The role of interfaces in polymeric light-emitting diodes

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Abstract
Langmuir–Blodgett films of poly(3-hexylthiophene) and poly(methyl methacrylate) have been used to prepare organic dc and ac light-emitting diodes(LEDs). The transient as well as steady state properties of the LEDs have been studied. In order to explain electroluminescence(EL), charge transport mechanisms, and the role of interfaces in organic LEDs, transient EL, luminance–voltage and luminance–current measurements have been performed both for single layer and multilayer devices. The transient measurements were performed by applying a single square-wave voltage pulse or a square-wave ac voltage. We have shown that the time delay observed in transient ELmeasurements of single layer LEDs is mainly due to accumulation of charge carriers and not due to carrier transit times. The interfaces were found to have a major role on the operation of organic LEDs.