Surface Passivation And Stability Of Metal Halide Perovskites Luminescent Materials

Metal halide perovskites(MHPs)have shown great potential for a variety of optoelectronic applications,especially for light emitting diodes(LEDs),due to their high luminous efficiency,tunable emission wavelength and high color purity.However,intrinsic instability of MHPs materials make them degradable upon exposure to polar solvents,light,heat and oxygen,which deteriorates their luminescent properties and inhibits their commercialization.It is imperative to develop MHPs materials with high stability and high luminous efficiency.Over the past few years,lots of efforts have been made to enhance the stability of MHPs and great success has been achieved.However,it is still insufficient for MHPs to be utilized in practical devices.In this regard,this thesis focuses on optimizing surface ligands and constructing passivation layers to improve the stability and photoluminescence performance of MHPs nanomaterials.The main contents are summarized as follows:(1)Surface passivation of CsPbBr3 nanocrystals by Pb Br2-adlayers through the precursor regulation.Gengrally,MHPs nanocrystals will tend to aggregate and dissolve,resulting in a great reduction in their luminescent intensity,when they are exposed to polar solvents.We have developed an easy way to encapsulate CsPbBr₃ nanocrystals with Pb Br₂-adlayers by employing didodecyl dimethyl ammonium bromide(DDAB)molecules as the only bromine source and capping ligands.X-ray photoelectron spectroscopy(XPS)and ¹H nuclear magnetic resonance(¹H NMR)spectra demonstrate that Pb Br₂-adlayers can be formed only in a lead-and bromide-rich circumstance.The successful encapsulation of Pb Br₂-adlayers enables dense packing of DDA⁺ligands on the nanocrystals surface.Both Pb Br₂-adlayers and densely packed DDA⁺layer can efficiently prevent enthanol molecules from etching CsPbBr₃ nanocrystals.These Pb Br₂-adlayer-encapsulated CsPbBr₃nanocrystals still retained 95%of their initial photoluminescence intensity after dispersing in the mixture of ethanol and n-hexane for 6 h,displaying high durability against ethanol.This work provides a new strategy for synthesis of stable CsPbBr₃ nanocrystals against polar solvents.(2)Self-passivation of CsPbBr3 nanocrystals through introducing bromide vacancies and ultraviolet irradiation.Upon exposure to ultraviolet light,all-inorganic MHPs nanocrystals experience aggregation and coalescence,resulting in photoquenching,due to the photoinduced halide ion migration.Consequently,the stabilization of halide ions at the grain surface may be a key role for conquering the intrinsic photoinstability of lead halide perovskite nanocrystals.In this work,surface stoichiometry has been modulated by lead oleate modification and the effects of variation in surface stoichiometry on photostabiltiy of CsPbBr₃ nanocrystals have been investigated.The experimental results show that Br-rich surface is detrimental for their photostability,while Br-deficient surface is favorable for photoactivation and following light-induced self-passivation.We reveal that coalescence will be dominant on illuminated Br-rich surface,while defects annihilation and self-passivation due to light-induced bromide ion migration predominant on illuminated Br-deficient surface.We provide evidence for the key role of surface stoichiometry in photostability of LHP materials and demonstrate the possibility for taking advantage of photoinduced ion migration to improve photostability of LHP materials.(3)Photoluminescence properties of self-trapping excitons in 2D RP tin-based perovskites.The luminescence efficiency and air stability of 2D RP tin-based perovskites(L₂Sn Br₄)are superior to those of 3D counterparts.Moreover,2D RP tin-based perovskites exhibit strong emission from self-trapping excitons.It is necessary for us to better understand the formation and behaviors of self-trapping excitons for improving the luminescence performance of 2D RP tin-based perovskites.We have chosen oleylamine,octylamine,hexylamine and butylamine(BA)as the organic ligands layer to prepare L₂Sn Br₄ perovskites by a modified injection method in this study.An obvious blue-shift in emission peak as well as shorter photoluminescence lifetime can be found in(BA)₂Sn Br₄,as compared with the other three samples.The detailed analyses on the temperature-dependent emission spectra indicate that 2D tin-based perovskites with short-chain amines have higher exciton-phonon coupling strength and lower longitudinal optical phonon energy than those with long-chain amines,which may lead to their different photoluminescence properties.