Fabrication Of Large Green Light InP-based Quantum Dots And Their Application In Electroluminescence Devices

Quantum dots(QDS)are an ideal luminescent material with the advantages of adjustable color,high color purity and high luminous efficiency.In order to meet the requirements of the next generation of high efficiency and wide color gamut display devices,it is necessaty to synthesize quantum dots with high brightness and narrow half-peak width.Although cadmium selenide(CdSe)and perovskite based quantum dots have high photoluminescent quantum yields(PLQY>90%)and narrow half-peak width(FWHM<20 nm),they contain toxic heavy metals,namely cadmium(Cd)and lead(Pb),which limit their practical application in the market.Indium phosphide(InP)quantum dots are typical Ⅲ-Ⅴ semiconductor nanoctystals with high absorption coefficient,low toxicity,large exciton pole radius and high carrier.Their spectral range can cover the entire visible region,and high color purity,high luminous efficiency,so that they are expected to replace the traditional Cd/Pb based quantum dots.Early InP-based quantum dots were mainly based on indium phosphide/zinc sulfide(InP/ZnS)core/shell structure,but the resulting InP/ZnS single-shell quantum dots had low half-peak width(FWHM>45 nm)and quantum yield(PLQY<80%),mainly due to the large lattice mismatch between InP and ZnS(7.7%).There is lattice distortion at the core-shell interface,and a large number of defects are generated,which limits the epitaxial growth of the thick shell.Therefore,the fluorescence properties of quantum dots can be significantly improved by inserting ZnSe(3.3%),which has a smaller lattice mismatch,between the InP nucleus and the outer layer of ZnS,thereby effectively reducing the interface distortion.The surface of InP quantum dot crystal core is easy to be oxidized,leading to the increase of defects,and there are many suspension bonds,which seriously affect its fluorescence performance.On the other hand,due to insufficient precursor activity,it is difficult to grow a thick shell to inhibit QLED Auger recombination and Forster energy resonance transfer,which seriously affects its device performance.These problems are more prominent in green light InP quantum dots,because InP crystal nuclei are smaller,specific surface area is larger,surface defects and hanging bonds will be more,and high-quality InP quantum dots synthesis is more difficult,which seriously restricts the development of InP green light quantum dots.At present,the optical performance of InP-based red quantum dots is close to that of CdSe-based quantum dots.The maximum external quantum efficiency of InP-based red quantum dots is more than 20%,and the maximum brightness of InP-based red quantum dots has reached 136090 cd/m2.However,InP green quantum dots are still far behind.The maximum QLED external quantum efficiency is 16.3%,and the maximum brightness of the device is only 15,606 cd/m2.Its performance is far from the requirements of commercial applications.In this paper,the precursors of InP were adjusted,such as phosphorus/indium(P/In),indium/myristic acid(In/MA)and indium/zinc(In/Zn).In this process,zinc ion and tri-n-octylphosphine(TOP)are used to slow down the nucleation and growth rate of InP nanoctystals.Myristate acid is used as the ligand.After optimizing the ratio of each precursor,high quality InP nanoctystals with uniform size distribution and controllable peak position are obtained.After that,zinc myristate(ZnMy2)was used to treat the surface of InP crystal core to remove excessive indium ions on the surface,thereby reducing impurities in InP crystal core and the subsequent growth of ZnSe shell,so that each component in the core-shell structure of quantum dots was controlled.Then,selenium-octadecene(Se-ODE)is added for coating ZnSe thin shell.Because ZnSe shell is relatively stable,it can not only passivate the defects on the surface of InP crystal nucleus,but also prevent the InP ctystal nucleus from being seriously oxidized during the purification process,so as to minimize the defects on the surface of InP crystal nucleus.Then zinc acetate(ZnAc2)and zinc stearate(ZnSt2)were mixed with zinc source and Se-ODE suspension as selenium precursor,and ZnSe shells with appropriate thickness were coated.Then,the ZnS shell was coated with S-ODE-TOP as the sulfur source,and the highfluorescence InP/ZnSe/ZnS green quantum dots with a fluorescence peak of 540 nm and a half-peak width of 42 nm were obtained.On this basis,the QLED device is constructed and the hole transport layer is adjusted.Finally,the EQE of 13.64%and the maximum brightness of 95489 cd/m2 are obtained,and the maximum brightness is about 6 times of that reported.In this paper,we also conducted a series of studies on the growth of ZnSe thick shell in InP/ZnSe/ZnS.First,we found that TOP can promote the growth of ZnSe shell,and then tried to add TOP in different synthesis processes to promote the reaction,and successfully obtained larger size of green light quantum dots based on ZnSe.Later,we studied the activity of different Se precursors and measured the growth process of ZnSe shell.It was found that the activity of Se-ODE-TOP was moderate,which could make the quantum dots grow more evenly,and keep the quantum dots at the right peak position and fluorescence intensity.After that,we further studied the ratio of TOP/ODE in the Se-ODE-TOP precursor to control the growth rate of ZnSe shell,so as to make it grow evenly and maintain high fluorescence intensity as far as possible.Finally,we obtained green quantum dots with a particle size of 9.9 nm and a maximum size of 11.1 nm.This is almost the largest size of InP-based green light quantum dots reported so far.Subsequently,we constructed largesize InP/ZnSe/ZnS QLED devices with green light quantum dots,but the properties of the devices decreased compared with those of reported green InP-based QLED.We speculated that there were two reasons for this.On the one hand,as the quantum dots themselves increased in size,surface defects and non-radiation recombination increased,leading to the deterioration of device performance.On the other hand,due to the mismatch between the transport layer materials in the device and the energy level of large quantum dots,the imbalance of electron and hole injection leads to the poor performance of QLED devices.