White emission in WOLEDs is typically generated from multiple emitters to cover a wide spectral range, e.g. the whole visible region from 380 nm to 780 nm. From the point of view of emitter materials (fluorophors or phosphors), WOLEDs can be classified into three types: all-fluorescent WOLEDs, all-phosphorescent WOLEDs and hybrid fluorescent/phosphorescent WOLEDs. Singlet-harvesting all-fluorescent WOLEDs have demonstrated long lifetimes [4] and [5], but their efficiencies are low because the singlet exciton generation efficiency is theoretically limited to 25% (except delayed fluorescence). All-phosphorescent WOLEDs incorporating transition-metal complexes as the phosphorescent emitters have attracted a lot of interest due to their ability to harvest both singlet and triplet excitons, resulting in nearly 100% internal quantum efficiency (IQE) [6], [7] and [8]. In the last few years, the power efficiencies (PE) of all-phosphorescent WOLEDs have broken 90 lm/W by introducing different light extraction technologies. The researchers in Leo's group reported an all-phosphorescent WOLED with 90 lm/W at a brightness of 1000 cd/m2[9]. UDC developed a device with 102 lm/W at 1000 cd/m2 in 2008, and then improved to 113 lm/W in 2010. In 2012, Panasonic Corp announced that their PE reached 142 lm/W at 1000 cd/m2. Very recently, NEC Lighting Ltd. announced at Lighting Fair 2013 (Tokyo) that they had developed a WOLED with a PE as high as 156 lm/W at 1000 cd/m2. Although the efficiency of some all-phosphorescent WOLEDs can surpass fluorescent tubes, and even their half-decay lifetimes can be over 10 000 h, the poor stability of blue phosphors is still a serious drawback for color stability. Recently, hybrid WOLEDs