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

Abstract

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 Cs 2 CO 3 , and a maximum efficiency of 58.5 cd A −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 11 660 cd m −2 , which is more than one order of magnitude higher than that of the device without the interlayer (≈775 cd m −2 ).