Highly efficient tandem organic light-emitting devices adopting a nondoped charge-generation unit and ultrathin emitting layers

Highly efficient non-doped tandem phosphorescent organic light-emitting devices (PhOLEDs) have been demonstrated by employing LiF/AL/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) as a charge generation unit (CGU) and ultrathin phosphorescent dyes as emitting layers (EMLs). It was found that the performance of the tandem PhOLEDs was significantly dependent on the Al thickness in CGU. Analyses regarding the current density-voltage characteristics of the CGU only devices indicate that the charge carriers are generated at the HAT-CN/NPB interface, and Al/LiF layers work as assistant electron injection layers to facilitate electron extraction from CGU and injection into the adjacent electron transport layer. From the capacitance-voltage and optical transparency characteristics of CGU with different thicknesses of Al interlayer, the thick Al layer is beneficial to charge separation, while it weakens the light output of the tandem device. Under the optimal Al thickness of 5 nm, more than twofold enhancement of current efficiency is achieved for the non-doped tandem blue device. Furthermore, the non-doped tandem white PhOLED was also developed showing a maximum current efficiency of 94.9 cd/A and maximum external quantum efficiency of 31.6%.