Investigation Of Pattern Films And Electroluminescence Devices Based On Protonation On Surface Ligands Of Quantum Dots

Quantum dots(QDs)with high photoluminescence quantum yield(PLQY),tunable emission wavelength,narrow emission width,and photochemical stability,have been widely used in solar cells,photodetectors,biomarker detection,light-emitting diodes(LEDs),and other fields.Especially,QDs display and lighting technology become one of the focus research fields.With the development of QDs synthesis technology and the optimization of the LEDs device structure,the external quantum efficiency(EQE)of quantum dots light-emitting diodes(QLEDs)has exceeded over 20%,even comparable to the current commercial organic light-emitting diodes(OLEDs).Generally,the surface of QDs contains a large number of organic ligands with long alkyl chains.Although,organic ligands can improve the stability of QDs solution and reduce the leakage current of QLEDs.The insulating property of organic ligands limits the carrier transmission of QLEDs and hinders the improvement of the QLEDs efficiency.Therefore,researchers have sought to improve carrier mobility by replacing insulating long-chain organic ligands with conductive short-chain inorganic ligands.In general,the driving force of anion ligand exchange reaction comes from the difference in binding energy,which largely depends on the chemical composition of the QDs surface.However,due to the unique chemical properties and coordination environment of the QDs surface,it is very challenging to realize ligands exchange through anion exchange directly.Based on the above problems,this thesis mainly investigates the QDs surface of carboxyl and amino functional groups of organic ligands.According to the hard and soft acid-base theory,the hard acid H proton and hard base carboxyl could combinate and desorb from the QDs surface by the principle of "hard acids prefer hard bases,soft acids prefer soft bases ".The exchange reaction of short-chain anions and long-chain ligands is completed after the anionic occupy sites.The trace amount of inorganic acid not only cannot corrode the QDs film and decrease the photoluminescence(PL)intensity but also can passivate the defects and promote the carrier transport capacity to improve the efficiency of the QLEDs.In addition,using trace hydrochloric acid vapor or photoinitiator patterned QDs films,which provides a new idea for color QDs display technology.The main contents of this thesis are as follows:1.Firstly,CdSe/ZnS QDs film treated by the HCl vaper not only could not destroy the fluorescence intensity of the QDs film but also showed anti-polar solvent properties.We found that H+can protonate the carboxyl and amino ligands on the surface of QDs and desorbed the ligands from the surface of QDs through analysis of the elemental kinds and the chemical environment on the surface of QDs films with HCl treatment.The inorganic short-chain ligands showed a higher conductivity of QDs film compared with the original QDs film.The performance of monochrome QLEDs also exhibited EQE enhancement based on HCl treatment QDs films.2.Based on the solvent resistance of CdSe/ZnS QDs film treated by HCl,we successfully fabricated the WQLEDs with red,green,and blue three-layer QDs films.However,the energy of blue light will be absorbed by red and green QDs with a smaller band gap with the existence of Forster resonant energy transfer(FRET),which is not conducive to white light emission.To balance the luminous intensity of red,green,blue three colored light and ensure the efficiency of blue light maximum output,we found that the arrangement of the function layer was the blue QDs/green QDs/red QDs which can avoid the quantum dot trap effect and decrease the blue light is absorbed.The maximum EQE of the WQLEDs device was 9.1%and the CIE coordinates were(0.34,0.33).3.Perovskite QDs come into view in recent years.Due to the water and oxygen instability of perovskite materials,it cannot be treated by acid solution directly.However,we found that MBT can change the emission color of CsPbBr3 perovskite QDs and make the PL emission wavelength blue shift under UV excitation by the photoinitiato 2-(4Methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine(MBT).Combined with the PL spectral blue shift phenomenon of two-dimensional perovskite material without ligands and the acidity of MBT in toluene solution,MBT can generate Cl radical under UV light which reacts with toluene to produce the HCl finally.Then,the HCl could protonate ligands on the surface of QDs to produce vacancy sites that can be occupied by Cl ions.The generated halogen mixed structure changed the PL spectra.In addition,we found that a group of photoinitiators containing Cl,Br,and I radicals can achieve a reversible transformation between blue and green light of perovskite QDs.The patterned luminescent films were fabricated by direct photolithography based on the perovskite QDs with MBT photoinitiator under UV.