The influence of MoOx gap states on hole injection from aluminum doped zinc oxide with nanoscale MoOx surface layer anodes for organic light emitting diodes

Abstract

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.

Investigation of MoOx/n‐Si strong inversion layer interfaces via dopant‐free heterocontact

“…Oxygen loss during MoO x deposition results in oxygen vacancies leading to the oxidation transitions in Mo and to generation of gap states by partial filling of the unoccupied 4d orbitals of molybdenum atoms . G 1 is associated with singly occupied Mo 4d orbitals, while G 2 feature is associated with doubly occupied Mo 4d orbitals . Previous works have already reported that gap states can be induced by interaction between MoO x layer and inorganic substrates or organic molecules .…”

Surface Modification of Al-Doped ZnO Transparent Conducive Thin Films with Polycrystalline Zinc Molybdenum Oxide

“…Li et al increased the WF of a Ga-doped ZnO (GZO) thin film from 3.29 to 4.87 eV through surface modification with a phosphonic acid of F 5 PPA; however, it has not been applied to any devices, and its effect on a device is still unknown. Finally, Jha et al proposed the surface modification of AZO with physisorption of transition-metal oxides (e.g., MoO 3– x , WO 3– x , or V 2 O 5– x ) and initially applied them to OLEDs to improve the hole injection efficiency. − Although a WF of 5.55 eV was achieved by utilizing the high WF characteristic of MoO 3– x , the large resistivity (10 5 to 10 6 Ω·cm) of MoO 3– x significantly increased the series resistance of OLEDs, resulting in an increase of ca. 30% in the threshold voltage .…”

“…Finally, Jha et al proposed the surface modification of AZO with physisorption of transition-metal oxides (e.g., MoO 3– x , WO 3– x , or V 2 O 5– x ) and initially applied them to OLEDs to improve the hole injection efficiency. − Although a WF of 5.55 eV was achieved by utilizing the high WF characteristic of MoO 3– x , the large resistivity (10 5 to 10 6 Ω·cm) of MoO 3– x significantly increased the series resistance of OLEDs, resulting in an increase of ca. 30% in the threshold voltage . This indicates that the high-resistance MoO 3– x modifier increases the surface WF of AZO at the cost of increasing the series resistance of the device, resulting in the deterioration of the device performance.…”

“…30% in the threshold voltage. 30 This indicates that the high-resistance MoO 3−x modifier increases the surface WF of AZO at the cost of increasing the series resistance of the device, resulting in the deterioration of the device performance. Consequently, it is imperative to develop a high WF and low-resistance surface modifier to improve the WF of AZO without increasing the series resistance of the device.…”