Design Of Internal Light Extraction Structure Based On Light Scattering Of Nanospheres And Construction Of Efficient QLED Devices

Quantum dot light-emitting diode（QLED）has the advantages of high color purity,long working life,low preparation cost and good stability,and is expected to be developed into a new generation of lighting and display technology.Although the internal quantum efficiency of QLED devices has been close to 100%,due to the existence of various optical losses in the devices,for example,Waveguide mode,surface plasmonic mode and substrate mode optical loss result in the maximum External Quantum Efficiency（EQE）of QLED devices is limited to about 20%,in which the waveguide mode optical loss mode within the device accounts for about 36.7%,and accelerate the eventual conversion of light into heat in the device,reducing the stability of QLED devices.It is not conducive to the construction and practical application of high performance QLED devices.Therefore,the development of efficient light extraction technology for QLED devices is of great significance and application value for improving the light output efficiency and stability of QLED devices.It is an effective method to improve the efficiency of light extraction in QLED devices to embed microstructures such as grating and surface ripples in the device,which can reduce the total reflection of light at the interface between Indium Tin Oxide（ITO）electrode and organic layer and extract waveguide mode light in the device.However,the commonly used micromachining methods to prepare internal light extraction structures,such as photolithography and etching,still have some problems,such as complicated process and strong angle dependence and electrical performance of the device is reduced.In this paper,polystyrene（PS）nanospheres and mesoporous silica dioxide（Si O₂）nanospheres are introduced at the interface of the functional layer by one step spin coating technology to change the interface structure of the functional layer.The internal light extraction structure can change the propagation path of the outgoing light and extract the waveguide mode light in the QLED device,so as to improve the light output efficiency of the QLED device.By changing the duty ratio,particle size and refractive index of the nanospheres in the Poly（3,4-ethylenedioxythiophene）-poly（styrenesulfonate）（PEDOT:PSS）hole injection layer,the influence of the nanostructure on the optical properties of the film,the electrical properties of the device and the electroluminescence properties and the regulation rules were studied,and the internal light extraction efficiency is greatly improved,among which the external quantum efficiency of the green device is increased by 67%.It provides an efficient method to improve the luminescence performance of QLED devices.The main work content of this paper is divided into the following two parts:（1）Effect of disordered nanospheres embedded in PEDOT:PSS films on film propertiesThe effect of 30、50 and 100 nm PS nanospheres on the morphology of PEDOT:PSS in hole injection layer and TFB in hole transport layer is investigated.The 30 and 50 nm PS nanospheres did not affect the structure of quantum dot layer,while the 100 nm PS nanospheres changed the morphology of quantum dot layer.The effect of the volume ratio between 30 and 50 nm PS nanospheres solution with solid content of 5 wt%and PEDOT:PSS solution on the photoluminescence properties of quantum dot films is studied.When the volume ratio of solution is 5.7%,50 nm PS nanospheres can increase the photoluminescence intensity of red,green and blue quantum dot films by 43%,39%and 23%,respectively.The influence of the particle size of PS nanospheres embedded in functional layer on the electrical performance of the device is explored.PS nanospheres with particle size of 30 and 50 nm will not have a great impact on the electrical performance of the device,while PS nanospheres with particle size of 100 nm will cause serious leakage current in the device.（2）Regulation of light extraction structure in nanospheres on luminescence performance of devicesThe regulating effect of the particle size of PS nanospheres embedded in the functional layer on the electrical and electroluminescent properties of QLED devices is studied.The peak EQE of green QLED devices with a particle size of 50nm is increased by 47%to 14.4%.The regulation effect of the volume ratio between 50 nm PS nanospheres solution with solid content of 5 wt%and PEDOT:PSS solution on the performance of QLED devices is analyzed and studied.When the volume ratio of solution is 5.7%,the duty ratio of PS nanospheres in PEDOT:PSS film is 4.1%.The internal light extraction structure can increase the peak EQE of green QLED by 67%to 16.36%.Then,the influence of 50 nm PS nanospheres on the performance of red and blue QLED devices is explored.When the solution volume ratio is 5.7%,the duty ratio of PS nanospheres is 4.1%,the peak EQE of red devices can be increased by 35%to 24.76%,and when the solution volume ratio is 7.0%,the duty ratio of PS nanospheres is 6.2%,the peak EQE of blue devices can be increased by 17%from 4.88%to 5.73%.The influence of the nano-structure embedded in the functional layer on the luminescence Angle characteristics of the device is analyzed.The effect of the optical extraction structure of mesoporous Si O₂ nanoparticles with low refractive index（1.46）on the performance of blue devices i studied.When the volume ratio of 50 nm mesoporous Si O₂solution to PEDOT:PSS solution is 4.3%,the peak EQE of blue devices is further improved from 4.27%to 5.63%,an increase of 32%.