Investigation On Device Structure And Charge Balance For High Performance Quantum Dot Light-Emitting Diodes

Quantum dot light-emitting diodes(QLEDs)are emerging as the excellent candidates and at the edge of large-scale commercial applications of next generation of the full-color display and the solid-state lighting based on their unique advantages of tunable colors,saturated emission,aesthetic flexibility and low cost.High efficiency and long lifetime red,green and blue QLEDs play a vital role in QLEDs technology commercialization.However,QLEDs are currently subject to charge imbalance,which affects the device performance in the practical brightness(100?1000cd/m2).Especially,device performance of blue QLEDs still lags far behind that of their green and red devices.The difficulties originate from blue quantum dots(QDs)inherently possesse a greater optical band gap required for blue emission,which presents increased challenges for hole and electron injection into the QDs.Therefore,the study on charge balance in the device is an important and unavoidable problem in order to prepare high-performance QLEDs and promote the applications of QLEDs technology.For this purpose,inverted multilayer QLED devices based on organic-inorganic hybrid charge-transporting layer(CTL)were studied to enhance the charge balance for the high device performance,mainly about material optimization,functional layer doping,and interface modification.Firstly,we introduced some fundamental work about the materials of QLED devices and the device structure.A series of characterization tests were carried out,including photophysical properties,the size,dispersiblity and crystallinity of the quantum dots.Then,we fabricated the inverted red QLED devices based on organic-inorganic hybrid CTL.It was found that the emitting layer substrate had an important influence on the film forming properties of the evaporated organic molecules CBP as hole-transporting layer(HTL)in our fabricated inverted QLED devices.Secondly,we reported a facile but effective method to improve the performance of QLEDs.A polymer poly(9-vinlycarbazole)(PVK)as a dopant,instead of a host,was introduced into the quantum dots emitting layer(EML)to achieve balanced charge recombination in the layer.In addition,doping PVK into the EML also benefits the film morphology of the HTL on the EML.With enhanced charge balance and improved morphology,a maximum current efficiency of 19.0 cd/A was achieved in our red color emitting diodes based on PVK-doped EML.The corresponding external quantum efficiency reached 16.8%,which is 1.6 times higher than that of the reference QLEDs based on pristine QDs EMLs.Thirdly,we adopteda polyethylenimine ethoxylated(PEIE)introduced into the ZnO nanoparticles film for inverted structure blue QLEDs.PEIE doped ZnO film facilitates the enhancement of both electron inject into blue QDs EML by lowering the work function of ZnO and charge balance by lowering conductivity of ZnO film.Power efficiency is 1.6 times higher than that of the reference QLED device.Using the optimized device structure,we exhibited the operated large-area flexible RGB-emitting devices.The Commission Internationale de l'Eclairage(CIE)color coordinates of each color are(0.69,0.31),(0.21,0.74),and(0.14,0.05),whose color gamut is larger than that of National Television Systems Committee(NTSC)standard(?115%of NTSC 1931).