Proceedings Article | 7 March 2007
KEYWORDS: Organic light emitting diodes, Polymers, Waveguides, Refractive index, Interfaces, Excitons, Photons, Electrodes, Absorption, Glasses
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.
