Research On Synthesis Of Stable And High Performance CsPbBr₃ NCs Via Low Toxic Solvents

Metal halide perovskite nanocrystals（NCs）have attracted the attention of the public with their unique optoelectronic properties,including high photoluminescence quantum yield（PLQY）,narrow full width at half maximum（FWHM）,tunable bandgap,high color purity and high defects tolerance,etc.Besides,perovskite NCs can be easily synthesized by solution processing,which is low-cost.The traditional methods for preparing CsPbBr₃ NCs（hot-injection（HI）method and ligand-assisted reprecipitation（LARP）method）have been well developed to synthesize high-quality perovskite NCs.However,not only toluene used in HI method but also N,N-dimethylformamide（DMF）and dimethyl sulfoxide（DMSO）used in LARP method to synthesize perovskite NCs are toxic.Such toxic solvents will seriously damage the environment and lead to environmental degradation.Therefore,it is urgent to develop a green solution-processed method to meet the need of eco-friendliness.The main difficulty in the synthesis of CsPbBr₃ NCs with low-toxic solvents is the choice of solvents.ThoughPbBr₂ is a kind of an ionic crystal,the solubility ofPbBr₂ in water is still very low（5 g/L）.As we know,CsPbBr₃ NCs are easily decomposed into ions in polar solvents and lose their optical properties.Therefore,to choose the low-toxic solvents,we not only need to consider the solubility of the precursor,but also that the CsPbBr₃ NCs can maintain their optical properties and stability in the solvent.To solve these problems,we have done the following work:（1）We used a low-toxic method to synthesize CsPbBr₃ NCs at room temperature.We synthesized CsPbBr₃ NCs in ethyl acetate/ethanol mixed solution（EA/Et）,and the fluorescence performance was significantly improved by adding tert-butylamine（TBA）during the synthetic process.After adjusting the components in the synthetic process,the CsPbBr₃ NCs with the best fluorescence performance were obtained with the absorption peak at 508 nm,the photoluminescence emission（PL）peak at 519 nm,and a PLQY of 41%.The CsPbBr₃ NCs’morphology is regular with an average size of 35±6 nm and clear lattice fringes.The Fourier Transform Infrared Spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）suggested there are a large number of TBA-Br ligands on the surface of CsPbBr₃,which passivated the surface defects of CsPbBr₃ NCs,reduced the nonradiative recombination,and enhanced the PL performance.（2）The stability of CsPbBr₃ NCs synthesized by low-toxic solvents was characterized and the mechanism was analyzed.During the long-term storage,the PLQY of the CsPbBr₃ NCs solution can be quickly increase to 1.7 times of initial value at 45 h（absolute PLQY 70%）,and the maximum enhancement can be achieved to 1.9 times at 750 h（absolute PLQY 78%）.Even after 1000 h,the relative PLQY of CsPbBr₃ NCs can still remain at 1.1 times of the initial value（absolute PLQY 45%）.Such amazing stability in polar solvents is dominated by a ripening process,which is the resulting balance of crystallization and dissolution processes of CsPbBr₃ NCs.In the solution state,the as-prepared CsPbBr₃ NCs existed a large number of surface defects,such as uncoordinated Pb²⁺or Br^-.Such defects can be easily etched by polar solvents,while the excess ions in the solution will slowly crystallize to form CsPbBr₃ NCs with fewer defects.Therefore,defects etching associated with decreasing size of CsPbBr₃ NCs greatly boosted PLQY at the beginning of the ripening process.At the end of the ripening process in one cycle,slow crystallization process formed CsPbBr₃ NCs with fewer defects.This ripening process will undergo periodic cycles,resulting in a large increase in PLQY and maintaining at a relatively high value.The XRD of the CsPbBr₃film shows inconspicuous change after more than ten days storage.The reason is that,once the film placed in low humidity,the pre-adsorbed moisture in the film can be slowly removed and the ions can be recrystallized.But in the relatively high humidity,moisture is preferred to dissolve TBA-Br,which is a sacrificial agent of moisture to protect CsPbBr₃ film against the moisture.Therefore,CsPbBr₃ NCs show good stability in both solution state and solid state.（3）Passivating the CsPbBr₃ NCs by tartaric acid ligands to improve their optical properties and stability.After passivation by tartaric acid ligands,the PLQY of CsPbBr₃ NCs increased rapidly from 41%to 80%,and the PL lifetime increased from 27.99 ns to 47.11 ns.After passivated by tartaric acid ligands,the size of CsPbBr₃ NCs hardly changed after repeatedly purifying,and the PL intensity decreased slightly.After three times purifying,the PL intensity can maintain 71.9%of the initial PL,and the stability in polar solvents is improved.The reasons for the improvement of optical properties and stability after passivation of tartaric acid ligands were:on one hand,the carboxyl groups on tartaric acid combined withPbof CsPbBr₃ NCs,which passivate the surface defects of CsPbBr₃ NCs.On the other hand,the product has a strong binding force with CsPbBr₃ NCs after the reaction,so the ligands can still be tightly combined with the surface of CsPbBr₃ NCs after repeatedly purifying,which improves the stability of CsPbBr₃ NCs.