Inorganic Perovskite CsPbBr₃ Quantum Dots:Stability And Light Emitting Applications

Inorganic cesium lead halide perovskite(Cs Pb X3,X=Cl,Br,I)quantum dots(QDs)are a class of semiconductor nanocrystals with excellent photoelectric properties,such as high photoluminescence quantum yield(PLQY),narrow emission spectrum and the emission spectrum tenability.They exhibit a wide potential application in optical field.Among the different members of the all inorganic perovskite family,CsPbBr3QDs is a very appealing material for optical applications owing to the outstanding optical properties,simple synthesis route and mass-produced.Unfortunately,the optical properties of these perovskite QDs can be affected with internal and external factors such as water,oxygen,light,high temperature,and inherent instability.It greatly restricts the potential applications of the Cs Pb X3 QDs.This dissertation focuses on the stability issue of CsPbBr3 QDs.In attempts to improve stability,we have carried out the ligand modification and doping strategy for CsPbBr3 QDs and explore their properties in the heating condition,polar and other special environments.Besides,we demonstrate the amplified spontaneous emission(ASE),the whispering-gallery(WGM)mode lasing and the light emitting diode(LED)based on the prepared CsPbBr3 QDs.The results show that the CsPbBr₃ QDs prepared by ligand modification and doping exhibit an enhanced stability and optical properties.The main research contents are summarized as follow:(1)Using a combination of theoretical calculations and experimental evidence,we analyse the stability of the ligand modified CsPbBr3 QDs.The surface ligand modification is an effective strategy to improve the stability of the QDs.Based on this view,we have developed a 2-hexyldecanonic acid with two branch to modify the CsPbBr₃ QDs.In theory,we have clarified the mechanism of the stability enhancement for ligand modified CsPbBr3 QDs by using density functional theory(DFT).Experimentally,we have synthesized the common CsPbBr3 QDs and the ligand modified CsPbBr3 though the hot-jection method.In order to investigate the stability performance of CsPbBr3 quantum dots with ligand modification,we have studied the two CsPbBr3 QDs solution stability in air,the thin film stability in air,the QDs solution stability in ethanol and water.After 70 days of storage in air,the common CsPbBr3 QDs has undergone a change from clarification to precipitation,while QDs solution with ligand modification still can maintain relative clear clarification.To further prove the stability of the photoluminescence(PL)properties,the ligand modified CsPbBr3 QDs film still maintains 94.3%of initial PL intensity even in air exposure for 28 days,while the PL intensity of common CsPbBr3 film reduces to 21.2%of initial PL intensity.In ethanol solution surrounding,the regular CsPbBr₃ exhibitis a severe decrease to 22.3%with respect to the initial value,while the ligand modified CsPbBr₃ QDs still maintained their 66%of initial PL intensity even after 90 min.In the water environment,the regular CsPbBr₃ QDs are almost completely degraded after storage for 7 days,while the ligand modified CsPbBr₃ QDs can still maintain a dispersed state in the solution and emit strong green fluorescence.The results of the performed expertiments prove again that the stability of the ligand modified CsPbBr₃ QDs has been significantly improved.(2)The investigations for the ASE and WGM lasing based on the regular CsPbBr₃QDs and the ligand modified CsPbBr₃ QDs.Based on the high optical gain properties of CsPbBr₃ QDs,the performance of the ASE and WGM lasing of CsPbBr3 quantum dots before and after ligand modification has been carried out.In comparsion with the common CsPbBr3 QDs,the ligand modified CsPbBr3 QDs exhibit low threshold values and high stabilities for both ASE and WGM lasing.The ASE threshold of ligand modified CsPbBr3 QDs thin films also decrease to more than 50%of common CsPbBr3 QDs thin films under both the one and two photon pump excitation.In comparison with the unmodified CsPbBr3 QDs,the lasing threshold of WGM lasers obtained from the modified CsPbBr3 QDs decrease obviously under one-photon and two-photon pump excitation,respectively.These results indicate that DA ligand modified CsPbBr3 QDs can obtain the high optical gain and excellent stability.This thesis has expanded the field of lasing application from the CsPbBr₃ QDs.(3)Realizing the green and the warm WLEDs based on the ligand modified CsPbBr₃ QDs.Using the ability of light conversion from the ligand modification CsPbBr₃ QDs,we have aimed to realize the pure green LED fabrication and the white LED application.Compared with common CsPbBr₃ quantum dot coated LED,the ligand modified CsPbBr₃ quantum dots coated LED exhibit a higher color purity,higher thermal stability and stronger PL intensity.In addition,the warm white light-emitting diodes(WLEDs)based on ligand modified CsPbBr₃ QDs exhibits a color rendering index of 93,a power efficiency of 64.8 lm/w,a CIE coordinate of(0.44,0.42)and correlated color temperature value of 3018 K.This thesis has promoted the research of LED based on ligand modified perovskite quantum dots.(4)Lightly doping of Sn²⁺into CsPbBr₃ QDs at room temperature and the application in white LED.Doping is one of the most important strategies for tunable photoelectric properties of the semiconductor materials.Based on the room temperature synthesis method,we have prepared the pure CsPbBr₃ QDs and the CsPbBr₃ QDs doped with Sn²⁺.We have explored their crystal structure,composition and optical stability.In addition,the QDs solution obtained by doping 20%Sn²⁺can reach a highest PLQY value.The warm LED based on the doping CsPbBr₃ QDs can be realized and exhibit a higher CRI value than that of the pure CsPbBr₃ QDs.This doping stratege possesses three obvious advantange,i.e.uniform doping,facile process and suitable for mass production.In summary,this thesis has explored ligand modification and doping strategy to prepare the CsPbBr₃ QDs,and explored their stability and the corresponding optical properties,thereby improving the lighting device performance.This thesis will contribute to the further development of CsPbBr₃ QDs in the optical field.