The Li3PO4/Al electrode: An alternative, efficient cathode for organic light-emitting diodes

In this contribution an alternative, efficient cathode for organic light-emitting diodes (OLEDs) is introduced consisting of a sub-nanometer thin lithium phosphate (Li3PO4) layer capped with aluminium. This cathode is competitive to the LiF/Al state-of-the-art cathode and performs equally well in small molecule and polymer OLEDs but yields a higher device lifetime. The functional principle of the Li3PO4/Al cathode is based on the in situ formation of a dipolar interface. Partly oriented AlPO4 dipoles are the reason for a potential drop at the interface and a work function lowering of about 500 meV. The work function of the Li3PO4/Al cathode is found to be comparable to the work function of calcium resulting in a promotion of electron injection. As the reaction products of the interface reaction are environmentally stable and chemically attached to the interface diffusion of these species into the organic layer is unlikely. This might be the reason for the observed enhanced device lifetime for devices with a Li3PO4/Al cathode.