Enhancement of current injection in inverted organic light emitting diodes with thermal annealing

The enhancement of current density in inverted organic light emitting diodes is achieved by thermal annealing after device process. The current-voltage characteristics of annealed devices are improved as compared to that of nonannealed devices. The current improvement is attributed to the increase in electron injection efficiency from the inverted cathodes. X-ray and ultraviolet photoemission spectroscopy were also carried out to investigate the origins of the enhancement. The result shows that the activation of doping effect of the inverted Alq(3) trilayers occurs after thermal annealing. However, the current density is still not compatible to that of normal devices with Alq3 trilayers as the cathode on the top. The reason is found to be related to the oxidation of aluminum cathodes, which are deposited first in the inverted devices. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3512911]