Study On Preparation And Properties Of Novel Lead-free Perovskite Photoelectric Crystals

As a new material in the field of photovoltaics and optoelectronic devices,lead-based organic-inorganic hybrid perovskite materials have become research hotspots in the perovskite fields due to their excellent optoelectronic properties.Pb produced by the decomposition of lead-based materials and devices would show large impacts on the environment,which limits their application.It is of great significance to explore new lead-free perovskite materials and characterize the basic properties of their crystals.The work of this thesis is mainly to study the growth and properties of organic-inorganic hybrid tin-based perovskite crystals CH₃NH₃Sn I₃,CH₃NH₃Sn Br₃,CH₃NH₃Sn Cl₃,CH₃NH₃Sn Br_xCl_3-x and all-inorganic double perovskite crystals Cs₂Ag Bi Br₆.The main research results are:?1?The CH₃NH₃Sn X₃?X=I,Br,Cl?type perovskites crystal were grown by the solution cooling method.The grown CH₃NH₃Sn I₃ and CH₃NH₃Sn Br₃ are of cubic structure with lattice constants a=5.9523?and 5.8930?,respectively.The CH₃NH₃Sn Cl₃ is of monoclinic structure with lattice constants a=5.6873?,b=8.2268?,c=7.9312?,?=93.12°.HRTEM test shows that the internal crystal quality of CH₃NH₃Sn I₃ single crystal is good.The absorption edges of CH₃NH₃Sn X₃?X=I,Br,Cl?crystals are:933nm?I?,632nm?Br?,and 351nm?Cl?,the corresponding band gap values are:1.33e V?I?,1.98e V?Br?and 3.57e V?Cl?.Thermogravimetric analysis and EDS analysis show that the stability of CH₃NH₃Sn I₃ and CH₃NH₃Sn Br₃ crystals in the oxygen environment is poor.First-principles calculations show that the band structure of the CH₃NH₃Sn X₃ is a direct band gap structure.The top of the valence band is mainly composed of the p-electron orbital of the outermost layer of the halogen element,and the bottom of the conduction band is mainly composed of 5p electron orbits of tin.?2?The solution cooling method was used to configure the CH₃NH₃Sn Br_xCl_3-x mother liquor at a ratio of 1:1 of CH₃NH₃Cl:Sn Br₂ at 90°C.The phase of the crystals precipitated during the cooling process was analyzed.The crystal precipitated at 60°C is CH₃NH₃Sn Br₃crystal phase.The crystal component precipitated at 45°C is CH₃NH₃Sn Br_2.67Cl_0.33,the lattice constants were a=13.3114?,b=7.5326?,c=4.9970?,and?=126.83°,which belong to the monoclinic structure.The absorption edge is 465 nm,and the band gap is 2.78 eV which is between the band gaps of CH₃NH₃Sn Br₃ and CH₃NH₃Sn Cl₃,indicating that the band gap of CH₃NH₃Sn Xr₃ can be adjusted by changing the halogen ratio.?3?The Cs₂Ag Bi Br₆ crystal was grown by the solution cooling method,which belongs to the cubic structure with a lattice constant a=11.2641?.HRTEM and SAED tests show that its crystallization performance is good.The experimentally obtained Cs₂Ag Bi Br₆ crystal has an absorption edge of 593 nm and a band gap value of 2.12 e V.Thermogravimetric analysis showed the thermal decomposition mechanism of Cs₂Ag Bi Br₆ crystal.Combined with first-principles analysis,it is an indirect band gap structure crystal and the calculated band gap value is 2.39 eV.