We devised a method of modifying indium-tin-oxide (ITO) electrodes for organic electroluminescent devices. It consists of deposition of a nanometer-thick Cu layer on the ITO electrode and an oxygen plasma treatment. By this modification, the surface of the ITO substrate is covered with a partly oxidized Cu layer (CuOx). The CuOx-coated ITO electrode possesses strong hole-injection ability, which leads to lowered operational voltage and high luminance from the devices consisting of tris(8-quinolinato)aluminum and diamine hole-transport layers. The hole-injection ability of the CuOx-coated ITO electrode is better than that of the ITO electrode modified by conventional methods, such as insertion of a Cu-phthalocyanine buffer layer. Moreover, the CuOx layer is effective to improve the durability of the devices.

Light-emitting properties of devices fabricated from a copolymer of p-phenylenevinylene derivatives, 98% 3-[4′-(3,7-dimethyloctyloxy)phenylene]-p-phenylenevinylene and 2% 2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene (MEH–PV), were investigated. The emission efficiency of the devices showed strong oscillatory dependence on the film thickness, and the frequency agreed with that expected from a localized emission near the indium–tin–oxide (ITO) electrode. Although holes are injected from the ITO electrode into the copolymer layer, they are probably trapped by the MEH–PV unit of the copolymer. On the other hand, electrons are easily transported through the copolymer layer. These are probably the reasons for the confinement of the light-emitting zone near the ITO/copolymer interface. The confined light-emitting zone leads to high emission. For comparison, the properties of devices based on a homopolymer were examined. In this case, the emission zone was distributed over a wide region, and the light-emission efficiency was low.

Impedance of space-charge-limited currents in organic light-emitting diodes with double injection and strong recombination A polymer blend approach to fabricating the hole transport layer for polymer light-emitting diodes Appl. Phys. Lett. 84, 3873 (2004); 10.1063/1.1737791Numerical simulations of the electrical characteristics and the efficiencies of single-layer organic light emitting diodesThe majority carriers of polymer-based light-emitting diodes are often considered to be holes. However, we demonstrate that the majority carriers of p-phenylenevinylene-copolymer-based devices, which show very high emission efficiency, are electrons. This conclusion is attained by comparing the current-voltage characteristics of these devices with those of electron-only and hole-only devices. The current due to electrons is proportional to the fourth power of the voltage. This result is interpreted in terms of the space-charge-limited conduction of electrons with field-dependent carrier mobility. The transport of holes in the hole-only devices is also attributed to the same mechanism. The mobility of holes under the normal operational conditions of the devices is estimated to be lower than that of electrons by an order of magnitude. This large difference in the mobility leads to carrier recombination near the interface between the copolymer and the anode, and thus to the high emission efficiency of the light-emitting devices.

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