Improvement of Performance of Organic Light-Emitting Diodes with Both a MoO3 Hole Injection Layer and a MoO3 Doped Hole Transport Layer**Supported by the National Natural Science Foundation of China under Grant No 11274402, the National Basic Research Program of China under Grant No 2012CB933704, the Natural Science Foundation of Guangdong Province under Grant No S2012020011003, and the Program for Changjiang

Abstract

We improve the performance of organic light-emitting diodes (OLEDs) with both a MoO3 hole injection layer (HIL) and a MoO3 doped hole transport layer (HTL), and present a systematical and comparative investigation on these devices. Compared with OLEDs with only MoO3 HIL or MoO3 doped HTL, OLEDs with both MoO3 HIL and MoO3 doped HTL show superior performance in driving voltage, power efficiency, and stability. Based on the typical NPB/Alq3 heterojunction structure, OLEDs with both MoO3 HIL and MoO3 doped HTL show a driving voltage of 5.4V and a power efficiency of 1.41 lm/W for 1000 cd/m2, and a lifetime of around 0.88 h with an initial luminance of 5268 cd/m2 under a constant current of 190 mA/cm2 operation in air without encapsulation. While OLEDs with only MoO3 HIL or MoO3 doped HTL show higher driving voltages of 6.4V or 5.8V and lower power efficiencies of 1.20 lm/W or 1.34 lm/W for 1000 cd/m2, and a shorter lifetime of 0.33 or 0.60 h with an initial luminance of around 5122 or 5300 cd/m2 under a constant current of 200 or 216 mA/cm2 operation. Our results demonstrate clearly that using both MoO3 HIL and MoO3 doped HTL is a simple and effective approach to simultaneously improve both the hole injection and transport efficiency, resulting from the lowered energy barrier at the anode interface and the increased hole carrier density in MoO3 doped HTL.