The Synthesis And Doping Effect Of Cesium Lead Halide Perovskite Nanocrystals

Due to the advantages in light absorption coefficient,carrier diffusion length and defects tolerance,organic-inorganic perovskite materials,such as CH₃NH₃PbI₃ and CH₄?NH₂?₃PbI₃,were used in the solar cells,and achieved rapid progress in improving the photoelectric conversion efficiency.Further researchs reveal that,organic-inorganic perovskite materials also have the properties of direct bandgap,tunable bandgap value,high photoluminence quantum yield?PLQY?and so on,which are essential for the high quality lighting application.This was confirmed by the application of organic-inorganic perovskite materials in the fields of white light illumination,laser and ligh emitting diode.In 2015,Protesescu et al.reported the preparation of CsPbX₃?X=Cl,Br,I?nanocrystals?NCs?by the replacing of organic molecules with Cs⁺,and the prepared CsPbX₃ NCs exhibit obvious enchament in stability,photoluminence quantum yield,color purity and tunablity in fluorescence wavelength.However,the stability of inorganic cesium lead halide perovskite materials is still not good enough;especially for the attacking of moisture,light and heat.And,the PLQYs in some wavelengths still need to be improved.Therefore,the further improving in stability and fluorescence efficiency of CsPbX₃ NCs decides the possibility of the large-scale application in light emitting devices.Doping,intentionally introducing heteroatoms into target lattices,is a general and effective method to adjust the basic physical properties of a semiconductor,and does not change the original crystal structure.So,the properties CsPbX₃ NCs can be adjusted by the doping treatment,and the stability and optical-electric properties may be enhanced at the same time.Based on the above considerations,we at first fabricated inorganic C_SPbBr₃ NCs,then try to dope C_SPbBr₃ NCs with halogen,Mn²⁺,Na⁺and other ions,and study the effects of doping on the fluorescence characteristics and stability of the NCs.In the experiment,CsPbX₃ NCs and Mn²⁺and Na⁺doped CsPbBr₃ NCs were prepared by solution methods.By the optimization of synthesis conditions,halogen composition and doping concentration,the emission properties and stability of the NCs were largely improved,which laid a foundation for subsequent device applications.The main achievements of this paper are as follows:?1?CsPbX3 NCs:Synthesis,halogen doping and emission propertiesAmong the CsPbX₃?X=Cl,Br,I?NCs,CsPbBr₃ NC is the most thoroughly studied one,with the best stability and the highest PLQY.Moreover,by doping CsPbBr₃ NCs with Cl^-and I^-ions,it is very convenient to modulate the emission wavelength from green fluorescence to short-wave blue and long-wave red fluorescence.So,CsPbBr₃ NCs with uniform size,good color purity and high PLQY were firstly prepared by a high temperature injection method.By adjusting the reatction temperature,growth time and surfactant concentration,the growth process and driving force of perovskite CsPbBr₃ NCs were explored.The fluorescence emission of CsPbBr₃ NCs can be continuously adjusted from 410 to 685 nm by the doping and substituting of halogen elements,with the highest PLQY of 93%,which indicates that high quality CsPbX₃?X=Cl,Br,I?NCs with all color in visible region were achieved in this part.?2?Mn²⁺doped CsPbBr₃ NCs:Synthesis and emission propertiesWhen exposed to moisiture,CsPbBr₃ NCs tende to decompose and dissolve,so the toxic Pb²⁺ions easily enter natural water and become a potential risk for human and animal's health.So,the developing of lead-free or lead-less inorganic perovskite NCs is necessary.The metal Mn has the advantages of low toxicity,low cost,abundance and easy to introdue doping energy levels.Therefore,the doping of Mn²⁺ions can be used to reduce the content of lead in perovskite NCs.On the base of the synthesis of high quality CsPbBr₃ NCs,we tried to dope them with Mn²⁺,and explore the realization of doping in NCs and the effect on the properties of perovskite NCs.In the experiments,Mn²⁺can be easily doped in CsPbCl₃ NCs by the adding of MnCl₂ in precursor.However,for the doping of CsPbBr₃ NCs with Mn²⁺,it does not work by the adding of MnBr₃ but MnCl₂,so the later was used to dope CsPbBr₃ NCs.It was found that the doping Mn²⁺ions can change the fluorescence emission wavelength of CsPbBr₃ NCs.And,with the increase of Mn²⁺doping,the intrinsic fluorescence of CsPbBr₃ NCs decreases gradually,while the fluorescence emission of Mn²⁺increases.Transient fluorescence results show that the doping of Mn²⁺causes the changing of carrier recombination paths in CsPbBr₃ NCs.Based on the above analysis,it was believed that the doped Mn²⁺ions form new doping levels in the energy band of CsPbBr₃ NC,so the photogenerated electrons in conduction band,at first transfer to the energy level of Mn²⁺and then return to the valance band by radiation transition,which no longer return to the valence band by direct radiation recombination,thus changes the color of fluorescence emission.What's more,the doping of Mn²⁺changes the formation energy of CsPbBr₃ NCs,which changes the crystal structure of CsPbBr₃ NCs from tetragonal phase with lower symmetry to cubic phase with higher symmetry.With the increase of doping concentration,a directional shift of the diffraction peaks in X-ray diffraction spectrum was observed,which indicates that the doping of Mn²⁺is realized by the replacing of Pb²⁺atoms wth larger ionic radius.The results of X-ray photoemission spectroscopy analysis also confirms that Mn²⁺doping causes the variety of chemical environment of the atomes in CsPbBr₃ NCs.It is noted that Mn²⁺doping amount in CsPbBr₃ NCs is as high as 70 mol%,which is of great significance in reducing the using of toxic Pb²⁺in CsPbBr₃ NCs.The improvement of thermal stability also helps the commercial application of perovskite NCs in optical devices.?3?Na⁺doped CsPbBr₃ NCs:Synthesis and emission propertiesConsidering the excellent optical properties of CsPbX₃ all-inorganic perovskite NCs,the main problem is the poor stability.So,we considered to dope perovskite NCs with chemically stable Na elements,to keep the original optical peroperties and enhance the stability.Compared with the fresh CsPbBr₃ NCs,a better color purity and a higher PLQY were achieved,accompanied by a gradual blue-shift of the emission peak with the increasing of Na⁺-doping.We hold that,on one hand,the passivation effect of Na⁺-doping substantially reduces the trap centers in NCs,which was confirmed by the increase of fluorescence lifetime.On the other hand,the density functional theory calculations indicate that the doping of Na⁺ions forms the energetically preferred substitution defect of Na_Pb,which increases the optical band gap of CsPbBr₃ and the diffusion barrier of Br^-vacancy in CsPbBr₃ structures.Experimentally,the as-synthesized Na⁺-doped CsPbBr₃ NCs demonstrate a substantially improved stability against ultraviolet light,heat,and environment oxygen/moisture.The above advantages allow us to use such CsPbBr₃:Na NCs as solid state phosphors for white light-emitting diode?WLED?applications.Finally,the prepared device exhibits a white light emission with Commission International de I'Eclairage color coordinates at?0.31,0.33?,a correlated color temperature of 6652 K,a color rendering index of 75.2,and high power efficiency of 67.3 lm/W.More importantly,the WLED working in direct current demonstrates remarkable operation stability in air ambient condition;and after a continuous operation for 500 h,only15%decay can be observed,showing great superiority to the reference device with fresh CsPbBr₃ NCs as the phosphor.The above results provide an effective strategy for the improvement of emission efficiency and stability of perovskite NCs,opening up enormous opportunities for their applications in lighting and display fields.