Improved color purity and efficiency of blue quantum dot light-emitting diodes

Quantum dot light-emitting diodes (QLEDs) are attractive to be used for new generation displays since their advantages of high color saturation and emission efficiency. However, the wide-bandgap blue light-emitting quantum dots (QLEDs) suffer from an undesirable emission from surface defects of ZnO nanoparticles which are widely used as an electron transport layer (ETL). Here, we firstly present an inverted structure of blue QLEDs with inserting an ultra-thin layer of polyethylenimine (PEI) between the ITO and ZnO layer to address such issue. The QLEDs with the inverted structure of ITO/PEI/ZnO/QDs/PVK/MoO3/Al (where PVK is poly (N-vinylcarbazole)) exhibit a pure blue emission with the CIE color coordinates of (0.14, 0.04). Meanwhile, the peak external quantum efficiency (EQE) of the blue QLEDs increases from 3.5% to 5.5%. The improved color purity and efficiency are attributed to decreasing oxygen vacancy defects of ZnO and electron accumulation at the interface of ZnO and QDs by means of modifying ZnO nanoparticles with PEI. The results may suggest a potential method of using PEI as the interfacial layer to fabricate high efficiency and pure blue QLEDs.