Research On Charge Balance Of Quantum Dot Light-emitting Diodes By Means Of Structural Optimization

Quantum dot light-emitting diodes(QLEDs)have become the most attractive light sources for the next-generation displays and solid-state lightings field due to their unique advantages,such as high color purity,high stability,high resolution,and relatively simple manufacturing process.However,QLED devices still face many serious challenges before they are widely used in practice,the main problem is the unbalanced transmission of electronic-hole in QLED devices,that is in QLED devices,the number of electrons injected into the quantum dot emission layer is much higher than the number of holes,the mismatch between electron and hole which limits the improvement of device performance.How to adjust charge balance to improve device performance is a key problem.It has important value and significance which using the method of structure optimization to promote the charge balance in the QLED device and improve the device performance.The purpose of this thesis is to promote the charge balance of QLED devices,from three aspects to optimize the structure of the device:hole transport layer(HTL),emission layer(EML),and electron block layer(EBL).The main contents and innovations of this paper are as follows:(1)The organic polymer is added into the hole transport layer to enhance the injection capability of the interfacial hole and improve the hole transport rate,so as to balance the charge carriers in QLED devices and develop efficient QLED devices.BTE is a kind of material with good hole transport function,which belongs to a kind of low polythiophene organic semiconductor material with high electron density and high charge carrier mobility.For the first time,it was mixed with poly-TPD as the HTL,and the influence of BTE doping on the morphology and hole transport performance of the HTL was test characterization and analyzed.The results show that the HTL of poly-TPD:BTE doped can enhance the hole transport rate,adjust the flatness and uniformity of the interface film between the HTL and the EML,significantly improve the charge transfer balance,the External Quantum Efficiency(EQE)of doped QLED devices was increased by 1.68 times to 16%,and increased the brightness by 1.71 times to 56290 cd/m².(2)Compared with quantum dots(QDs),quantum rods(nanorods)due to their larger aspect ratio(length/diameter),it is more difficult to obtain a closely arranged film with each other when prepared by spinning coating method,compared with spherical and cubic shaped QDs.Doping organic matter F₄TCNQ in nanorods EML can improve the arrangement of nanorods in the EML and the quality of the film,and preparation out the nanorods film of high quality and close arrangement,ensure good contact between EML and other functional layers,a fast channel was established for hole transport between HTL and EML,increase the transport rate of holes,promote charge balance.As a result,doping F₄TCNQ in nanorods EML can significantly improve the performance of QLED devices,make the EQE of red nanorod-LEDs reached 11.6%and the brightness reached 16222 cd/m²,which are 1.68 times and1.84 times higher than pure nanorods-LEDs devices,respectively.(3)The wideband gap organic polymer Spiro-OMe TAD was used as EBL to block electrons and balance charge carriers transport.There are two options:(1)The organic polymer Spiro-OMe TAD with high lowest unoccupied molecular orbital(LUMO)energy was used as EBL,which is placed between HTL and QDs EML to block the transport of some electrons from the QDs EML to HTL,promoted the charge balance of devices,and some quantum dot layer luminescence caused by energy transfer after excitons are formed in HTL is reduced,and efficient green QLED device is prepared.The results show that compared with traditional devices,the EQE of new green QLED devices increased by 87%to 11.87%.Brightness increased by106%to 53055 cd/m².(2)The organic polymer spiro-OMe TAD was used as EBL and inserted between QDs EML and the electron transport layer(ETL),increase the energy barrier between the electron injection from ETL to QDs EML,so as to avoid excessive electron injection into QDs EML.Moreover,because of the excellent hole transport capability of Spiro-OMe TAD,it enables EBL to block excessive electron injection,maintain high current density,and have good hole transport capability,thus better promoted the balance of charge carrier transport and improved device performance.Compared with traditional devices,the EQE of green QLED devices using Spiro-OMe TAD increased by 1.93 times to 14.6%,and the brightness increased by 1.37 times to 55760 cd/m².