Improve the Charge Balance of White Phosphorescent Organic Light Emitting Diodes Using Co-Doped Electron Transport Layer in Emitting Layer

White phosphorescent organic light-emitting diodes (WPHOLED) with high efficiency and low driving voltage were achieved by incorporating an electron transport material (3TPYMB) into a hole transport-type host (TCTA) as a mixed-host structure. For electrons, the emitting layer is nearly barrier-free until they reach the region of exciton formation, which keeps the driving voltage low. Therefore, improved the charge carrier balance within the emitting layer and enhanced the power efficiency of device. White PHOLED at a luminance of 1000 cd/m2 shown a driving voltage of 4.38 V, luminance efficiency of 36.1 cd/A, and power efficiency of 26.4 lm/W was observed. Furthermore, the power efficiency can be improved to 34.27 lm/W, and luminance efficiency to 46.7 cd/A by attaching a brightness enhancement film (BEF).