Preparation And Properties Of Lead-free Halide Perovskites Nanocrystals

A three-dimensional hybrid halide perovskite of the general formula ABX₃,where A is a small organic/inorganic cation(such as CH₃NH³⁺,H₂NCH₂NH²⁺,Cs⁺),B is a metal(Pb²⁺,Sn²⁺,Ge²⁺),and X is a halogen(I^-,Br^-,Cl^-)has become a revolutionary semiconductor material for photovoltaic and optoelectronic device applications.This phenomenon resulted from the fusion of a variety of interesting physical properties,including extremely high light absorption,small effective mass of holes and electrons,long diffusion length,high charge carrier mobility,and potential ferroelectricity.Although the lead-based 3D hybrid perovskite has made great progress in photovoltaic applications,there are still some disadvantages that are not conducive to its actual commercialization in the future,especially the presence of lead in its chemical composition.This toxicity issue has caused serious concern about the practical feasibility of lead-based hybrid perovskites.Their large-scale application seriously pollutes the environment.In this regard,many design strategies have been proposed to solve this problem.The most straightforward method is to use other Group IVA metals(such as Sn and Ge)instead of lead to assemble new lead-free candidate materials.However,their photovoltaic equipment performance is not as good as similar products based on Pb.The design strategy of element substitution of Pb has been extended to the family A₂B'B''X₆of halogenated double perovskites(HDPs).Two Pb²⁺cations in the crystal lattice can be replaced by monovalent and trivalent metal cations to construct non-lead heterovalents.A large number of studies have shown that for these HDPs high efficiency comparable to CH₃NH₃Pb I₃,a long carrier diffusion length and a suitable band gap can be achieved.From a structural point of view,the A₂B?B??X₆structure of HDPs provides a feasible way for the substitution of different metal cations with different oxidation states at the B position,and the substitution of various organic and inorganic cations and halide changes at the A position.This structural flexibility of HDPs can provide rich and interesting physical properties,including narrow band gap,high photon absorption,benign mechanical stability and photoluminescence.Therefore,after conducting preliminary research,these surprising advantages of HDPs have aroused people's interest in its optical performance and stability.In this paper,the ligand-assisted reprecipitation method was used to successfully synthesize Cs₂Na Bi_1-xSb_xCl₆(x=0,0.25,0.5,0.75,1)and Cs₂Na_1-xAg_xBi Cl₆(x=0,0.05,0.1,0.2,0.4,0.6,0.8,1)two kinds of nanocrystals.Through a series of characterization methods,the crystal structure,optical properties and stability were deeply explored.(1)Near-ultraviolet luminescence Cs2Na Bi0.75Sb0.25Cl6 perovskite colloidal nanocrystals with high stabilityIn this work,the room temperature ligand-assisted reprecipitation method was used to synthesize Cs₂NaBiCl₆and Cs₂Na Bi_0.75Sb_0.25Cl₆nanocrystals.Both X-ray diffractometer and transmission electron microscopy tests proved that the two have good crystallinity and no obvious lattice defects.By adjusting the ratio of surface ligand oleic acid,the optimal reaction conditions were 3%oleic acid,and the fluorescence quantum yield of the Cs₂NaBiCl₆perovskite nanocrystals obtained under this condition was 39.05%.When the chemical formula is Cs₂Na Bi_0.75Sb_0.25Cl₆,the fluorescence quantum yield was increased to 46.57%.In addition,a transient fluorescence lifetime test was performed on the mechanism of Sb doping,which proved that the enhancement in fluorescence quantum yield was attributed to the increase in recombination pathways.At the same time,we systematically studied the air stability,water stability of the Cs₂Na Bi_0.75Sb_0.25Cl₆ nanocrystalline solution and powder air stability.Compared with similar perovskites,it has better stability and shows great potential for photoelectric applications.(2)Synthesis and properties of silver-doped Cs2NaBiCl6 perovskite nanocrystalsIn this paper,Cs₂Na_1-xAg_xBi Cl₆(x=0,0.05,0.1,0.2,0.4,0.6,0.8,1)nanocrystals were synthesized by an efficient and simple room-temperature ligand-assisted reprecipitation method.In this method,Cs Cl,Na Cl,Bi Cl₃,and Ag Cl were dissolved in DMSO in a certain proportion to form a precursor solution,and then 100?L of the precursor solution is added dropwise to the anti-solvent isopropanol,and the desired product was obtained after centrifugation.Through the combination of ultraviolet-visible absorption,fluorescence emission,powder X-ray diffractometer and other characterization methods,the microscopic morphology,crystal structure and opticial properties of Cs₂Na_1-xAg_xBi Cl₆(x=0,0.05,0.1,0.2,0.4,0.6,0.8,1)were explored.In addition,this paper also used transient fluorescence spectroscopy to investigate the effects of Ag ion doping on the dynamics,and further explains the mechanism of fluorescence emission enhancement.In short,Cs₂Na_0.6Ag_0.4Bi Cl₆nanocrystals show good optical properties and stability,and are expected to be used in future device development such as LEDs.