Thrust 2 - Charge and Energy Transfer at Interfaces
The carefully controlled preparation of organic interfaces, achieved in Thrust 1, permits the research of Thrust 2 to make more thorough interpretation of electrical and optical measurements in “real” device structures and using STM and conductive probe AFM techniques.
P3HT mobility in straight nanopores. McGehee has shown that poly-3-hexylthiophene (P3HT) has much larger charge carrier mobility when confined in straight nanopores than when confined in disordered nanopores. The efficiency of straight-nanopore bulk heterojunction solar cells would then greatly benefit from this higher mobility. Miller fabricated templates using block copolymers and reactive ion etching with varying pore-size, periodicity, and pore-depth. Preliminary experiments have so far reproduced earlier results, underscoring the potential for future experiments to deepen our understanding of polymer mobility inside straight pores.
Vertical-nanopore titania/P3HT bulk heterojunction solar cells: McGehee is collaborating with Prof. DeSimone at the University of North Carolina to fabricate vertical-nanopore titania by nanoimprinting sol-gel titania using Teflon-like molds. These titania nanostructures are then used to make vertical-nanopore bulk heterojunction cells. The templating method allows us to vary pore-size, periodicity and pore-depth, while also allowing suitable modifications of the titania-P3HT interface in a separate step.
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Figure 4: Cross-sectional SEM image showing a vertical-pore template into which P3HT can be infiltrated. |
Long-range resonant energy transfer for improved exciton harvesting. McGehee and Frechét are developing a novel route to harvest excitons over long distances to increase efficiencies of solar cell architectures currently limited by poor exciton harvesting. Their approach to overcome limited exciton migration is to use hetero-energy transfer by choosing appropriate energy donor-acceptor pairs with large coulomb-coupling. By relying on the asymmetry in geometry and by properly calculating the rate of exciton transfer from a donor chromophore to an array of energy acceptor chromophores, they have shown both experimentally and theoretically that excitons can be harvested well over 25 nm from the donor-acceptor heterointerface. They are currently screening materials to use this exciting approach to fabricate highly efficient organic photovoltaics.
Field effect transistors based on polydiacetylene monolayers. The chemistry and film preparation achieved in Thrust 1 permits the formation of sheets of conjugated chains extending in parallel between the source and the drain of the FET. Field-effect transistor operation has been observed in these polymer monolayers, the first such demonstration. By varying the channel length in the FETs, Scott and Miller have shown that the current is injection limited. Isocyanide terminated (di)acetylenes, synthesized by Swanson, have been used to improve the coupling of the conjugated polymer backbone to the metal electrodes, with preliminary results giving about 30x increase in current.
STM imaging and current-voltage studies of isocyanide SAMs. Conjugated isocyanide molecules offer an alternative to thiols as coupling groups between organic semiconductors and metal electrodes. Using phenylene di-isocyanides synthesized by Swanson, Liu and Scott have shown, via STM imaging, that monolayers on gold are dense but disordered. These experimental results are in agreement with theoretical studies by Galli (see below) which indicate low energetic barriers to molecular tilt. Current-voltage measurements at selected points on the same films show that conductance is about an order of magnitude higher than in comparable thiol monolayers.
