Electron-transporting layer doped with Cesium azide for high performance phosphorescent and tandem white organic light-emitting devices

Cesium azide was employed as an effective n-dopant in the electron-transporting layer (ETL) of organic light-emitting devices (OLEDs) owing to its low deposition temperature and high ambient stability. By doping cesium azide onto 4,7-diphenyl-1,10-phenanthroline, a green phosphorescent OLED having best efficiencies of 66.25 cd A−1, 81.22 lm W−1 and 18.82% was realized. Moreover, the efficiency roll-off from 1000 cd m−2 to 10 000 cd m−2 is only 12.9%, which is comparable with or even lower than that of devices utilizing the co-host system. Physical mechanisms for the improvement of device performance were studied in depth by analyzing the current density–voltage (J–V) characteristics of the electron-only devices. In particular, by comparing the J–V characteristics of the electron-only devices instead of applying the complicated ultraviolet photoelectron spectrometer measurements, we deduced the decrease in barrier height for electron injection at the ETL/cathode contact. Finally, an efficient tandem white OLED utilizing the n-doped layer in the charge generation unit (CGU) was constructed. As far as we know, this is the first report on the application of this CGU for fabricating tandem white OLEDs. The emissions of the tandem device are all in the warm white region from 1213 cd m−2 to 10870 cd m−2, as is beneficial to the lighting application.