Ligand Exchange Of Alloyed Quantum Dots For Display Applications

Solution processed semiconductor nanocrystals,also known as quantum dots(QDs),possess many advantages such as high quantum efficiency,tunable emission spectra and narrow full width at half maximum(FWHM).Electroluminescence(EL)and photoluminescence(PL)applications based on QD materials have been rapidly developed.Precise regulation of the QDs'properties to meet the requirements for lighting and display applications has become a subject of intense interest.Surface ligands have significant influences on the synthesis,optoelectronic properties and dispersibility of QDs.By precisely control of surface ligands,QDs can be more compatible with processing solvents for fabricating composite fluorescent materials and white light emitting diodes with multiple emission layers.Furthermore,alcohol-soluble QDs obtained by ligand exchange show better film formability,which is crucial for printing based display applications.In this work,based on alloyed CdSe QDs with well-developed synthetic route in our group,we investigated the dispersibility of the QDs in different solvents by exchanging surface ligands.A series of QDs capped with mercaptan ligands were developed and their EL and PL applications were explored.The main achievements are summarized as follows:(1)CdSe QDs were synthesized using high temperature injection method.Then the surface ligands were exchanged also through high temperature injection under inert environment.The obtained hydroxyl-terminated QDs changed from non-polar solvent soluble to alcohol-soluble and maintained excellent optical properties.Subsequently,we explored the EL and PL applications of the QDs with exchanged surface ligands.(2)Second ligand exchange was achieved by converting hydroxyl-terminated CdSe QDs with mercaptan ligands through high temperature injection method under inert environment.Surface ligands with different chain length and spatial conformation can be regulated this way,thus controlling the properties of QDs.(3)Inorganic salt and QD composite material was obtained by blending hydroxyl-terminated QDs with sodium nitrate and calcium chloride and co-crystalizing with precipitants.The applications in light emitting diodes of this composite were explored.What's more,hydroxyl-terminated QDs are employed in EL device.Reduced charge injection barrier and enhanced charge transfer rate were achieved with the short-chain ligands.The optimized device showed maximum luminance of 10,000 cd/m~2,turn-on voltage of 4 V and current efficiency of 2 cd/A.