Optical Properties And Stability Of CsPbBr₃ Perovskite Based On Post-synthesis Method

Metal halide perovskite nanocrystals are one of the most promising optoelectronic materials because of their excellent opto-electronic properties.Perovskite nanocrystals are usually obtained by chemical synthesis,during which organic ligands were introduced as stabilizers for the nanocrystals.However,perovskite nanocrystals are ionic crystals with weak bonding between the surface and the ligand,which makes the ligand in dynamic equilibrium and highly susceptible to exfoliation.Therefore it seriously affects the optical properties and stability of nanocrystals.Two strategies have been proposed to address this problem.One is to synthesize highly stable perovskite nanocrystals by introducing ligands that are more tightly bound to the nanocrystal surface during the synthesis process.The other approach is to add a new ligand into the synthesized perovskite nanocrystals for exchanging with the original ligand.The morphology and phase structure of the nanocrystals can be regulated,as a result the optical properties and stability of the nanocrystals can be finally improved.Therefore,in this thesis,we will choose the post-synthesis strategy to introduce new ligands for surface modification of perovskite nanocrystals and change study of the effects of the morphology and phase structure on the optical properties and stability of nanocrystals,the main research contents and results are as follows:CsPbBr₃-Cs₄Pb Br₆ nanocrystals materials with mixed phase were obtained by adding dual-ligand of oleylamine-tetradecylphosphonic acid（OLA-TDPA）to CsPbBr₃perovskite nanocrystals through ligand post-treatment method.The photoluminescence quantum yield of the CsPbBr₃-Cs₄Pb Br₆ perovskite nanocrystal prepared at the optimal ratio（1:1:15 molar ratio of CsPbBr₃,TDPA and OLA）was up to 78%with a fluorescence lifetime of 476 ns.The as-obtained nanocrystal remained stable for at least25 days at room temperature environment and 5 heating between 293 K and 328 K heating-cooling cycles.The formation of dual phase nanocrystals undergoes two stages of surface passivation/dissolution and recrystallization:in the first stage（t≤1 h）,the OLA-TDPA ligand forms（RNH₃）₂PO₃ X-type ligands to exchange with ligands onthe nanocrystal surface,and the new ligands can bind tightly with Pb²⁺on the nanocrystal surface at a high content,which thus reducing the density of defect states on the nanocrystal surface and improving the quantum yield and fluorescence lifetime of CsPbBr₃-like spherical perovskite nanocrystals.;In the second stage,a small amount of hexagonal phase Cs₄Pb Br₆ nanocrystals were generated due to the recrystallization of CsPbBr₃ NCs as a result of precipitating Pb Br₂ from solution,and finally the nanocrystals with the dual phase coexistence of CsPbBr₃ and Cs₄Pb Br₆ were obtained,which improved the stability of the nanocrystals.Monoclinic phase CsPbBr₃ and hexagonal phase Cs₄Pb Br₆ coexisting nanocystals were prepared by adding quaternary ammonium salt ligands to CsPbBr₃ NCs.It was found that the degree of phase transition of CsPbBr₃ NCs increased with the shortening of alkyl chain length and the increase of TOAB concentration.In addition,the longer the alkyl chain length,the higher the fluorescence intensity and PLQY value.When the TOAB concentration was 0.003 mol/L,the CsPbBr₃ NCs were transformed into mixed-phase nanocrystals with the coexistence of CsPbBr₃ and Cs₄Pb Br₆,in which CsPbBr₃NCs accounted for 8.4%.In this case,the PLQY was imporved from 24%to 65%compared to the untreated CsPbBr₃ NCs.At the same time the storage stability,light stability and thermal stability were enhanced.The main reason for significantly improvement of the performance is the replacement of Cs⁺in CsPbBr₃ NCs by TOA⁺,while the larger size of TOA⁺leads to lattice distortion and induces the formation of hexagonal phase Cs₄Pb Br₆ NCs.Moreover,the ionic ligand can also exchange with the OAm⁺ligand that is protonated on the surface,which is tightly encapsulated on the nanocrystal surface after exchange.Therefore the surface defects can effectively be passivated.The study can provide a reference value for promoting the application of highly efficient and stable perovskite nanocrystals.