| In the past decade,Quantum dots(QDs),a kind of nano luminescent material,have attracted great attention of major scientific research institutions and display enterprises,because of the excellent characters including gap tunability,narrow line width,high stability,simple synthetic method and solution-processed ability,which caters for display and solid lighting field.The tremendous progress of quantum dot light-emiting diode(QLED),a kind of active electroluminescent technology utilizing QDs,in device structure design,operating mechanism,synthetic routes have been achieved in recent years,which make QLED a next generation display technology.So far,the maximum external quantum efficiency(EQE)of three primary colors(red,green,blue)QLEDs are all over 20%,their performance of luminance,colour purity and device operating lifetime are comparable to or even surpass those of the best OLEDs(Organic light-emitting diodes).Since QLEDs in inverted device structure can be connected with nether TFT(Thin film transistor)drive circuit directly,catering for product production process and fabricating QLED by all solution process matches the advanced production technology,such as ink-jet printing and transfer printing,all-solution processed inverted QLED has become the focus of research in recent years.At present,the performance of red inverted QLED still fall behind that of red conventional QLED,so,we aim at realizing the fabrication of all-solution processed inverted red QLED with high efficiency and improving the its performance by different strategies in this dissertation.In inverted QLEDs,elecrton injection from ZnO layer to QD emission is easier than hole injection from hole trnasport layer,because of higher electron mobility and matched energy level with QD.This unblanced injection will cause severe nonradiative recombination,which makes poor devcie performance.To solve this problem,strategies from two angles were used to optimize the device performance:(1)Adding TADF(Thermally activated delayed fluorescence)material in invered QLED to capture leaking electrons,and the performance of QLED is improved by the additional emission mechanism,FRET(F?rster resonance energy transfer)process,between TADF donors and QDs accepters.(2)Using organic electron transport material to act as the electron barrier layer to balance charge carriers in emission layer by limiting electron injection.Better charge carriers balance in QD layer will suppress Auger recombination,leading to better device performance.The specific contents are described as follows:(1)To make use of excessive electrons that leak through the emission layer,for the first time,TADF material was applied to inverted QLED to replace phosphorescent materials in similar strategy.Blue TADF material,DMAC-DPS(Bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]solfone),of different thickness were inserted into QD and TAPC interface as charge carrier harvesting layer.According to this mehod,the green QLEDs with structure of ITO/ZnO/QDs/DMAC-DPS/TAPC/Mo O3/Al were fabricated on the basis of the control green QLED.The excitons formed in DMAC-DPS layer can transfer their energy to green QD via FRET process,which enhances the utilization of excitons in device,leading to improving luminance and efficiency of QLED.Moreover,hole injection can be promoted when the thickness of TADF layer is less than 10 nm because of matched HOMO(The highest occupied molecular orbital)energy level of DMAC-DPS with QD and TAPC.When the thickness of DMAC-DPS layer is 5 nm,the best device performance was obtained,the maximum luminance(Lmax)of the optimal device is 63458 cd/m2,and the maximum current efficiency(CEmax)is 10.44 cd/A,which is 41.7%and 16.4%higher than that of the base device without DMAC-DPS layer,respectively.Besides enhancing hole injection,DMAC-DPS layer can also reduce the electrons density accumulated at interface QD and TAPC because of matched LUMO(the lowest unoccupied molecular orbital)energy level,leading to the improved device roll-off character at high current density.This work demonstrates that TADF materials have the same ability to improve QLED performance as phosphorescent materials.(2)For the first time,we use the TADF doping strategy to improve performance of the all-solution processed inverted QLED.All-solution method can simplify the preparation process,shorten the time cost.And the performance of red inverted QLED still fall behind that of red conventional QLED,so on the premise of verifying the feasibility of the TADF strategy in the first work,we aim to realize the preparation of high efficiency all-solution inverted red QLED and optimize its performance by TADF doping strategy.At first,We used isopropanol solution of(Mo O3)12·H3PO4·(H2O)x to replace the Mo O3 powder prepared by evaporation method,so as to ensure that the device was prepared by all-solution process.The solution can be used as the precursor solution of Mo O3 layer,and the Mo O3 layer can be formed after spinning coating and annealing.By this method,we have prepared standard inverted QLED with device structure of ITO/ZnO/QDS/TFB/Mo O3/Al.However,due to large leakage current,the device performance is very low,the Lmax and CEmax are only 3220 cd/m2 and 0.523 cd/A,respectively.Therefore,a second hole transport material PVK(Poly(9-vinylcarbazole))with low electron mobility was added to the device to reduce the leakage current.In order to ensure that the PVK layer is not destroyed during the preparation of TFB(Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl))layer,an orthogonal solvent method was adopted.Use 1,4-dioxane and p-xylene dissolve PVK and TFB respectively,inverted red QLED with double hole transport layers(Double HTLs-QLED)was prepared,the device structure is ITO/ZnO/QDs/PVK/TFB/Mo O3/Al.With the addition of PVK layer,the leakage current of the device decreases greatly,and luminance and efficiency of the device are increased to 25092 cd/m2 and 6.8 cd/A,respectively.In order to further optimize the device,TADF doping strategy was adopted.The TADF material,2Cz PN(4,5-Bis(carbazol-9-yl)-1,2-dicyanobenzene),whose emission spetrum matches with the absorption spectrum,was doped into the PVK layer according to different mass ratios to prepare the devices(ITO/ZnO/QDs/PVK:2Cz PN/TFB/Mo O3/Al).In the optimal doping ratio(PVK:2Cz PN),the Lmax and CEmax of the best device achieved 35352 cd/m2and 11 cd/A,compared with the Double HTLs-QLED,the percentage increase is 40.8%and 61.7%,respectively.(3)In the third part,we adopted the strategy of directly regulating the electron injection to improve the performance of the device from the perspective of balancing charge carriers in QD emission layer.The Double HTLs–QLED was set as the control device and the organic material,Tm PPB(1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene),was inserted between ZnO electron transport layer and QD emission layer as an electron blocking layer(EBL)to inhibit electron injection and balance the charge carriers in QD layer.The electron current can be ajusted by thickness of this Tm Py PB layer.The device structure is ITO/ZnO/Tm Py PB/QDs/PVK/TFB/Mo O3/Al.When the concentration of Tm Py PB solution is 6 mg/m L,the thickness of the Tm Py PB EBL is the best,the charge carriers in the device is the most balanced,leading to the best device efficiency.The Lmaxand CEmax of the device are increased from 31916 cd/m2 and 8.2 cd/A to 46674 cd/m2 and13.1 cd/A,the percentage increases are 46.2%and 59.7%,respectively.In addition,through comparing the PL intensity decay curves at the emission peak wavelength of QD in different films,we prove exciton qunching process which often happens in QLEDs with ZnO nano particles electron transport layer(ETL)caused by its trap energy level do not exist in our device with ZnO ETL made by precursor solution method. |
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