Glass substrates precoated with 90 nm ITO with resistance of 15~20 Ω per square were successively cleaned with acetone, isopropanol, detergent and deionized water in ultrasonic bath. After that, all substrates were dried in an oven maintaining at 70 °C. In order to improve the hole-injection process in WOLED devices, all substrates were treated by O2 plasma for 10 minutes. All of the WOLED devices were fabricated by vacuum deposition method under a pressure < 5 × 10 -4 Pa in the Fangsheng OMV-FS 450 vacuum deposition system. Organic materials, LiF and Al were deposited at the rates of 1~2 Å s -1 , 0.1 Å s -1 and 5 Å s -1 , respectively. All the devices had their luminancevoltage-current density characteristics and EL spectra measured in a PhotoResearch PR670 spectroradiometer, along with a Keithley 2400 Source Meter. The effective emitting area of the devices was 9 mm 2 , determined by the overlap between anode and cathode. All the characterizations were conducted at room temperature in ambient conditions without any encapsulation, as soon as the devices were fabricated.