Microcavity Structure On The Performance Optimization Of Quantum Dot Electroluminescent Devices

Semiconductor quantum dots（QDs）have exhibited great application prospects in the field of light-emitting diodes due to their unique advantages,such as high luminous efficiency,strong photochemical stability,narrow emission peak width and continuously adjustable peak position.Compared with organic light-emitting diodes（OLEDs）,quantum-dot light-emitting diodes（QLEDs）with wider color gamut,higher stability and lower energy consumption,have become a strong competitor in the next-generation of high-performance display and lighting.Although QLEDs are outstanding in terms of color purity and luminous efficiency,their typical full width at half-maximum（FWHM）is located at 20～40nm,and ultra-high color purity cannot be achieved only by optimizing the monodispersion and particle size distribution of QDs.External quantum efficiency（EQE）is an important indicator for evaluating QLEDs application performance,while light outcoupling efficiency is an important parameter that affects EQE.A large number of photons are lost by substrate mode,waveguide mode,surface plasma mode and surface light absorption of metal cathode during the operation of QLEDs.Only 20～30%of photons escape from the device,leading to low light outcoupling efficiency,and seriously restricting the improvement of the theoretical limit EQE of the device.Therefore,QLEDs technology still has great space for improvement in color purity and luminous efficiency for future applications.The introduction of external microlenses or internal nanostructures in QLEDs can regulate the propagation direction of the light emitted from the quantum dot emitter in each functional layer of the device to achieve the purpose of increasing the outgoing light,thereby improving the device luminous efficiency.For example,the microlenses can increase the critical total reflection angle of the glass/air interface,which increases the proportion of light with a refraction angle greater than the critical total reflection angle.However,it is difficult to ensure that the nanoscale structure is not damaged during the transfer printing process,reducing light extraction efficiency.The internal nanostructures may cause the film unevenness or even discontinuous when the subsequent functional layer is deposited,which greatly increases the difficulty of the preparation of the defect-free functional layer,which is not conducive to the electrical transport during the working process of the device,thus leading to the degradation of the device performance.In addition,this strategy can only improve the luminous intensity and efficiency of the device,and cannot narrow the spectral emission linewidth.Microcavity can solve the problem of low light outcoupling efficiency of QLEDs by changing light propagation direction and reducing light loss.The emission FWHM is obviously narrowed and its intensity is enhanced by inhibiting the light in other wavelength and enhancing the light along the axis direction,thus improving the color purity of the device.Moreover,the planar microcavity structure can be completely compatible with the current solution construction method of QLEDs,which is an effective way to further improve the color purity and luminous efficiency.The main objective of this paper is to improve the luminous efficiency and color purity of QLEDs.A planar microcavity structure is formed by introducing distributed Bragg mirror（DBR）composed of Ti O₂and Si O₂with large refractive index difference into the device together with Al.We adjust and optimize the microcavity structure to achieve a high match between electroluminescence（EL）peak wavelength and microcavity resonant wavelength.The high EQE and narrow emission microcavity QLEDs（MQLEDs）can be constructed.The main research content is as follows:（1）Construction of high-efficiency and narrow emission red MQLEDs.Firstly,the reflectance range and reflectivity of DBR are regulated by adjusting the thickness and pair of Ti O₂and Si O₂,so that the DBR structure matching the red QLEDs is preliminarily designed.The reflectance of 4 pairs of Ti O₂/Si O₂can reach more than 95%in a wide red wavelength region.The hole injection layer（HIL）,as the cavity length tuning layer,can regulate the cavity length of MQLEDs and the relative position of the emitting layer.However,the required thick HIL will affect the electrical properties of the device,so we increase the thickness of the Si O₂in the fourth pair to serve as a space layer to jointly share the regulation effect of cavity length and resonate cavity wavelength,thereby weakening the impact of HIL on device performance.The axial EQE of the final device is increased from 28.7%to 35.5%,and EL emission peak is located at642 nm,and the FWHM is narrowed from 24 nm to 11 nm,achieving an obviously improvement in the efficiency and color purity of red devices.（2）Construction of high-efficiency and narrow emission green MQLEDs.Based on the study of red MQLEDs,this chapter introduces the DBR-Al microcavity structure without space layer into green QLEDs.The high reflectance of DBR in the green light area is initially realized by optimizing the thickness of 3pairs of Ti O₂/Si O₂.Also the cavity length of the microcavity and relative position of the emission layer are controlled through HIL,so that the peak of resonant cavity is consistent with the EL spectrum of normal green QLEDs.The axial EQE of the final device is increased from 17.7%to 27.8%,and the current efficiency achieves 69%improvement from 77 cd A^-1to 130 cd A^-1,and the EL FWHM is narrowed from38 nm to 20 nm.When Ti O₂/Si O₂is 4 pairs,the higher reflectance increases the narrowing effect of microcavity effect on the spectrum.The FWHM of the device is narrowed to 14 nm,which can match the display requirements of ultra-high color purity.However,its high reflectivity affects the optical output performance of the device to a certain extent,resulting in the insufficient improvement of device efficiency.In order to explore the general applicability of the proposed method,blue MQLEDs have also been preliminarily explored,and the results show that this strategy can also optimize the axial light outcoupling efficiency and color purity of blue devices to some extent.