Interface properties of a Li3PO4/Al cathode in organic light emitting diodes

Abstract

Recently Li3PO4/Al has been introduced as an alternative cathode for the commonly used LiF/Al system for organic light emitting diodes (OLEDs) due to its competitive electron injection properties. In the present article the interfaces of the organic semiconductor with the Li3PO4/Al bilayer cathode are investigated using photoelectron spectroscopy to elucidate the origin behind the efficient electron injection. Therefore, a thick Li3PO4 layer was vacuum deposited onto an indium tin oxide substrate and characterized in order to learn about the stoichiometry of evaporated Li3PO4. During evaporation Li3PO4 decomposes, forming a layer consisting of P2O5 and LiPO3. In a second step the interface between Li3PO4 and Alq3 [tris(8-hydroxyquinoline) aluminum] was investigated, whereupon Li3PO4 coverage Alq3 molecules decompose, forming aluminum oxide or aluminum phosphate leaving 8-quinolinol molecules behind. A similar reaction occurs at the Li3PO4/Al interface where again an oxidation of the metallic aluminum points toward the formation of aluminum oxide or phosphate. A work function lowering of up to 180 meV observed once Al was covered by Li3PO4 is likely to be caused by the polar nature of all generated or deposited species. A simple estimate of the surface potential drop caused by those dipoles can account for the work function lowering and explain the highly efficient electron injection property of the Li3PO4/Al cathode in an OLED.