Energy Focus: PV composites with enriched semiconducting SWNTs display prolonged charge separation
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+V Electrodes +V
−V QD
UV exposure of liquid photoresist
EOFC manipulation −V PDMS Glass slide cover
375 nm UV laser beam
A microfluidic device for precisely positioning quantum dots on a surface using electroosmotic flow control (EOFC). A UV laser is used to polymerize the fluid and trap individual quantum dots in place. Reproduced with permission from Nano Lett. 10 (11) (2010) DOI: 10.1021/nl1029557; p. 4673. © 2010 American Chemical Society.
Energy Focus PV composites with enriched semiconducting SWNTs display prolonged charge separation
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urrent organic photovoltaic (OPV) solar cells utilize bulk heterojunctions (BHJs) comprised of conjugated polymers acting as electron donors, and derivatized fullerenes acting as electron acceptors. With electron mobility dependent on fullerene clustering and hopping transport, solar power conversion efficiencies (PCEs) now approach 8%. The properties of single-walled carbon nanotubes (SWNTs) have led to speculation that superior PCEs would result if they were to replace the fullerene phase in OPV solar cells. However, the maximum PCE achieved by SWNT-composite solar cells is only about 0.5%. This low performance has been hypothesized to be due, in part, to the relatively large fraction of metallic (m-) nanotubes in as-produced samples—about one-third, with the remainder semi-conducting (s-) nanotubes— because m-SWNTs lack a true bandgap and can therefore act as recombination centers for charge-carriers or excitons, lowering charge-separation efficiency. One obstacle to a systematic investi-
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MRS BULLETIN
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VOLUME 36 • JANUARY 2011
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precise, sub-pixel, imaging algorithm, a specific QD can be manipulated into position with more and more slight adjustments to the flow, while other QDs drift in a divergent fashion. When the QD has reached the desired location, the spot is automatically irradiated with a UV laser, which causes cross-linking of the photoresist around the QD and effectively encapsulates it. The nature of the photoresist fluid, comprised of a photoinitiator, a water-soluble acrylic monomer, and a viscosity modifier, is crucial to the process. Using high concentrations of the monomer causes segregation of the QDs near the glass surface, providing an element of control over the third dimension and enhancing the actuation effect. Immobilizing the QDs allows succes-
sive objects to be moved without disturbing them, and introduces the possibility of surface patterning. The team demonstrates this, as well as the ability to spectrally select for QDs, by organizing a 3 × 3 array of alternating color QDs, 5 μm apart. The technique could easily be extended to positioning any imageable nanoparticle on water-compatible surfaces, and should prove to be a powerful tool to the nanotechnologist. Tobias Lockwood
gation of this hypothesis has been the controllable incorporation of only m-SWNTs or s-SWNTs into photovoltaic blends with semiconducting polymers. Recently, however, J. Blackburn, J. Holt, and a team of researchers at the National Renewable Energy Laboratory pr
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