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Search: WFRF:(Friend R. H.) > (2010-2014)

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1.
  • Howard, Ian A., et al. (author)
  • Charge Recombination and Exciton Annihilation Reactions in Conjugated Polymer Blends
  • 2010
  • In: Journal of the American Chemical Society. - 0002-7863. ; 132:1, s. 328-335
  • Journal article (peer-reviewed)abstract
    • Bimolecular interactions between excitations in conjugated polymer thin films are important because they influence the efficiency of many optoelectronic devices that require high excitation densities. Using time-resolved optical spectroscopy, we measure the bimolecular interactions of charges, singlet excitons, and triplet excitons in intimately mixed polyfluorene blends with band-edge offsets optimized for photoinduced electron transfer. Bimolecular charge recombination and triplet−triplet annihilation are negligible, but exciton−charge interactions are efficient. The annihilation of singlet excitons by charges occurs on picosecond time-scales and reaches a rate equivalent to that of charge transfer. Triplet exciton annihilation by charges occurs on nanosecond time-scales. The surprising absence of nongeminate charge recombination is shown to be due to the limited mobility of charge carriers at the heterojunction. Therefore, extremely high densities of charge pairs can be maintained in the blend. The absence of triplet−triplet annihilation is a consequence of restricted triplet diffusion in the blend morphology. We suggest that the rate and nature of bimolecular interactions are determined by the stochastic excitation distribution in the polymer blend and the limited connectivity between the polymer domains. A model based on these assumptions quantitatively explains the effects. Our findings provide a comprehensive framework for understanding bimolecular recombination and annihilation processes in nanostructured materials.
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2.
  • Yim, Keng-Hoong, et al. (author)
  • Phase-Separated Thin Film Structures for Efficient Polymer Blend Light-Emitting Diodes
  • 2010
  • In: NANO LETTERS. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 10:2, s. 385-392
  • Journal article (peer-reviewed)abstract
    • We report laterally and vertically phase-separated thin film structures in conjugated polymer blends created by polymer molecular weight variation. We find that micrometer-scale lateral phase separation is critical in achieving high initial device efficiency of light-emitting diodes, whereas improved balance of charge carrier mobilities and film thickness uniformity are important in maintaining high efficiency at high voltages. The optoelectronic properties of these blend thin films and devices are strongly influenced by the polymer chain order/disorder and the interface state formed at polymer/polymer heterojunctions.
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