Preparation Of Large-Size Red-Light Alloy Quantum Dots And Their Performance In Electroluminescent Devices

Quantum-dots Light-Emitting Diodes(QLED)are expected to become the next-generation lightemitting display technology after CRT,LCD,and OLED due to their good optical advantages.Through the research on the whole field of quantum dots in the past forty years,researchers have reached a certain consensus on the preparation process of materials,the structural design of devices,and the evaluation standards of optical properties.However,although quantum dots have made breakthroughs in the research of long life,high brightness,high efficiency,and no roll-off,the red,green,and blue quantum dots have different degrees of short life and high efficiency under high brightness.There are a series of problems such as rolloff of external quantum efficiency,long life and high efficiency.At the same time,this thesis found through research that the optical properties of quantum dot materials inevitably have the above-mentioned problems under the premise that the particle size does not exceed 20 nm.Since quantum dots are a material with a relatively large specific surface area,in the process of epitaxial growth with the help of organic ligands,the smaller the particle size of the quantum dots,the larger the specific surface area,and thus the larger the proportion of organic ligands on the surface.When the quantum dots are spin-coated on the test film,this organic ligand will act as a resistor during the carrier migration process,hindering the transport of carriers.Therefore,this paper proposes to increase the particle size of quantum dots to reduce the specific surface area of quantum dots,thereby reducing the influence of surface ligands on carrier migration and recombination.The traditional way of preparing quantum dots is to grow the shell layer by layer based on a smaller crystal nucleus,so the usual way to increase the particle size of quantum dots is to increase the thickness of the shell layer.However,as the thickness of the quantum dot shell increases,the crystal stress inside the interface cannot be effectively released,thus affecting the quantum yield.Compared with quantum dots with a core-shell structure,quantum dots with an alloy structure have a flatter energy level barrier,and at the same time,the proportion of lattice defects at the interface is small,which is conducive to the preparation of large particle size quantum dots.Based on this,this paper uses red quantum dots as a breakthrough to carry out related research on large-size alloy quantum dots.The main work is divided into the following two parts:(1)ZnCdSe/ZnSeS/ZnS fully gradient alloy structure quantum dots.The synthesis route of alloying is adopted to increase the particle size of quantum dots.Firstly,high-quality ZnCdSe alloy nuclei were prepared by adjusting the reaction temperature,solvent concentration,reaction time and other conditions.Secondly,a ZnSeS alloy shell layer with a suitable thickness is grown outside the ZnCdSe crystal nucleus to form ZnCdSe/ZnSeS structure quantum dots.Finally,through the fine regulation of the ZnS layer,an ultra-thin ZnS layer is grown outside the ZnCdSe/ZnSeS quantum dots to improve the stability of the quantum dots.From the analysis of the energy level structure of quantum dots,ZnSeS,as a wide bandgap material,can well confine the electron wave function and hole wave function of the ZnCdSe crystal nucleus in the nucleus.At the same time,the ZnSeS valence band energy level position is higher than that of ZnS,which helps to improve the hole injection efficiency.A series of characterization shows that the ZnCdSe/ZnSeS/ZnS gradient alloy structure quantum dots have a particle size of 17 nm,uniform particle size and tight arrangement,quantum yield over 95%,and high color purity.The mapping test shows that the quantum dots have good penetration of Se and Zn.In addition,the cross distribution of Se and S elements in the ZnSeS layer eliminates the lattice difference between the ZnCdSe crystal nucleus and the interface of the ZnS layer,which indicates a high degree of alloying of quantum dots.XRD and other related characterizations were carried out for different shell thicknesses,and the XRD test spectral lines(100),(002),and(101)angles of quantum dots changed in different trends.It is analyzed that the ZnSeS layer has the characteristics of polarized luminescence through the directional release of the internal stress of the crystal nucleus,which is of great significance for further improving the quality of quantum dots and device performance.Finally,the performance test of quantum dots was carried out by constructing a QLED device.The results showed that the ZnCdSe/ZnSeS/ZnS quantum dots prepared under the optimal conditions had a peak external quantum efficiency of 30.3% at a turn-on voltage of 1.91 V,and a brightness of 342715 cd/m2.(2)ZnCdSe/ZnCdSe/ZnS super large crystal nucleus quantum dots.The main idea of the above method is to increase the particle size of the quantum dots by growing a shell with a wider bandgap.Although ZnCdSe/ZnSeS/ZnS quantum dots have achieved good results in the research of fluorescence intensity,fluorescence quantum yield,alloying,etc.,the problem of fluorescence intensity decline appears in the process of increasing shell thickness,which shows that This method cannot take into account the optical properties of quantum dots while increasing the particle size.In addition,due to the above method,the goal of 20 nm large particle size quantum dots has not been achieved.Therefore,this chapter changes the design idea.Based on the research on the ZnCdSe high-quality alloy nucleus of ZnCdSe/ZnSeS/ZnS quantum dots,through a series of controls,the ZnCdSe layer is successfully grown outside the ZnCdSe nucleus to form a ZnCdSe/ZnCdSe super large nucleus quantum point.In addition to the super large crystal nucleus of ZnCdSe/ZnCdSe,the outermost layer of ZnS was further finely regulated,and ZnCdSe/ZnCdSe/ZnS quantum dots with a particle size of 21 nm were prepared.According to XRD test analysis,the quantum dots of this structure have more obvious directionality in the(100)direction.By analyzing the directionality of quantum dots,a way of thinking is provided for the study of quantum dot luminescence properties: that is,the directionality of quantum dots can be improved through the regulation of materials,and then the external quantum efficiency can be improved.Finally,the luminescence performance of ZnCdSe/ZnCdSe/ZnS quantum dots was tested by building a QLED device platform.The results showed that when the turn-on voltage was 1.67 V,the peak external quantum efficiency of the device reached 32.24%,which exceeded the highest value reported so far.