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 (∼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.
The effective workfunction of Al doped ZnO films (AZO) increased from 4.1 eV to 5.55 eV after surface modification with nanoscale molybdenum sub-oxides (MoOx). Hole only devices with anodes consisting of 3 nm of MoOx on AZO exhibited a lower turn-on voltage (1.5 vs 1.8 V), and larger charge injection (190 vs 118 mA/cm2) at the reference voltage, compared to indium tin oxide (ITO). AZO devices with 10 nm of MoOx exhibited the highest workfunction but performed poorly compared to devices with 3 nm of MoOx, or standard ITO. Ultraviolet photoelectron, X-ray photoelectron, and optical spectroscopies indicate that the 3 nm MoOx films are more reduced and farther away from MoO3 stoichiometry than their 10 nm equivalents. The vacancies associated with non-stoichiometry result in donor-like gap states which we assign to partially occupied Mo 4d levels. We propose that Fowler-Nordheim tunneling from these levels is responsible for the reduction in threshold voltage measured in devices with 3 nm of MoOx. A schematic band diagram is proposed. The thicker MoOx layers are more stoichiometric and resistive, and the voltage drop across these layers dominates their electrical performance, leading to an increase in threshold voltage. The results indicate that AZO with MoOx layers of optimal thickness may be potential candidates for anode use in organic light emitting diodes.
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