White OLED with high stability and low driving voltage based on a novel buffer layer MoOx

White organic light emitting diodes ( WOLEDs) with copper phthalocyanine (CuPc), 4,4', 4''- tris( N-3-methylphenyl-N-phenyl-amino) triphenylamine ( m-MTDATA), tungsten oxide ( WO3) and molybdenum oxide ( MoOx) as buffer layers have been investigated. The MoOx based device shows superior performance with low driving voltage, high power efficiency and much longer lifetime than those with other buffer layers. For the Cell using MoOx as buffer layer and 4,7-diphenyl-1,10-phenanthroline(Bphen) as electron transporting layer ( ETL), at the luminance of 1000 cd m(-2), the driving voltage is 4.9V, which is 4.2V, 2V and 0.7V lower than that of the devices using CuPc ( Cell-CuPc), m-MTDATA ( Cell-m-MTDATA) and WO3 ( Cell- WO3) as buffer layers, respectively. Its power efficiency is 7.67 Lm W-1, which is 2.37 times higher than that of Cell- CuPc and a little higher than that of Cell- m-MTDATA. The projected half-life under the initial luminance of 100 cd m(-2) is 55 260 h, which is more than 4.6 times longer than that of Cell- m-MTDATA and Cell- CuPc. The superior performance of Cell- MoOx is attributed to its high hole injection ability and the stable interface between MoOx and organic material. The work function of MoOx has been measured by the contact potential difference method. The J-V curves of 'hole-only' devices indicate that a small hole injection barrier between MoOx/N,'-bis(naphthalene-1-yl)-N,N'-bis( phenyl)-benzidine ( NPB) leads to a strong hole injection, resulting in a low driving voltage and a high stability.