Inverted Solution Processable OLEDs Using a Metal Oxide as an Electron Injection Contact

摘要

A new type of bottom-emission electroluminescent device is described in which a metal oxide is used asthe electron-injecting contact. The preparation of such a device is simple. It consists of the deposition of athin layer of a metal oxide on top of an indium tin oxide covered glass substrate, followed by the solutionprocessing of the light-emitting layer and subsequently the deposition of a high-workfunction (air-stable)metal anode. This architecture allows for a low-cost electroluminescent device because no rigorousencapsulation is required. Electroluminescence with a high brightness reaching 5700 cd m–2 is observedat voltages as low as 8 V, demonstrating the potential of this new approach to organic light-emitting diode(OLED) devices. Unfortunately the device efficiency is rather low because of the high current densityflowing through the device. We show that the device only operates after the insertion of an additionalhole-injection layer in between the light-emitting polymer (LEP) and the metal anode. A simple modelthat explains the experimental results and provides avenues for further optimization of these devices isdescribed. It is based on the idea that the barrier for electron injection is lowered by the formation of aspace–charge field over the metal-oxide–LEP interface due to the build up of holes in the LEP layer closeto this interface