Improved hole injection and performance of multilayer OLED devices via electrochemically prepared-polybithiophene layers

Stable and neutral polybithiophene (PbT) thin layers with well-defined morphology and a high degree of smoothness have been prepared by electropolymerization of 2,2'-bithiophene. This semiconducting contact layer on an ITO anode improves remarkably hole injection between ITO and the naphthyl-substituted benzidine derivative (NPB) hole-transport layer. Vapor-deposited NPB/Alq3 bilayer devices with the incorporated injection layer show a sharp reduction of the operating voltage (about 4 V), as well as an enhanced luminous efficiency and luminance, compared to the NPB/Alq3 structure without a PbT layer. It is demonstrated that the use of an electropolymerization step in the production of vapor-deposited small molecular OLEDs is an alternative approach to improve the performance of OLEDs.