Organic-Oxide Cathode Buffer Layer in Fabricating High-Performance Polymer Light-Emitting Diodes

Spin-casting or thermal evaporation in vacuum of a salt-free, neutral, organic-oxide ultra-thin film as a buffer layer with an aluminum (Al) cathode has become an alternative approach for fabricating high-performance organic and polymer light-emitting diodes (O/PLEDs). [Guo et al., Appl. Phys. Lett. 2006, 88,113501 and Appl. Phys. Lett. 2006, 89, 053507] The electroluminescence efficiency of phenyl-substituted poly(para-phenylene vinylene) copolymer-based PLEDs is 0.16 cd A(-1) when Al is used as the device cathode, but is approximately two orders of magnitude higher, 14.53 cd A(-1), when an organic oxide/Al composite cathode is used. The polymer/metal junction in PLEDs with and without depositing an ultra-thin organic oxide interlayer is studied by X-ray photoelectron spectroscopy. Experimental results indicate that the deposition of an Al electrode causes the oxidation at the surface of the light-emissive polymer layer. Introducing an organic-oxide cathode buffer layer suppresses the oxidation and the diffusion of the Al atoms into the functional polymer layer. The formation of a carbide-like (negative carbon) thin layer, which accompanies interfacial interactions, is critical to the injection of electrons through the Al cathode. The balanced charge injection is responsible for the substantially improved device performance. This process is specific to the organic oxide/Al interface, as revealed by a comparison with similar device configurations that have Ag as the electrode, in which no significant interaction in the interface is observed.