Study On High Efficiency And High Stability Quantum Dots For Display Applications

Quantum dots（QDs）are emerging semiconductor nanomaterials with excellent optoelectronic properties,and have great application prospects in the next generation of advanced lighting and display technologies.In the key project of"New Display and Strategic Electronic Materials"in the"14th Five-Year Plan"of China,QDs are regarded as the key technology for the development of new display materials.However,it is still a great challenge to realize the long-term efficient and stable operation of QDs in display devices.The main factors affecting the efficiency and stability of QDs are light,heat,moisture,and oxygen,which will induce ligand shedding or surface oxidation of QDs,and form defects that capture electrons and holes on the surface of QDs.Thereby affecting the radiative recombination process of QDs,which will reduce their luminous efficiency and stability.Focusing on this key problem,we propose some new preparation methods and composite structures of QDs to prepare highly efficient and stable QD materials.The outline of our studies is briefly introduced below:（1）A strategy of doping titanium ions in ZnS shells was designed to improve the stability of QDs.In the previous reports,the oxidation of surface elements of QDs by moisture and oxygen has been regarded as the main culprit of the performance degradation of QDs.Therefore,blocking or delaying the contact between moisture/oxygen and QDs will effectively improve the stability of QDs.Herein,we improve water and oxygen’s difficulty in oxidizing QDs by introducing titanium ions into the shell of ZnS.The results showed that titanium ions were successfully doped into the shell of ZnS and formed titanium sulfide,which gave the QDs better stability.After 96 hours of accelerated aging,the titanium-doped QD sample retained 51.4%of its initial performance,while the optical performance of the undoped QD sample decreased to 24.5%of its initial value.In addition,the reasons for the stability improvement of titanium-doped QDs are explored.The binding energy of the Ti-S bond（1.715 eV）is stronger than that of the Zn-S bond（1.14 eV）and the doped titanium ions form titanium sulfide in the ZnS shell.As a result,when water oxygen diffuses to the surface of the QD,the Ti-S bond requires more energy to be broken than the Zn-S bond.（2）The sol-gel method and powder atomic layer deposition（ALD）process were introduced to realize the dual physical packaging of SiO₂ and Al₂O₃ for QDs,which improved the stability by hundreds of times for the QD fluorescence microspheres（QLuMiS）.Previous studies have shown that direct ALD coating Al₂O₃ or sol-gel coating SiO₂ is challenging to provide adequate protection for QDs.The former is due to the alkyl on the surface of QDs preventing the growth of Al₂O₃ and forming a complete coating.The latter is due to incomplete hydrolysis of the residual hydroxyl group by SiO₂ precursors,which leads to its inner space being full of void and not dense enough.Herein,we design a two-step strategy:coating the QDs with SiO₂ and then depositing Al₂O₃ on the surface of SiO₂-coated QDs.The hydroxyl group generated by the hydrolysis of SiO₂ provides the adsorption site for the ALD deposition of Al₂O₃.This strategy not only solves the problem of SiO₂ coating is not dense but also solves the problem of Al₂O₃ deposition integrity,which significantly improves the stability of quantum dots.The results show that the prepared QDs@SiO₂@Al₂O₃ QLuMiS show very high stability.After 1056 hours of accelerated aging,its performance remains above 86%of the initial value,while QDs@SiO₂ decreases to 28%of the initial value.Similarly,the performance of QDs@Al₂O₃ and initial QDs samples with direct deposition of Al₂O₃ decreases to 24%and 5.7%of the initial value,respectively,after only 240 hours of aging.By calculation,the accelerated aging half-life（L50）of the prepared sample is 4969 hours,and the L50 life of the photoconversion film is 10739hours,which is more than 11 times and 621 times higher than that of QDs@SiO₂ and initial QDs samples,respectively.（3）In this work,polypropylene（PP）substituted polymethyl methacrylate（PMMA）for the preparation of QD photoconversion film was proposed,and the reason for the optical performance attenuation of QDs induced by polar groups in the polymer material is revealed.Compared with PP,PMMA can be dissolved by some common solvents due to its strong polarity,so it is widely prepared into QD photoconversion film by solvent volatilization method,while PP has weak polarity and is difficult to be dissolved by the solvent,so it needs to be prepared into QD photoconversion film by thermal processing.The experimental results show that the stability of QDs in the PP film prepared by thermal processing（PP-H）is significantly better than that of the PMMA film prepared by thermal processing（PMMA-H）and the PMMA film prepared by solvent volatilization method（PMMA-S）.After 216 hours of blue light accelerated aging,QDs can maintain more than 90%of the initial PLQY in the PP-H film,and only63%of the initial PLQY in the PMMA-H film.For the PMMA-S film,the PLQY of QDs quickly attenuates to 25%of the initial value.Those results exhibit that PP material and related thermal processing methods can promote the stability of QDs.Firstly,we found that the surface of the QD photoconversion film prepared by thermal processing is flatter and denser than that prepared by solvent volatilization,which is conducive to reducing the influence of water and oxygen on QDs.Secondly,PP has better hydrophobicity and lower water-oxygen permeability than PMMA,which can also increase the stability of QDs.Most importantly,we found that PP has less effect on the optical performance of QDs than PMMA,so QDs are more stable in PP.PP is composed of weak polarity groups,which can hardly induce the ligand shedding of the QDs,while the ester group in PMMA has strong polarity,which is easy to induce the ligands on the surface of QDs to fall off,and form dangling bonds to become defects of electron capture,thus leading to the decline of the optical performance of QDs.