Room Temperature Preparation And Optical Properties Of Highly Stable All-inorganic Lead Halide Perovskites

All-inorganic lead-halide perovskite materials（CsPbX3,X=Cl,Br,I）are widely used in light-emitting diodes（LEDs）,collectors and solar cells due to their tunable emission band,wide color gamut and high quantum yield.However,the poor stability to water,light and heat due to the unique structure and defects of perovskite materials limits their application and commercialization.On the other hand,most of the previously reported preparation methods for perovskites usually required highly toxic and corrosive reagents or harsh experimental conditions（e.g.,high temperature,inert gas protection,etc.）.In view of the above two problems,we propose in the present thesis a facile,environment-friendly and scalable protocol to synthesize bromine-based perovskite materials at room temperature.Then,KBr was introduced in the precursors in improve the optical properties of the perovskite materials.Finally,we prepared highly stable red emissive perovskite materials by introducing 3-mercaptopropionic acid（MPA）or thioglycolic acid（TGA）at room temperature by ligand-assisted reprecipitation method（LARP）,and studied the optical properties.The main results are as follows:（1）The perovskite materials were successfully synthesized by a simple room temperature solvent evaporation procedure without addition of any ligands.The representative sample with PL emission wavelength of around 516 nm was achieved by adjusting the ratio of cesium and lead,and the types of solvents.The morphological and structural characterizations indicated that the typical powder sample was in a unique sheet-like structure containing CsPbBr₃ and Cs₄PbBr₆ nanocrystals embedded in Pb（OH）Br.Most importantly,the phase and morphology can be converted to a micro-rod-like structure containing CsPb₂Br₅ nanoparticles embedded in Pb（OH）Br after soaking the as-synthesized sheet-like sample in water at room temperature.Concomitantly,the luminescence intensity and stability of optical performance were greatly improved.The photoluminescence quantum yield of the perovskite material before and after water treatment increased from 2.40%to 27.14%,suggesting an increase of around 11 times.The mechanism of the phase transition is explained by the morphology and crystalline characterization of samples before and after water treatment.Benefiting from the protection of as-formed Pb（OH）Br shell,the perovskite material showed excellent stability upon exposing to water,UV light and high temperature environment.（2）Based on the above-mentioned research experience,we introduced KBr as a complexing agent to dissolve PbBr₂ in water.The reaction of transparent PbBr₂ solution with Cs-precursor solution led to the formation of green emissive perovskite materials.The as-synthesized perovskite material can restore its optical performances after more than ten cycles of soaking-in-water to evaporation-drying.The dried powder sample still maintained the photoluminescence intensity,showing a unique recovery of luminescent properties.The mechanism on the recovery of optical propertieswas further explored.The LED device fabricated by using the represent perovskite material exhibited an external quantum yields（EQEs）and luminous efficiency of 5.7%and 20.7 Lm/W,respectively.（3）The highly stable red emissive CsPbBr_xI_3-x perovskite nanocrystals were prepared by using an improved ligand-assisted reprecipitation（LARP）method at room temperature.The introduction of either 3-mercaptopropionic acid（MPA）or thioglycolic acid（TGA）as a stabilizing ligand improved the optical properties of the resulting perovskite materials.The emission wavelength of the as-formed MPA-CsPbBr_xI_3-xand TGA-CsPbBr_xI_3-xcan be tuned from 590 nm to 655 nm by changing the molar ratio of PbBr₂ to PbI₂,and the amount of MPA or TGA added in the reaction system.The PL intensity still maintains more than 90%of the initial intensity after storing under an ambient environment for60 days.Besides,the thin film of CsPbBr_xI_3-x perovskite nanocrystals still maintained their photoluminescence properties after annealing at 100°C and soaked in water for 14 days,indicating a good optical stability.