Inverted Solution Processable OLEDs using a metal oxide as electroninjection contact

摘要

A novel class of bottom emission electroluminescent device is described in which a metal oxide is used as the electron injecting contact. The preparation of such a device is simple, and consists of the deposition of a thin layer of a metal oxide on top of an indium tin oxide covered glass substrate, followed by the solution processing 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 as no rigorous encapsulation is required. Electroluminescence with a high brightness level reaching 6500 cd/m~2 is observed at voltages as low as 8 V, demonstrating the potential of this new approach to OLED devices. Unfortunately the device efficiency is rather low caused by the high current density flowing through the device. We show that the device only operates after the insertion of an additional hole injection layer in between the light emitting polymer and the metal anode. A simple model that explains the observed experimental results and provides avenues for further optimization of these devices is described. It is based on the idea that the barrier for electron injection is lowered by the formation of the space charge field over the metal oxide-light emitting polymer interface due to the build up of holes in the light emitting polymer layer.