Study On Electron Injection In High-performance Blue Quantum-dot Light Emitting Diodes

Due to several superior properties,colloidal quantum dots(QDs)have obtained sufficient attention and achieved great progress during the past 30 years.Among them,II-VI semiconductor quantum dot materials are ideal for preparing light-emitting diodes(LEDs)because of excellent quantum yields,color purity and adjustability.Since the first appearance of quantum dot LED(QLED)in 1994,great progresses have been made on material synthesis,device structure and preparation process,which provide QLEDs with comparable performance to traditional organic LEDs now.These technological advances are accompanied by a more in-depth understanding of the physical processes behind the quantum dot electroluminescence process.However,because of difficult charge injection induced from big band gap of quantum dots(QD)layer,the performance of blue QLED is obviously inferior to that of green and red ones till now.In this paper we introduce an partial oxidized aluminum cathode of low work function and high stability from simple autoxidation process of an Al cathode.Together with ZnCdS/ZnS core-shell structure quantum dots,blue QLED of remarkable performance is fabricated.Highest luminance and current efficiency of our device reach 13000 cd/m2 and 1.1 cd/A respectively.With an electroluminescence(EL)spectrum peak of 457 nm and a full width at half maximum(FWHM)of 20.8 nm,the device exhibits pure deep blue emitting light.We also carry out several characterizations for the device with autoxidation-made Al:Al2O3 cathode,including XPS,UPS,TRPL,and etc.Results of these characterizations confirm the promotion effects of charge injection and effective exciton recombination from the process of autoxidation.This process is cheap,easy to carry out and repeatable,therefore provides a novel method for fabricating EIL/cathode in QLED.On the basis of the Al:Al2O3 cathode,ZnO nanoparticles were inserted between the quantum dot layer and the cathode as an electron transport layer to fabricate the LED device,and the conditions of preparation of the ZnO layer were adjusted,further improving the device performance.By controlling the condition of ZnO layer annealing to control the balance of carrier injection,a high performance blue LED quantum dot LED device with luminance of 37000 cd/m2 and current efficiency of 4.5 cd/A was fabricated.As far as we know,the luminance of the device is by far the record of blue QLED.By comparing the properties of devices with different ZnO annealing conditions and different cathodes,we have ensured the influence of preparation conditions of ZnO towards the defect density and the electron mobility.This method of controlling the carrier injection balance does not need to change the original structure of the device.The carrier injection balance of different device structures can be optimized by a simple annealing method,which is enlightening for the follow-up study of QLED devices.