Synthesis Of Cesium Lead Halide Perovskite Quantum Dots And Their Application In Down-conversion LED Devices

Quantum dots are nanoscale particles composed of limited numbers of atoms where the motions of electrons are confined in all directions.One of the most outstanding properties of quantum dots is the quantum confinement effect,where their band gap would be altered along with the shrinking or expansion of particles.In addition,quantum dots perform size effect,surface effect and quantum tunnelling effect,which enables them to hold vital roles in optoelectric devices.The cesium lead halide（CsPbX₃）perovskite quantum dots embrace distinguish features of high quantum yield,high monodispersity,narrow size distribution,high stability,simple synthesis methods,short photoluminescent lifetime and tunable emission spectra alongside different chemical compositions,and these make them candidate materials for next generation optoelectronic devices including light-emitting diodes,photo detectors,quantum dot lasers and solar cells.In this dissertation,beginning with the synthesis method of CsPbX₃ quantum dots,modifications were made in the synthesis method,and studies were carried out in reaction process and their properties.Down-conversion mono-color and white-light LEDs were fabricated based on CsPbX₃ quantum dots.Main achievements are listed as followed.（1）CsPbBr₃ quantum dots were synthesized in 1-octadecene using octadecanoic acid and oleylamine as binding groups.Characterization results showed evident exciton absorption behaviors.The quantum dots had high absorption ratios in UV-vis spectra region and narrow emission spectra of 21 nm with high quantum yield of 86%.The quantum dots had uniform crystalline morphology which were square-shaped and cubic phase crystalized.The particles had equivalent size with their Bohr diameter of 7nm.Experiment results showed that the reaction was quite fast which completed after about 3s and reaction time had affections on their spectra.Along with the reaction time,the size of quantum dots would be more uniform.The employment of octadecanoic acid could effectively lower the reaction temperature and excellent products were achieved in the range of 80℃-160℃ with strongest emission at 120℃.（2）We studied the anion-exchange in CsPbBr₃ quantum dots with zinc halide salts providing as anion sources and high-quality exchange products were obtained.The reaction could be carried out under room temperature without the protection of inert gas.The exchange reaction was extremely fast which completed in seconds.The emission spectra of quantum dots could cover the whole visible spectra region from 410 nm to 700 nm by simply regulating the addition of halide source.The exchange reaction was able to obtain Cs Pb Cl3 quantum dots which were difficult to synthesize by direct reaction method,as well as strong luminescent cubic-phase Cs Pb I3 which showed orthorhombic phase under room temperature and the quantum dots were especially stable in surrounding environment.We tracked the reaction process and found that the emission position could be regulated by changing the relative concentration of various anions in the solution.The reaction proved to be reversible,indicating a fast interaction between crystals and surrounding environment.The exchange reaction proceeded in two periods including fast surface exchange and slow internal exchange.The stability of the exchange products was attributed to the high stability of parent quantum dots and the preservation of surface ligands.A hypothesis was raised concerning the evolution of full width at half maximum peak widths from short to long wavelength where the size fluctuation had different affections on the band gap width.（3）Mono-colour light-emitting diodes and white light LED were packaged using the anion-exchange products as the downconversion layer.The results showed a high colour purity of up to 99% and high luminous efficiency of up to 21.5lm W-1 of the mono-colour LEDs.The LEDs also showed high stability to driving current.As to WLED,the application of red-emission anion-exchange quantum dots raised their colour rendering index to 90.5%,lowered their correlated colour temperature to 3561 K,and the operation of the resultant WLED stayed stable under different driving current,which makes conventional phosphor more applicable.