Optical Design For High Efficiency Quantum-dots Light Emitting Diodes

The first chapter introduces the research background.Colloidal semiconductor quantum dot is a kind of nanocrystalline material that is synthesized and dispersed in solution.The quantum confinement effect induced by nano-size enables some novel properties that macro-crystals do not have.In recent years,many innovative research achievements and industrialization progress have been made in the fields such as lighting,display,solar cells,biological detection,chemical catalysis and quantum information.Especially,quantum dot is a hopeful material in the display field because of their narrow light width,adjustable luminous wavelength,simple synthesis and high stability.Compared with the mature liquid crystal display(LCD)and the current popular OLED,mini-LED and micro-LED,quantum-dot light-emitting diode(QLED)has its unique advantages.Therefore,improving the efficiency and performance of QLED is meaningful and has attracted a lot of research attention.However,in the research on improving the efficiency of QLED devices,most of the works only focus on improving the internal quantum efficiency(IQE)but pay less attention to its light extraction efficiency.In order to make up for the lack of research in this area,we focus on improving the out-coupling efficiency of QLED through optical design.The second chapter is mainly about the optical modeling and theoretical calculation of the QLED structure and characteristics.Through the optical modeling of QLED structure,we find that the results obtained by different calculation methods are different,so it is very important to choose the appropriate calculation method for the correct analysis and optimization of QLED optical design.Through appropriate simulation methods,we calculate the optical mode distribution in QLED,and combined with the existing research work,we summarize the methods of improving efficiency through optical design in QLED into two types:structural manipulation that requires extra structure and non-structural manipulation that optimizes the existing structure.Through comparative analysis,we find that structural regulation can obtain higher light efficiency but usually comes with the inefficient electrical efficiency and additional cost,while non-structural regulation is a kind of means with lower cost and more applicability.In the non-structural manipulation methods,the calculation shows that dipole orientation has a better effect on enhancing the out-coupling efficiency than the refractive index,thickness and photon recycling.In the third chapter,we mainly introduce our work to enhance the optical efficiency of QLEDs by controlling the polarization direction of quantum dots.Controlling the orientation of the dipoles in the emitting layer has a significant effect on the efficiency of QLEDs.In view of this,we analyze and point out two methods to regulate the orientation of the dipole in the quantum dot.By explaining the formation mechanism of dipole orientation in quantum dots,we find that the crystal field formed by lattice structure has a strong regulating effect on dipole orientation,and the tunability of lattice structure itself is one of the advantages of quantum dots compared with other luminescent materials.Furthermore,by utilizing the lattice tunability of quantum dots,we apply asymmetric strain to the lattice by growing non-uniform shells,and the strained lattice changes the energy level structure of CdSe/CdS core/shell quantum dots then produces strong in-plane dipole polarization.Finally,the dipole orientation in the quantum dot film is detected and determined by using the self-built polarization emission(back-focal-plane imaging)and polarization absorption(polarizationresolved PLE)detection devices,which prove that the directional emitting quantum dot has the potential to enhance the optical efficiency of QLED.