Glass substrates pre-coated with a 90-nm-thin layer of indium tin oxide (ITO) with a sheet resistance of 20 Ω per square were thoroughly cleaned for 10 minutes in ultrasonic bath of acetone, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol, and then treated with O2 plasma for 5 min in sequence. Organic layers were deposited onto the ITO-coated substrates by high-vacuum (< 5 × 10 -4 Pa) thermal evaporation. Deposition rates were controlled by independent quartz crystal oscillators, which are 1~2 Å s -1 for organic materials, 0.2 Å s -1 for LiF, and 5 Å s -1 for Al, respectively. The emission area of the device is 3 × 3 mm 2 as shaped by the overlapping area of the anode and cathode. All the device characterization steps were carried out at room temperature under ambient laboratory conditions without encapsulation. The luminance-voltage-current density characteristics and EL spectra for monochromatic OLEDs were measured in a dual-channel Keithley 2614B source meter and a PIN-25D silicon photodiode. The luminance-voltage-current density characteristics and EL spectra of all the WOLEDs were measured in a PhotoResearch PR670 spectroradiometer, along with a Keithley 2400 Source Meter.