7 March 2007 Light extraction from OLEDs: the waveguide perspective
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Organic light emitting diodes are deemed to be valuable light sources for displays and general illumination in the future. In the last decade remarkable progress was made concerning materials, lifetime, and production techniques. One of the remaining challenges is to increase the light extraction efficiency of these devices. Due to the internal structure of the diodes, the light emitted by electron-hole recombinations within the electro-luminescent layer may be transferred into several channels: to slab guided modes, to substrate guided modes, and, to some limited extent only, to air propagating modes, directly. In order to get some physical insight where and why photons may be trapped in the slab geometry, a modal analysis is helpful. In configurations where metallic materials are used, the transversal magnetic light is prone to be annihilated by coupling to surface-plasmon polaritons which may have propagation length of a few microns only. A high index of refraction of the emitting material and an indium-tin-oxide electrode, eventually,contribute to light confinement within guided modes for both polarisations. In order to evaluate what parts of the dipole-like emission are transferred to what channel several methods can be used, e.g. the Finite-Difference-Time-Domain-Method, a rigorous coupled wave analysis including internal sources, and an approach based on Green-functions. Although the latter method is restricted to a plane geometry, it is a quite fast tool, which is well suited for the optimisation of a layered structure. To maximize the emission of the organic diodes, two basic strategies are self-evident. The first is to aim at a restricted coupling to guided waves. The second one addresses the recycling of light from unavoidable guided modes to air propagating modes. This can be achieved e.g. by scattering, by gratings, and by lens arrays, respectively.
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Norbert Danz, Dirk Michaelis, and Christoph Wächter "Light extraction from OLEDs: the waveguide perspective", Proc. SPIE 6475, Integrated Optics: Devices, Materials, and Technologies XI, 64750J (7 March 2007); doi: 10.1117/12.700569; https://doi.org/10.1117/12.700569

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