Research Of Key Technologies Of Inkjet-printed QLED Devices

Quantum dots light-emitting diode(QLED)is an emerging inorganic nanocrystalline self-luminous technology.Quantum dots can generate three primary colors of red,green and blue,and the spectra have a narrow half-peak width of 28 nm,which can achieve high color purity.With the breakthrough in the printed organic light emitting diode(OLED)display panels,QLED displays technology with the same process is becoming the focus in the development of next-generation large-size new-type display technology.However,the current research on inkjet-printed QLED devices is still in its early stage.There are still some problems affecting the performance of inkjet-printed QLED devices,such as the problems of interlayer solvent erosion,ink spreading on film layers,imperfect hole transport material systems,and the instability at the interface of different printed layers.In addition,in order to prepare all solution-processed QLED devices,the top electrode is also required to be printed in order to replace the vacuum evaporation process and eliminate the dependence of QLED fabrication on high vacuum environment.In order to solve the problems associated with the inkjet printed QLED devices,the present thesis conducted thorough investigations and accomplished the following three innovative achievements:1.A new printable hole transport material has been developed by mixing the poly(9,9-dioctylfluorene-alt-N-(4-sec-Butyl phenyl)-diphenylamine(TFB)with the home-synthesized cross-linked small molecule hole transport material 4,4'-bis(3-vinyl-9H-carbazol-9-yl)-1,1'-biphenyl(CBP-V)for inkjet-printed QLED devices.By optimizing the mixing ratio and film thickness,the blended hole transport material has both the advantages of high mobility associated with the TEB and the deep HOMO(highest occupied molecular orbital)energy level associated with the CBP-V,which can reduce the hole injection barrier to the quantum dots layer and increase the recombination rate in the quantum dot layer.It also has the excellent solvent resistance and the stability at the interface between quantum dots layer and hole transport layer can be improved,hence improving the device performance considerably.Compared to red QLED devices using traditional hole transport material TFB,QLED devices with the blended hole transport layers improved the external quantum efficiency from 15.9%to 22.3%,and the T90 lifetime and T70 lifetime were extended from 5.4 h and 31.1 h to 39.4 h and 148.9 h.Based on this,the inkjet-printed red QLEDs were fabricated using the new blended hole transport layer,with the external quantum efficiency of the inkjet-printed red QLEDs reached 16.89%.The external quantum efficiency of the inkjet-printed red QLEDs based on TFB was only 6.7%.2.The previously developed cross-linkable small molecule hole transport material 9,9'-(9,9-dimethyl-9H-fluorene-2,7-diyl)bis(3-(((4-of vinylbenzyl)oxy)methyl)-9H-carbazole)(BV-FLBC)was further optimized to allow lower cross-linking temperature.The effects of cross-linking temperatures on the surface energy and charge mobility were systematically studied.By reducing the cross-linking temperature,the surface energy of the films was increased from 38.3 mN/m to 47.2 mN/m,thereby improving the performance of inkjet-printed QLED devices.The external quantum efficiency of the inkjet-printed green QLEDs was improved from 0.4%to 8.4%by reducing the cross-linking temperature.3.In order to realize all-solution processed QLED devices,the top electrode needs to be printable as well.The conductive inks based on silver nanoparticles are normally sintered at high temperature to achieve high conductivity,which often leads to damage to the QLED functional layer.To replace the traditional high temperature oven sintering process,photonic sintering using xenon' lamp intense pulse light' was investigated,particularly for the 'QLED ' devices with aluminum electrodes.It was found that the intense pulse light sintering could locally cause the reaction between aluminum and the oxygen in the electron transport layer to form a ultrathin alumina layer,which effectively reduced the electron injection efficiency and made the electrons and holes more balanced,thereby improving the device efficiency without damaging the functional layers.With the optimized sintering energy,a silver electrode with a square resistance of 1.39 ?/? was obtained without destroying the surface morphology of the silver electrode and all-solution processed QLED devices have been realized.