Electro-optical performance of molybdenum oxide modified aluminum doped zinc oxide anodes in organic light emitting diodes: A comparison to indium tin oxide

Organic light emitting diodes (OLEDs) have been extensively researched for applications in solid state lighting and emissive flat panel displays. Along with improved device lifetime and efficiency, significant cost reductions are required for OLED technology to find widespread use. The transparent conducting oxide (TCO) anode, specifically indium tin oxide (ITO), is a significant contributor to device cost. Low cost alternatives to ITO such as aluminum doped zinc oxide (AZO) are therefore of technological interest. Basic bilayer OLEDs employing AZO surface modified with approximately 3 nm of molybdenum sub-oxide (MoO _x ) anodes (AZO/3 nm MoO _x ) exhibited peak luminance (L), power efficiency (PE), luminous efficiency (LE), and external quantum efficiency (EQE) of 8959 Cd/m², 3.3 lm/W, 4.1 Cd/A, and 1.4%, respectively. The corresponding values for devices with standard ITO anodes were 8080 Cd/m², 2.9 lm/W, 3.1 Cd/A, and 1.0%, respectively. In all devices studied, tris(8-hydroxyquinolinato)aluminum (AlQ₃) was the emissive and electron-transport layer, and N,N'-bis(naphthalene-1-yl)– N,N'-bis(phenyl)-benzidine (NPB) was the hole transport layer. Ultraviolet photoelectron spectroscopy (UPS) measurements revealed an effective workfunction of 4.8 eV for the composite AZO/MoO _x anode, compared to 4.6 eV for ITO. Further, UPS of the MoO _x revealed the presence of a defect band below the fermi level, which is due to the filling of Mo 4d states, and indicates the presence of Mo⁴⁺, Mo⁵⁺ and compensating oxygen vacancies. These donor-like gap states in MoO _x lower the hole injection barrier from AZO, which in turn reduces the turn-on voltage compared to as-deposited AZO (i.e., neat AZO). The improved device performance compared to ITO is ascribed to better charge balance.