations. For long pump–probe delay times we identify an excess photobleach which we use as a chirality specific probe for the density of triplet excitons. We show that triplet energy transfer occurs between the (6,5) and the (7,5) chirality with a transfer time of about 70 ps. In contrast, no evidence of triplet exciton transfer is observed between (7,5) and (8,4) tubes. We find that the triplet yield is reduced at higher excitation densities, which points to singlet excitons as precursors for triplet states.