Fabrication Of Light Emitting Diodes Based On ZnCdSe/ZnS Core-shell Quantum Dots By Regulating Charge Injection Layer And Emitting Layer

Due to the unique photoluminescence?PL?and electroluminescence?EL?properties,the inorganic semiconductive quantum dots have been applied into a wide range of fields,such as quantum dots light-emitting diodes?QLEDs?,solar cells,lasers,biomedical labeling and so forth.Compared to the conjugated molecules?polymers?or inorganic phosphors,quantum dots are considered as one of the most promising emitters for next-generation displays and solid-state lighting because of their color purity,stability and color tunability.As one of the most important applications of quantum dots,QLEDs have more obvious advantages than traditional organic light-emitting diodes?OLEDs?and liquid crystal displays?LCDs?.Since the report of the first QLEDs two decades ago,the brightness and luminous efficiency of QLEDs have been greatly improved through continuous exploration of many years,and it is expected to be a new generation of optoelectronic devices.Usually,the QLEDs with a multilayered structure consist of patterned ITO/hole injection layer?HIL?/hole transport layer?HTL?/quantum dots light emitting layer/electron transport layer?ETL?/metal electrode?Al,Ag and so on?,were fabricated by all-solution process.The properties of each layer have different effects on the performance of QLEDs.Poly?3,4-ethylenedioxythiophene?/polystyrene sulfonate?PEDOT:PSS?was always used as HIL.However,the electrical conductivity of the PEDOT:PSS film is usually too low?usually below 1 S/cm?to meet the requirements of high-performance QLEDs.In addition,it is known that the performance of multilayer QLEDs can be greatly affected by different kinds of hole transport materials.Generally,a desired hole transport material needs to have an appropriate low-lying highest occupied molecular orbital?HOMO?level and high hole mobility to facilitate hole injection and transport into the light emitting layer.Otherwise,the device performance will decrease,in the result of the imbalance of electron and hole injection.As the light emitting material,the shell thickness of ZnCdSe/ZnS core-shell quantum dots is one of the important factors that affect the optical and electrical properties of quantum dots.Quantum dots with thinner shell thickness are unstable,and easy to be affected by the surrounding medium and electric field when used as light emitting material in QLEDs.While,for quantum dots with thicker shell thickness,it is difficult to inject and transfer charges among the quantum dots.Based on the above problems,this paper carried out the following work:?1?In order to improve the electrical conductivity of PEDOT:PSS,we modified PEDOT:PSS by doping Au nanoparticles?NPs?.The effect of Au NPs on the performance of QLEDs based on Au-doped PEDOT:PSS as HIL were carefully assessed.In this paper,we chose ZnCdSe/ZnS core-shell quantum dots as light emitting material,PEDOT:PSS with varying concentrations and sizes of Au NPs as HIL,to fabricate QLEDs with simple structure.The effect of Au NPs on the device performance is evaluated by the optical,electrical and morphological properties of QLEDs.The devices with the best performance were obtained with a 22 nm-sized Au NP dopant and an OD value of 0.21,of which the maximum external quantum efficiencie(?_EQE)and current efficiency are 8.2%and 29.1 cd/A,respectively,corresponding to80%improvement compared with that of the devices without Au NPs.The results provide the possibility to further improve the performance of QLEDs and may accelerate the application of QLEDs in lighting and display.?2?The shell of core-shell quantum dots not only ensures the preparation and stability of high-performance core-shell quantum dots,but also has a significant impact on the carrier injection barrier in the QLEDs.The injection barrier is mainly composed of two factors.One is the energy level difference between carrier transport layer and shell of core-shell quantum dots.The main function of the barrier is influencing the balance and efficiency of carrier injection and the turn-on voltage of QLEDs.The other is the injection barrier formed by the carrier imigration path caused by the shell thickness of core-shell quantum dots.The barrier mainly affects the balance of carrier injection,which is ultimately reflected in the efficiency of QLEDs.In this paper,we have reported QLEDs based on Zn_1-xCd_xSe/ZnS core-shell quantum dots.Along with optimizing the shell thickness,hole transport materials?TFB,poly-TPD,PVK,TCTA,CBP?and thickness of HTL,light emitting layer and ETL,we achieved significant improvements in device performance as a result of increasing the quantum dot shell thickness to above 5 nm.By using poly[9,9-dioctylfluorene-co-N-[4-?3-methylpropyl?]-diphenylamine]?TFB?as HTL with a 38 nm thick quantum dots light emitting layer,the QLEDs show maximum current efficiency of 53.4 cd/A and peak?_EQE of 15.4%,all of which are well maintained over a wide range of luminances.Such as,the?_EQE can be maintained above 8%in the range of 200 cd/m²-50000 cd/m².Most importantly,at initial brightness of4000 cd/m²,these devices also possess long lifetimes with T₅₀?the time at which the brightness is reduced to 50%of its initial value?of 113 h.This result provides a new way to improve the life of QLEDs in practical application.