Publication Date:
2015-08-12
Description:
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 (MoO x ). Hole only devices with anodes consisting of 3 nm of MoO x on AZO exhibited a lower turn-on voltage (1.5 vs 1.8 V), and larger charge injection (190 vs 118 mA/cm 2 ) at the reference voltage, compared to indium tin oxide (ITO). AZO devices with 10 nm of MoO x exhibited the highest workfunction but performed poorly compared to devices with 3 nm of MoO x , or standard ITO. Ultraviolet photoelectron, X-ray photoelectron, and optical spectroscopies indicate that the 3 nm MoO x films are more reduced and farther away from MoO 3 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 MoO x . A schematic band diagram is proposed. The thicker MoO x 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 MoO x layers of optimal thickness may be potential candidates for anode use in organic light emitting diodes.
Print ISSN:
0021-8979
Electronic ISSN:
1089-7550
Topics:
Physics
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