Improving The Light Output Coupling Efficiency Of Inverted Quantum Dot Electroluminescent Devices

Inorganic semiconductor quantum dots(QDs)have the advantages of luminescence covering all wavelengths from ultraviolet to near-infrared,high color saturation,narrow luminescence spectrum,low-cost solution processing and high quantum yield(nearly 100%).Make it a very attractive semiconductor light-emitting material.Due to the strong quantum confinement effect in QDs,people can change the size and composition of quantum dots by optimizing the synthesis method to design the emission wavelength and color purity(or half-width of the spectrum)of QDs.Because QDs light-emitting materials have such excellent characteristics,quantum dot semiconductor light-emitting devices(QLEDs)with QDs as the light-emitting layer have become an important development target for the next generation of lighting and display devices.Since Alivisatos et al.first reported QLED devices with Cd Se quantum dots as the light-emitting layer,the performance of QLED devices has been significantly improved,and the external quantum efficiency(EQE)has increased from less than 0.01%to more than 20%.Therefore,for device efficiency,the optimization space of materials and device structure is very limited.Through the EQE analysis of the device,the optical output coupling efficiency of the device is an important parameter to improve the performance of the QLED device.Because the refractive index of the QDs luminescent material in the QLED device is relatively large,and the refractive index of ITO and glass is relatively small,the total reflection effect between the interfaces of each layer of the device makes most of the light confined inside the device.Eventually,it is lost through absorption and plasma,resulting in the optical output coupling efficiency of the device at 20%.Therefore,how to enhance the optical output coupling efficiency has become a research hotspot.In order to solve this problem,a ZnO film with micro-nano structure was prepared in this paper,and an inverted QLED device was prepared using it as an electron transport layer.Compared with conventional ZnO nanoparticle film-based devices,this ZnO micro-nano structure film has a textured structure.Its strong light scattering effect reduces the total emission between the interfaces,thereby improving the light output coupling efficiency and EQE of the device.The details are as follows:1.Prepare a ZnO film with a micro-nano structure by a solution method,use a scanning electron microscope(SEM)to characterize the surface morphology of the film,and analyze the physical mechanism of the micro-nano structure to enhance light output coupling.At the same time,the electrical properties of the film were tested,confirming its effectiveness as the electron transport layer of QLED devices.2.Based on the ZnO micro-nano structure film,an inverted QLED device was fabricated.Characterize the current density-voltage,brightness-voltage,capacitance-voltage and transient electroluminescence characteristics of the device,The influence of the micro-nano structured ZnO electron transport layer on the electrical and luminescence characteristics of the device was evaluated,and the reasons for the improvement of the performance of the QLED device were analyzed.3.In the end,we fabricated an efficient inverted red QLED device with a maximum brightness of 223100 cd/m~2and a maximum current efficiency of 20.7 cd/A.Compared with the devices using ordinary ZnO nanoparticles as the electron transport layer,the increase was 41.7%and 30.1%,respectively.This result confirms that the ZnO electron transport layer with micro-nano structure prepared by us is more suitable for the preparation of inverted QLED devices,and it has the advantages of simple preparation process,low cost and good repeatability.