| Quantum dot light emitting diodes(QLED)have attracted more and more attentions because of its the excellent material stability,continuous adjustable light-emitting wavelength with the size of QDs,narrow light-emitting spectrum and all-solution fabriction method.Adopts with the "sandwich" multi-layer structure,a large fraction of light(80%)is lost due to the different mechanisms takes place inside the device including substrate mode,indium-tin-oxide(ITO)/organic waveguide mode(WG),and surface plasmon-polariton(SPP)mode.As a result,only about 20% of the light can be emitted from the device.The light trapped in the devices can convert to heat,which can influence the efficiency and longevity of the device.Thus,through improving the light extraction efficiency and converting the non-radiative coupling into radiation coupling of the light,accompanied with reducing the internal heat accumulation of the device are the key factors for improving the external quantum efficiency and lifetime of the device.In order to increase the light extraction efficiency of the QLED device,it has become an important means to introduce different micro-nanostructures into the device and then improving the external quantum efficiency of the device.According to the morphology of the micro-nanostructures,it can be divided into periodic structure and non-periodic structure.Periodic structure can enhance the light output of the device at a specific angle while non-periodic micro-nanostructures have no selectivity to the wavelength of light and can enhance the light output by scattering properties of the structures.However,due to its large disorder roughness,it is easy to cause atomization of the light source.It is very important to find micro-nanostructures which can not only change the spectral characteristics of QLED devices,but also improve the light output efficiency of QLED devices.In this thesis,periodic nanostructures with grid structure are constructed by nanoimprint lithography,and non-periodic nanostructures with wrinkles are constructed by Reactive ion etching(RIE)techniques to enhance the light output of green QLED devices.The influence of nanostructures on the light output efficiency is studied in depth without changing the properties of devices.In order to systematically study the characteristics of micro-nanostructure for the enhancement of the light output efficiency of devices,the Finite-difference-time-domain(FDTD)method is used to simulate the QLED devices with or without micro-nanostructures.The characteristics of micro-nanostructure at visual angle were tested by Fast Fourier transform(FFT).The mechanism for the enhancement of the light output efficiency of QLED devices with micro-nanostructure were explained by the experimental results.The main content of this thesis is divided into three parts:(1)Fabrication of a variety of micro-nanostructures by using nanoimprint lithography combined with reactive ion etching techniques:The grating structure was constructed by nanoimprinting technique: two-dimensional grating structure was obtained by twice nanoimprinting technique,700 nm period and 45 nm height grid structure was finally obtained on glass substrate.RIE technique is used to construct wrinkles structure.By controlling the power and reaction time of RIE,the initial height of wrinkles structure is 35 nm at 10 seconds,and then the depth of the wrinkles pattern can increased about 15 nm every 10 seconds.Composite structure was constructed by nanoimprint lithography combined with RIE techniques.After two-dimensional grating structure constructed by nanoimprint lithography composite micro-nanostructures with 700 nm period and heigh was obtained by different etching times of RIE.(2)Improved light outcoupling efficiency in green quantum dot light emitting diodes:With the introduction of grid structure,the transmittance of the substrate increases to 88.71%,and the maximum brightness of green QLED devices increases from 151500 cd/m2 to 155900 cd/m2,but has angle-dependent characteristics.The wrinkles structure can increase the transmittance of the substrate to 88.65%.the brightness of QLED devices increased from 151000 cd/m2 to 154900 cd/m2,and the EQE from 10.52% to 12.22%.The grid composite wrinkles structure can increase the transmittance of the substrate to 90.48%,the brightness of QLED devices increased from 122400 cd/m2 to 178700 cd/m2,and the EQE from 12.29% to 17.94%.At the same time,the peak position of the EL spectrum and color rendering index are not affect by the structures and greatly reduces the angle dependence of QLED devices.(3)Simulating the light output properties of QLED devices by FDTD methods:The model of QLED device is established by FDTD method,and then the micro-nano structure is attached on the substrate of the QLED device.The device with or without micro-nanostructure is simulated by FDTD at 518 nm wavelength of green light.Dipole light source can be used to simulate the total reflection of light between the interface layer and air.Comparing with various simulation results,the lattice composite wrinkles structure can make the photon enhancement at the interface of air and ITO substrate,which can effectively extract light from various directions.Multiscale micro-nanostructures proposed here can magically tune the spatial emission profile to comply with the lambertian radiation pattern is expected to be of great potential for the development of efficient outcoupling structures for QLED. |
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