Properties of organic light-emitting devices with a rubrene sub-monolayer inserted between electron- and hole-transport layers

Organic light-emitting devices with a rubrene sub-monolayer were fabricated. When the rubrene sub-monolayer was inserted at the interface of Al-hydroxyquinoline complex and diamine layers, the devices showed advantageous properties. They showed high electroluminescence efficiency, and the efficiency was high even at low cur-rent densities. The efficiency showed strong oscillation depending on the thickness of the organic layer. Furthermore, the current-voltage curve of the device shifted toward a lower voltage than that of a device without the rubrene layer. These properties are discussed in terms of hole trapping by rubrene molecules existing at the interface of the organic layers, and confinement of light emission in a very narrow region.