Highly Efficient Inverted Organic Light-Emitting Diodes Adopting a Self-Assembled Modification Layer.

Abstract

Inverted organic light-emitting diodes (IOLEDs) can be integrated with low-cost n-channel thin-film transistors for use in active-matrix OLEDs (AMOLEDs). However, the electron injection from conventional indium tin oxide (ITO) cathode to the upper electron transport layer usually suffers from a large injection barrier. To improve the electron injection efficiency, the electron injection layers (EILs) of ZnO modified by a self-assembled monolayer arginine (Arg) were developed to construct efficient IOLEDs. ZnO/Arg EILs present an ultralow work function (WF) of 2.35 eV, which is lower than that of ZnO modified by poly(ethylenimine) (PEI) (2.77 eV). The mechanism of low WF is attributed to the generation of strong molecular dipoles and interface dipoles at the interface of ZnO/Arg. The green fluorescent IOLEDs with ZnO/Arg present a low turn-on voltage (Von) of 3.5 V and a maximum current efficiency (CEmax) of 4.5 cd/A. Especially, the device possesses a half-life of 3600 h at an initial luminance of 1700 cd/m2, which is 36 times as long as that of the IOLEDs with ZnO/PEI as EILs. Furthermore, the green phosphorescent IOLEDs show a Von of 3.5 V, a CEmax of 59.1 cd/A, and a maximum external quantum efficiency (EQEmax) of 16.8%. At a luminance of 10 000 cd/m2, the efficiency roll-off of the device is only 6.3%.