Efficient Quantum-dot Light-emitting Diodes Based On Sol-gel ZnO Electron-transporting Layer

Owing to the unique properties of colloidal quantum dots(QDs),such as high photoluminescence(PL)quantum yield,wide tunable emission wavelength range,narrow emission spectrum and low-cost solution processing,the quantum-dot light-emitting diodes(QLEDs)are attracting the attention of both academia and industry and gradually become one of the candidate technologies for next-generation lighting and display applications.Since the Alivisatos research group used quantum dots as the light-emitting layer to prepare QLEDs in 1994,QLEDs have been developed for nearly 30 years,and some breakthrough results have emerged one after another.Nowadays,high-performance QLEDs have been achieved with multilayer device structures with metao oxide electron injection/transport layer(EIL/ETL),light-emitting layer(EML),organic hole injection/transport layer(HIL/HTL),and electrode contacts.The external quantum efficiency(EQE)of red QLED has exceeded 30%and the operation lifetime of red QLED has been over 120 million hours at a brightness of 100cd/m².Owing to the multilayered structure of QLEDs,each layer has a critical impact on device performance.For the electron transport layer,inorganic metal oxides with high mobility such as Sn O₂,Ti O₂and ZnO are used.Among them,ZnO nanoparticles(n-ZnO)are especially popular due to their excellent electron injection/transport capabilities and good stability,attracting more and more researchers'attention.In2011,Qian et al.used n-ZnO as the QLED electron transport layer,obtaining the highest brightness of red,green,and blue QLEDs.Since then,n-ZnO as electron transport layer has achieved great success in QLEDs.However,the ZnO nanoparticles need to be synthesized by hydrolysis processes at high temperature and stored under low temperature conditions,which lead to high-cost and time-consuming for the commercialization.Moreover,due to the deep valence band energy of the core/shell Cd Se/Zn S QDs the hole injection barrier of the hole transport layer to the quantum dot is very large in QLEDs,which will cause unbalanced injection of electrons and holes.Excessive electron injection will cause exciton quenching through Auger recombination.This is also a typical challenge faced by QLEDs with ZnO nanoparticles as the electron transport layers.In this paper,a simple sol-gel method was used to prepare low-cost zinc oxide films(s-ZnO)as electron transport layer instead of n-ZnO.By modifying a thin polyethyleneimine(PEI)layer on the surface of the s-ZnO film,the performance of QLED is greatly enhanced with the peak current efficiency reaching 14.3cd/A,which is 21%higher than the device with n-ZnO.The main research contents of this thesis are as follows:(1)S-ZnO and n-ZnO were prepared by sol-gel method and hydrolysis method,and s-ZnO film and n-ZnO film were obtained by spin coating and annealing processes.The electrical properties and film characterization measurements of two kinds of ZnO films and corresponding QLEDs were carried out.We found that the s-ZnO film had better properties than the n-ZnO film,and the device performance of s-ZnO was close to that of n-ZnO,which verifies the feasibility of s-ZnO as an electron transport layer(2)QLEDs with high performance,high color purity,and good repeatability were obtained by inserting a PEI modifying layer between s-ZnO and QDs.Through AFM and contact angle characterization of s-ZnO and s-ZnO/PEI films,it is proved that the insertion of PEI greatly improves the smoothness and wettability of s-ZnO films,thereby obtaining high-quality QD films.Through the characterization of electrical properties,impedance spectroscopy and transient electroluminescence spectroscopy,the injection and transport process of carriers inside the device are discussed,and the mechanism of this improved method is demonstrated:PEI can passivate the surface defects of s-ZnO films and suppress excessive electron injection,balance the carrier concentration inside the device,which promote the radiative recombination efficiency of excitons,thereby improving the device performance.In summary,we successfully prepared low-cost and high-efficiency QLEDs,and demonstrated that s-ZnO/PEI can be used as the electron transport layer to replace n-ZnO.It lays a good foundation for the next application of s-ZnO/PEI to green and blue light devices.