Synthesis Of Blue CdZnSeS Gradient Alloy Quantum Dots Based On Composition And Shell Control And Applications Of QLED

Quantum dots(QDs)is a unique luminescent material,which has the advantages of adjustable luminescent wavelength,saturated emission color,near 100% quantum yield,high optical stability and thermal stability,and can be processed by solution method.These excellent properties make quantum dots have good application prospects in many fields.Quantum dots light-emitting diodes(QLED)have also attracted extensive attention from researchers.With the optimization of quantum dots synthesis technology and structure,as well as the improvement of device structure and charge transfer layer material,the efficiency,brightness and stability of devices have been significantly improved.But the overall performance of blue devices still lags behind that of red and green.Compared with the red and green quantum dots,the highest occupied molecular orbital(HOMO)of the blue quantum dots is lower,and the energy level barrier between the hole transport layer(HTL)and the HOMO of the quantum dots is larger,resulting in difficult hole injection,excessive electron injection,unbalanced charge injection and other problems,resulting in poor overall performance of the device.In building components,in order to effectively reduce the hole injection barrier,improve the injection of holes,we not only can select the lower valence band of the hole transport layer or stepwise of hole transport layer to improve the injection of holes,but also can choose a barrier layer between the QDs and electron transport layer(ETL)to inhibit excessive injection of electrons,improve the balance of charge injection,and then improve the efficiency of the device.Recently,researchers have found that the performance of blue devices can be improved by optimizing the charge transfer material,but the research on this method is still in the preliminary stage and needs to be continuously tried and optimized.At the same time,we find that the performance of the device can be improved by selecting a quantum dots matching the energy level of the hole transport layer only by designing and synthesizing the structure of the quantum dots.After years of research,researchers have found that quantum dots with high quantum yield and high stability are the key to achieve high efficiency and high brightness of blue QLED.In general,the cladding of quantum dots can effectively improve the performance of quantum dots and alleviate the poor stability of quantum dots caused by the erosion of water and oxygen caused by long time exposure to air,thus improving the stability of quantum dots.When building blue light-emitting devices,quantum dots are mostly based on Cd ZnS,because the alloy structure of the quantum dots is easy to obtain high quality blue quantum dots.However,due to the large potential barrier of hole injection,the injection efficiency of such devices is low.Although sometimes higher efficiency is achieved by optimizing charge transfer material and device structure to improving the carrier injection balance,its brightness is low and it is difficult to achieve a high level of brightness and efficiency at the same time.Based on this,we based on CdZnSeS quantum dots and adjusted the energy levels of the luminescent layer quantum dots by component regulation.Explore the impact on device performance.This paper mainly carries out work from the following two aspects:(1)Blue CdZnSeS/ZnS core-shell quantum dots were prepared by the synthesis route of phosphorus-free compounds based on the regulation of components and the growth of shells.When the ratio of S to Se is 3:1 and covers 7 layers of ZnS,it can obtain the luminescence peak position of 468 nm and the full width at half maximum 29 nm.And the quantum dots are used as luminescent layers to construct the devices.The maximum brightness of the devices is 3315 cd/m2,the maximum external quantum efficiency is 0.865%,and the maximum current efficiency is 0.743 cd/A.(2)CdZnSeS gradient alloy quantum dots were synthesized by using trioctylphosphine(TOP)as the solvent of Se and S to form TOP-Se/TOP-S as the precursor.After regulation the components of CdZnSeS gradient alloy quantum dots,when Zn: Cd ratio is 20:1 and S:Se ratio is 23:1,the high quality blue gradient alloy quantum dots with a luminescence peak of 487 nm,quantum yield of 87% and the full width at half maximum of 25 nm can be obtained.The ZnS shell layer was grown on top of the optimized components quantum dots.With the coating of the quantum dots shell layer,the peak position of the quantum dots blue moved to 483 nm,and the high color purity of CdZnSeS could be maintained.The full width at half maximum of CdZnSeS/ZnS core-shell quantum dots was also 25 nm.Then,these two quantum dots were used as the luminescent layer to construct the device.Compared with the fluorescence spectrum,the electroluminescence peak of CdZnSeS quantum dots has redshifted only 2 nm and the full width at half maximum of 27 nm.,the maximum current efficiency,maximum brightness and maximum external quantum efficiency of the CdZnSeS quantum dots reached 7.66 cd/A,12630 cd/m2 and 7.23% respectively.The CdZnSeS/ZnS coreshell quantum dots were used as the luminescent layer to construct the blue QLED,and the maximum current efficiency,maximum brightness and maximum external quantum efficiency of the device were 10.47 cd/A,19010 cd/m2 and 8.24%,respectively.Moreover,the electroluminescent peak of the device was 484 nm(compared with the photoluminescent peak which was only redshifted by 1 nm),and the full width at half maximum of was 26 nm.There was no significant redshift in the electroluminescent spectrum of the device as the voltage increased.This is mainly because we synthesized CdZnSeS/ZnS core-shell quantum dots with high quality and high stability.