High Efficiency Composite Metal Oxide-Polymer Electroluminescent Devices: A Morphological and Material Based Investigation (original) (raw)

Polymer-based electronics has developed rapidly over the last decade. In particular, the phenomenon of electroluminescence in conjugated semiconducting polymers spurred wide interest in the field. Successful demonstration of basic optoelectronics devices such as lasers, polymer light emitting diodes (PLEDs), thin film transistors, photovoltaics (PVs), and optical sensors have been realized in research laboratories, and some are already incorporated into commercial applications. However, there is still scope for improvement in terms of device stability and specifically correct choice of charge injecting/transporting layers for LEDs. The conventional PLED structure employs low work function metal electrodes which require hermetical encapsulation since they can not operate in ambient conditions. Even relatively stable Mg-Ag cathodes have been found to degrade gradually in air due to oxidation. [7] State-of-art PLEDs use poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as the hole injecting anode and Ca-Al bilayers as the electron injecting cathode. As an alternative, metal-oxides can be employed as charge transport and injection layers, as has been illustrated for charge collection electrodes in photovoltaic diodes. These metal-oxides have advantages of exceptional stability, mechanical and electrical robustness, low cost, transparency in the visible region, solution processable fabrication and the potential to control the film morphology and interfacial electronic structure through sol-gel and surface chemistry. Composite oxide-polymer based diodes are good substitutes to improve device stability, [11, and as we demonstrate here can compete well with conventional PLED architectures. In addition to being unsusceptible to oxidization, metal oxides also provide good double heterojunction structure for charge carrier confinements. Here, we present a comprehensive study of a variety of metal oxides in mesoporous and compact forms for electron injection in highly luminescent composite oxidepolymer light emitting diodes (COPLEDs). We demonstrate that the morphology of the polymer film cast on the metal oxide surface critically influences the ensuing photoluminescence and electroluminescence efficiency, and that wave-guiding is also extremely effective and causes an enhancement in the angular emission intensity. We report the highest efficiency of 2.8 Cd A À1 employing a compact ZnO electron transport and injection layer, and a MoO 3 hole injection layer. This demonstrates that these metal oxide inter-layers are real contenders to replace conventional low work function metals and conducting polymer electrodes currently employed in emissive polymer devices.