In this study, we investigated the performance characteristics optimized by molybdenum trioxide (MoO3) used as a buffer layer between an anode and a hole transport layer. MoO3, which acts as a carrier ladder, allows for easy hole injection. The small difference between the highest occupied molecular orbital (HOMO) level of N,N'-Bis (3-methylphenyl)-N,N'- diphenylbenzidine (TPD) and the conduction band of MoO3 leads to an easy carrier transfer effect. It was found that when the thickness of MoO3 as a buffer layer was 15 nm, the current density was improved by about 3 times and the luminance by about 3.87 times at a voltage of 7 V, compared to those of a basic device without a buffer layer. The 60% lifetime of the device was improved by a factor of 3.17 and 1.55 at the MoO3 layer thickness of 5 nm and 15 nm, respectively, compared to that of the basic device. It was found that when the thickness of MoO3 is more than 15 nm, the thickness of MoO3 acts as a resistance component, thus reducing the current density and luminance characteristics. The electrical and optical characteristics of the device were determined by varying the thickness of the hole-transport layer TPD to be 25, 35, and 55 nm with the fixed layer thickness of MoO3 to 15 nm which showed a high current density and luminance. The current density, luminance, and lifetime of the organic light-emitting diodes were improved by about 2.6 times, 3 times, and 5.5 times, respectively, at the TPD layer thickness of 25 nm and the MoO3 layer thickness of 15 nm.

Keywords: Organic Light-Emitting Diodes, Buffer layer, Molybdenum trioxide.