Novel Solution-Processed ZnO-Based Electron Injection Layer for Organic Light-Emitting Diodes

A novel solution-processed zinc oxide (ZnO)-based electron injection layer (EIL) has been developed with an easy method. The inverted organic light-emitting diode (OLED) with pristine ZnO as EIL shows improved electron injection property compared to that with thermally evaporated Cs2CO3, and a maximum efficiency of 58.5cdA(-1) has been achieved for a green phosphorescent OLED. However, when the ZnO is applied to a solution-processed OLED using poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as emissive layer, it is found that the luminance remains at a low level although the current increases very fast. The low luminance is mainly attributed to the inefficient hole blocking property of ZnO, resulting in unbalanced electron-hole pairs in the emissive layer. Hence, a mixed interlayer of ZnO and polyethyleneimine ethoxylated (PEIE) is introduced to modify the ZnO layer. The optimized device shows a maximum luminance of 11660cdm(-2), which is more than one order of magnitude higher than that of the device without the interlayer (approximate to 775cdm(-2)).