Effect of the top electrode work function on the rectification ratio of light-emitting diodes (LEDs) based on poly(3-octylthiophene)

Abstract

Electroluminescence efficiency and current-voltage characteristics of conjugated polymer poly(3-octylthiophene) (P3OT) light emitting diodes (LEDs) are discussed in terms of the presently popular band model. Devices were studied in forward and reverse mode of operation, with different types of top contacts to show the dominance of the top electrode workfunction on device properties. Changing the workfunction of the top contact with only 0.9 eV (from Au to Al) results in a change in the preferred current direction, corresponding to a change in the rectification ratio of the diodes of effectively 6 orders of magnitude. When Au is used as a top electrode, no electroluminescence was observed, because the barrier for electron injection is too high. The observation of light emission in the reverse mode of operation for ITO/P3OT/Al is a result of direct carrier tunneling into the transport bands at high electric fields.