Highly efficient inverted organic light-emitting diodes using composite organic heterojunctions as electrode-independent injectors

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode have attracted increasing attention for display applications because of their easy integration with the n-type transistors based on low-cost and highly-uniform amorphous silicon (a-Si), and transparent amorphous oxide semiconductors (TAOSs). Up to date, indium tin oxide (ITO) has been widely used as the transparent electrode, but the dogged issue of using ITO as the cathode in IOLEDs is the large energy barrier for electron injection due to its high work function. In this work, a kind of organic charge generation layer (CGL), comprising of a p-type semiconductor/bulk heterojunction (BHJ)/n-type semiconductor (p/BHJ/n), is introduced on the ITO cathode to fabricate high performance red, green and blue IOLEDs. It is found that the utilization of the composite organic heterojunction CGL as an electron injector greatly enhanced the electron injection, thus significantly improving the electroluminescence efficiency of the resulting IOLEDs. More importantly, their performances are independent of the work function of the used cathode. It is experimentally demonstrated that the electrons injected into the emitting layer are from the generated charges in composite organic heterojunction CGLs, which are completely different in the injection manner from electrodes in conventional OLEDs. It is believed that our studies provide a promising method to fabricate high performance IOLEDs regardless of the choice of electrodes, which will benefit to integrate IOLEDs on the n-type TFTs.