The Li3PO4/Al bilayer: An efficient cathode for organic light emitting devices

In this contribution an efficient cathode material for organic light emitting diodes (OLEDs) is introduced consisting of a thin layer of the metal salt lithium phosphate (Li3PO4) deposited between the organic semiconductor and an Al cathode. The bilayer cathode Li3PO4/Al enables a device performance of small molecule based OLEDs competitive to the benchmark cathode LiF/Al. While current densities and luminances of both systems are alike, the use of Li3PO4 substantially increases the device lifetime. It will be shown that the improved device characteristics can be ascribed to a stably enhanced electron injection. We demonstrate that neither a field enhancement across the Li3PO4 layer due to accumulated holes nor a possible charge transfer doping by the Li3PO4 is the reason for the improved electron injection. Investigation of the Li3PO4/Al interface by Kelvin probe techniques disclosed a work function lowering of the cathode that facilitates electron injection and finally explains the improvement in performance of respective organic light emitting diodes. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3112005]