Low driving voltage and long lifetime organic light-emitting diodes with molybdenum oxide (MoO3) doped hole transport layer

Abstract

We report low-operating-voltage, long-lifetime organic light-emitting diodes (OLEDs) with a molybdenum-oxide (MoO$_3$)-doped N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine ($\alpha$-NPD) layer between the indium tin oxide and the $\alpha$-NPD. The current density-voltage-luminance characteristics and the lifetime were studied for various MoO$_3$ doping concentrations and thicknesses. The devices with $\alpha$-NPD layers of various MoO$_3$ doping concentrations and thicknesses exhibited lower driving voltage and higher power efficiency compared with the undoped devices. Furthermore, the device with MoO$_3$-doped hole transport layer showed a longer lifetime and a much reduced operational voltage variation. The optical absorption spectra of the MoO$_3$-doped $\alpha$-NPD layer showed two new peaks, around 500 and 1400 nm, indicating that charge transfer complexes had formed between MoO$_3$ and $\alpha$-NPD. The p-type doping of $\alpha$-NPD by MoO$_3$ improved hole injection. In addition, AFM images show that the morphology of the MoO$_3$-doped $\alpha$-NPD layer improved with increasing MoO$_3$ doping concentration. Therefore, the improved device performance can be attributed to the higher hole-injection efficiency and the improved morphology of the MoO$_3$-doped $\alpha$-NPD layer.