Photophysics of Conjugated Polymer Thin Films and Semiconducting Single-Walled Carbon Nanotubes

1. Photophysics of Conjugated Polymer Thin Films and Semiconducting Single-Walled Carbon Nanotubes Sumit Mazumdar, University of Arizona, DMR 0705163 Triplet Excitations in Carbon Nanostructures: We show that the energy difference ST between the lowest optical singlet and the lowest triplet excitons in semiconducting single-walled carbon nanotubes (S-SWCNTs) with diameter ~ 1 nm and graphene nanoribbons (GNRs) with widths ~ 2 nm are more than an order of magnitude smaller than in the -conjugated polymers such as poly(para-phenylenevinylene) (PPV) (see Table 1). Figures 1 (a) and (b) present the spatial probability distributions for an electron and a hole being on atoms i and j (horizontal and vertical axes) in a PPV chain for a singlet and a triplet excitons, respectively. Figures 1 (c) and (d) are the equivalents of Figures 1 (a) and (b) for the (21,1) (M. Ezawa, Phys. Rev. B 73, 045432 (2006)) armchair graphene nanoribbon (AGNR). The spatial extent of the triplet exciton is nearly the same as that of the singlet exciton in AGNRs (and S-SWCNTs), in contrast to that in PPV, in which the triplet exciton exhibits strong spatial confinement. d in all cases is the average exciton size. K. Aryanpour, S. Mazumdar and H. Zhao, submitted to Nano Lett. arXiv: 1108.3880 Table 1 Figure 1

2. Photophysics of Conjugated Polymer Thin Films and Semiconducting Single-Walled Carbon Nanotubes Sumit Mazumdar, University of Arizona, DMR 0705163 Broader Impacts Education: One graduate student, two post-doctoral fellows, and four undergraduate students were trained in the program. The graduate student completed his Ph.D. and following a postdoctoral research position is currently employed as an assistant professor at the University of Miami. Impact on Organic Photovoltaics: The small ST in S-SWCNTs and AGNRs may have practical applications in the area of Photovoltaics. Intersystem crossing in systems with small ST is expected to be rapid, in which case a significant fraction of the optical excitations may end up in the spin triplet states with long lifetimes. Enhanced photoinduced charge-transfer from S-SWCNTs and AGNRs to acceptor molecules is therefore a distinct possibility. Impressive performance in blends of S-SWCNTs and C60, with S-SWCNTs as the donor materials has already been obtained. It is conceivable that spin-triplet participation is behind this dramatic improvement. ‏Enhanced photoinduced charge-transfer may be possible from triplet excitons with long lifetimes, giving higher performance for solar cells