Mechanisms Of Azo Workfunction Tuning For Anode Use In Oleds: Surface Dipole Manipulation With Plasma Treatments Versus Nanoscale Wox And Vox Interfacial Layers

Zinc oxide (ZnO) and aluminum doped zinc oxide (AZO) are potential low cost alternative anodes to indium tin oxide (ITO) for organic light emitting diodes (OLEDs). However, their smaller workfunctions compared to ITO lead to higher hole injection barriers, and methods of tuning their workfunctions are of significant technological interest. Ultraviolet and x-ray photoelectron spectroscopies together with density functional theory based first principles calculations indicate that with CFx plasma treatments, increased workfunction can be achieved by -CF3 or -F adsorption on ZnO surfaces, due to creation of a surface dipole moment with electron transfer to F. Modification of AZO surfaces with nanoscopic (similar to 3 nm) VOx and WOx layers yielded workfunction increases due to the larger workfunction of the add-layers. Deviations from stoichiometry (oxygen vacancies) result in reduced metal cations (W5+, W4+, V4+, and V3+), leading to partial filling of the metal d band, and formation of associated gap states. Current-voltage characterization of hole-only devices reveals that the increased workfunction of the surface modified anodes facilitated improved band alignment and hole injection compared to as-deposited AZO. The luminous efficiency (LE), power efficiency (PE), and external quantum efficiency (EQE) of OLEDs with AZO/WOx anodes were 62%, 100%, and 85% better than ITO. OLEDs with AZO/VOx anodes exhibited 62%, 75%, and 85% better LE, PE, and EQE, than ITO, respectively. The enhanced performance is ascribed to improved hole injection, charge balance, and radiative recombination efficiency. Thus, the results describe two physical mechanisms by which the workfunction of inexpensive alternatives to ITO can be tuned to yield comparable or enhanced performance. Published by AIP Publishing.