Abstract
We report an experimental and theoretical study of the effects of interference in polymeric light-emitting diodes (LEDs). These effects are due to the complex optical structures of the devices, which include many layers of materials with different refractive indices, and are of considerable importance since they affect spectral distribution and intensity of the absorption and emission in a significant way. By way of comparison, they can also provide a flexible, noninvasive optical probe of the electroluminescent processes, such as, for example, the spatial distribution of the recombination inside the LED. In this paper we analyse single-layer diodes with indium-tin oxide (ITO) and Al electrodes, where poly( p-phenylene vinylene) (PPV) is the luminescent polymer. We find that photo-induced excitation of the radiative species produce different spectral shapes depending on the excitation energy (and hence on the profile of excited chromophores) which we can describe in terms of interference phenomena. The theoretical analysis is conducted by means of multilayer stack theory and transfer matrix calculations, and takes into account additional quenching effects due to In contaminations from the ITO electrode. The theoretical results are in good agreement with the experiment.
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