New High Efficient Quantum Dot Light-emitting Diode Based On Polymer Optimization

Quantum dot light-emitting diode(QLED)is one of the most promising candidates in nextgeneration displays and solid-state lightings owing to its high color purity,tunable emission wavelengths,flexible,good photo-stability,and ease of fabrication.The external quantum efficiency of red and green QLEDs is comparable to that of OLEDs,but the performance of blue and white QLEDs is still relatively backward.How to further improve the performance of QLED devices is a key issue to be solved.It is of great value and significance to optimize the quantum dot lightemitting diodes and improve the performance by means of structural design and process adjustment.A organic functional layer were added aiming at improving the luminous efficiency of QLED devices in this paper.And structural optimization is mainly concentrated in three aspects: optimization of electron transport layer,optimization of hole transport layer and optimization of multi-layer quantum dot light-emitting layer.The main work content and innovations of the thesis are as follows:(1)Use of organic material as an electron blocking layer to achieve charge carrier transport balance and efficient QLED devices.Here,we report an organic polymer pre-PBO with a higher LUMO energy level as an electron buffer layer,inserted between the quantum dot light-emitting layer and the electron transport layer.Thereby increasing the barrier of electron transition to the quantum dot light-emitting layer,reducing excessive electron injection into the quantum dot light-emitting layer.We have found that the multifunctional electron blocking layer hinders excessive electron injection and has a good charge carrier transport balance while maintaining high current density.At the same time,it also limits the transfer process of electrons from the quantum dot luminescent layer to the electron transport layer,so that it has better radiation recombination ability.The external quantum efficiencies of the red and blue QLED devices prepared by this method were 16.7% and 17.5%,respectively.(2)The use of organic materials as a hole transport layer for increasing the transport rate of holes in a QLED device increases the recombination rate of excitons,which is the most effective method for improving device efficiency.At present,the HOMO level of the hole transport layer material applied to the QLED is generally around-5.4 eV,which causes a large energy barrier difference between the hole transport layer and the quantum dot light-emitting layer(red light quantum dots have a minimum HOMO level of at least-6.0 eV).This leads to a larger barrier to be overcome when the hole transits from the transport layer to the quantum dot lightemitting layer.The organic material pre-PBO used by us has a lower HOMO level(-5.8 eV)and is closer to the HOMO level of the quantum dot,which can be used as a medium for hole transport to prepare a device with two hole transport layers.Therefore,the energy barrier overcome by the hole transition is reduced,the hole transport rate of the device is increased,and the recombination probability of the excitons in the quantum dot light-emitting layer is increased,and the performance of the device is improved.And the maximum external quantum efficiency of the red QLED device prepared by this structure was 16.6%.(3)The preparation of a novel multi-luminescent layer device is realized by using an organic substance as a buffer layer.Based on the organic polymer pre-PBO has the ability to transport electrons and holes,and in QLED devices,the transport of carriers can be balanced.We use pre-PBO as a buffer layer between the quantum dot luminescent layers to prepare a multi-layer quantum dot luminescent layer QLED device.By using the superior energy level structure of pre-PBO,it can not only increase the light-emitting area of the device,but also it can balance carrier transmission and improve device performance.We used this method to prepare a dualemitting red-light QLED device and a three-layer quantum dot light-emitting layer white light QLED device.The quantum dot layers were separated by an organic polymer pre-PBO as a buffer layer.And the external quantum efficiencies of the two devices were 13.2% and 17%,respectively.