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  • 2000-2004  (2)
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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 1073-1081 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electroluminescence (EL) pattern emitted through the surface and edge of the glass substrate of two efficient polymer light-emitting diodes (LEDs) has been characterized. The surface emission is nearly Lambertian, while the edge emission comprises discrete substrate reflection and leaky waveguide modes. A simple "half-space" optical model that accounts for optical interference effects of the metal cathode–reflector is developed to extract the location and orientation of the emitting dipoles from these patterns. Numerical simulations for a range of polymer and metal refractive indices show that the surface out-coupling efficiency ξ of the internally generated photons can be greater than the 0.5 n−2 relation (where n is the refractive index of the emitter layer) valid for isotropic emitters that are not subjected to optical interference effects. When the emitting dipoles are optimally located for maximum rate of surface emission, the model predicts ξ to vary as 0.75 n−2 for the isotropic case, and as 1.2 n−2 for the in-plane case. For our LEDs, we found that the EL arises from in-plane dipoles that are on average almost optimally located away from the cathode. Using this result, the internal EL quantum yield is estimated to be close to 50% of the free-space photoluminescence yield of the emitter for the devices. This indicates excellent injection balance and recombination efficiency of the charge carriers. By also taking into account of optical interference effects on the radiative rate, we deduced that the lower limit for the probability of forming an emissive singlet exciton from electrical injection is 35%–45% in these conjugated polymers. This greatly exceeds the 25% probability from spin-degeneracy statistics. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 77 (2000), S. 2500-2502 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Colloidally grown CdSe nanocrystals with epitaxial ZnS shells show highly efficient, size-tunable luminescence. We report the incorporation of films of these core-shell nanocrystals into wavelength-scale, high-Q, planar microcavities. Under optical excitation, we find that emission from the nanocrystals couples to the discrete optical modes of the microcavity. The broad free-space emission spectrum of the nanocrystals is modified by the presence of the microcavity, giving a series of sharp emission lines with wavelengths determined by the cavity dimension. Our experiments demonstrate that microcavities with semiconductor emitters can be conveniently fabricated using spin-coating techniques. We find that, at room temperature, the microcavity emission spectrum is independent of excitation intensity for excitation densities up to approximately one electron–hole pair per nanocrystal. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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