Study On Component Gradient Regulation Of InP-Based Core-Shell Quantum Dots And Electroluminescent Devices

With the advantages of high fluorescence quantum yield（PLQY）,continuously tunable emission spectrum,high color purity,and good photochemical stability,semiconductor quantum dots are considered to prominent candidates for next-generation optoelectronic display materials.After nearly 30 years of development,numerous synthesis methods for high-quality semiconductor quantum dots have been developed and are widely applied in display lighting,printing,biomedicine,semiconductor lasers,solar cells,and other fields.Among many semiconductor quantum dot materials,cadmium（Cd）-based quantum dots are the most rapidly developed,with the external quantum efficiency（EQE）of red,green and blue light-emitting diodes based on Cd-based quantum dots exceeding 20%respectively.However,with the increasing demand for environmental protection in lighting display technology,the development and application of heavy metal free and environmentally friendly luminescent materials are urgent.At present,the most studied environmentally friendly quantum dots are indium phosphide（InP）-based quantum dots and zinc selenide（ZnSe）-based quantum dots.Among them,the peak EQE of red InP-based and blue ZnSe-based QLED has exceeded 20%and the device lifetime(T₅₀)has reached tens of thousands and millions of hours respectively,meeting the basic requirements for industrial applications.However,compared with red InP-based quantum dots,the size of InP nuclei usually needs to be controlled in the range of 1.5-2.5 nm due to the small band gap of InP materials（1.35 e V）in order to ensure that the fluorescence peak position of InP-based quantum dots lies in the green range（510-550 nm）.However,the smaller the size is,the larger the specific surface area is.And the strong tetrahedral covalent bonding coordination of III-V quantum dots leads to the surface of InP nuclei being highly susceptible to oxidation（InPO_xor In₂O₃）with less than 1%PLQY and unstable in air.In order to maximize the passivation of quantum dots,ZnS,ZnSe,GaP and other materials are usually used to grow the shell layer to construct core-shell structured quantum dots.But the large lattice mismatch between the core-shell materials easily leads to distortion of the core-shell material lattice and introduction of interfacial defects.So the size of green InP-based core-shell structured quantum dots is usually less than 10 nm and the size distribution is not uniform.After film formation,the F?rster resonance energy transfer（FRET）between the particles leads to a rapid decrease in the PLQY of the film compared with the solution.For the device,the structure of electroluminescent devices constructed based on this light-emitting material mainly refers to Cd-based QLED.But the interaction mechanism between the quantum dot luminescent layer and the charge transport layer,as well as the attenuation mechanism of device performance,is not very clear,resulting in the imbalance of electron and hole injection,which makes it difficult to effectively improve the device efficiency and stability.Based on the above analysis,this thesis selects non-toxic InP quantum dots as the basis and chooses a suitable shell layer structure for thick shell layer cladding of InP nuclei to increase the quantum dot size while maintaining a high PLQY.Meanwhile,the effect of thick and thin shell layers on quantum dot device performance is explored,and the QLED device structure is optimized to improve the carrier injection balance.The main contents include the following two parts:（1）Aiming at the instability of InP crystal core and the lattice mismatch of core and shell materials,the core and shell structure of green InP-based quantum dots was selected.Firstly,the nucleation conditions of InP（nucleation temperature and In:Znratio）were adjusted to prepare high-quality InP crystals.To ensure the stability of InP crystal nucleus in air during purification,a thin ZnSe shell layer was wrapped outside the nucleus.The green InP/ZnSe/ZnS core-shell quantum dots were synthesized based on the InP/ZnSe/ZnS structure of red InP-based quantum dots with mature synthesis conditions,and the shell cladding conditions suitable for green InP-based quantum dots were optimized.It has a peak position at 535nm and a PLQY up to 95%,but the FWHM is as wide as 40 nm and the size was only 7 nm.Overall,the optical properties of the quantum dots is not good.The peak EQE for constructing QLED devices based on this light-emitting material is only 2.54%,and the maximum luminance is 23916 cd m^-2.To solve the problem of low PLQY and small size of this structure,an alloy gradient ZnSeS shell layer was introduced into the quantum dot core-shell structure.After optimizing the Se and S ratio to 3:1,green InP/ZnSe/ZnSeS/ZnS core-shell quantum dots with better optical properties were obtained.The peak EQE of the QLED constructed based on this material is 4.68%and the maximum luminance is increased to38501 cd m^-2.The optimal structure of green InP-based quantum dots was finally determined as InP/ZnSe/ZnSeS/ZnS.（2）To address the problems of low PLQY of small-size quantum dot films and unbalanced charge injection in the devices,firstly,the green large-size InP/ZnSe/ZnSeS/ZnS core-shell quantum dots was prepared by screening the suitable Znsource precursors,optimizing the ZnSeS shell layer cladding method,continuously adding precursors in the gradient shell layer of ZnSeS alloy and extending the reaction time.It has a fluorescence peak position of 532 nm,FWHM of 37.5 nm,PLQY of 83%,and a particle size of 11.6 nm,which is highly crystalline.Compared with thin-shell quantum dots,thick-shell quantum dots have better photochemical stability.Their film fluorescence lifetime decreases less and the film PLQY is two times higher than the thin-shell quantum dots.which demonstrates that the thick shell layer can effectively suppress the FRET between quantum dots.Secondly,in order to improve the charge injection balance and increase the radiation compounding efficiency within the quantum dot light-emitting layer,PF8Cz with shallower LUMO energy level was used as the electron transport layer to inhibit the electron leakage from the light-emitting layer to the electron transport layer.At the same time,a PVP blocking layer was introduced between the electron transport layer and the quantum dot emitting layer to limit the injection of some electrons.The peak EQE and current efficiency of the optimized QLED device are 13.59%and 55.69 cd A^-1,respectively,and the maximum luminance can reach 114698 cd m^-2.