Emission Mechanism of Double-Insulating Organic Electroluminescence Device Driven at AC Voltage

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Abstract
We demonstrate charge recombination electroluminescence (EL) that is observed without charge injection from external electrodes in double-insulated organic EL devices. Double-insulating organic thin-film (EL) devices, indium–tin–oxide electrode (ITO)/polymer insulator layer/ambipolar EL layer/ITO or gold nanoparticles layer/ambipolar EL layer/polymer insulator layer/Al electrode, were fabricated. The ambipolar EL layer was made of a poly(N-vinylcarbazole) (PVK)/2.5-bis(4-naphthyl)-1,3,4-oxadiazole (BND)/coumarine-6 film or a poly-(2-methoxy-5-(2-ethylhexyloxy)-1.4-phenylenevinylene) (MEH-PPV) film. For the polymer insulating layer, a poly-4-vinylphenol (PVP) film, which was formed via spin coating from a 2-propanol solution, was used. ITO nanoparticles and thiol-passivated gold nanopartilces were used as ambipolar charge generation materials. When the devices were driven at ac voltage, a uniform surface emission was observed. Experimental results showed that this emission is due to the recombination of holes and electrons that are generated from ITO or gold nanoparticles embedded in organic layers.