Study On The Stability Mechanism And Modification Of Chalcogenide CsSnI₃

With the continuous consumption of fossil resources such as coal and petroleum,the development and utilization of renewable energy is imminent.Solar energy has attracted wide attention from researchers because of its inexhaustible supply.At present,solar cells are the main way to use solar energy.Most of the solar cells on the market are silicon-based solar cells.Although silicon-based solar cells have high photoelectric conversion efficiency,they are costly and difficult to develop and utilize.Perovskite solar cells have low cost and simple preparation process,and the photoelectric conversion efficiency is already comparable to that of silicon-based solar cells on the market.However,typical high-performance perovskite solar cells currently use Pb-based organic-inorganic hybrid perovskite as the light absorbing layer,such as methylammonium lead iodide（MAPb I₃）,formamidine lead iodide（FAPb I₃）and other perovskite materials.Due to the toxicity of Pb element,Pb-based perovskite solar cells may cause serious harm to human health and the ecological environment.In addition,since the A-site ions in the organic-inorganic hybrid Pb-based perovskite are macromolecular organic groups,this will result in poor stability of the perovskite materials in light,humidity and heat.Therefore,it is particularly important to find an alternative inorganic lead-free perovskite material.This thesis takes the inorganic lead-free B-γ-CsSnI₃ material as the research object.By using advanced spherical aberration corrected transmission electron microscope（AC-TEM）,scanning electron microscope（SEM）,focused ion beam-scanning electron microscope dual beam system（FIB/SEM）,X-ray diffractometer（XRD）,X-ray photoelectron spectroscopy（XPS）,ultraviolet-visible spectrometer（UV-Vis）,ultraviolet photoelectron spectroscopy（UPS）,the structure and properties of B-γ-CsSnI₃ perovskite were systematically studied and characterized from macroscopic,micro-nano,and even atomic scales.The main research results of this thesis are as follows:（1）The stability of B-γ-CsSnI₃ in air and its degradation mechanism were systematically investigated.The degradation of B-γ-CsSnI₃ films was found to occur at30°C and 40%RH.The B-γ-CsSnI₃ films were gradually transparent from black and finally completely degraded.By XRD characterization we found that the B-γ-CsSnI₃orthogonal phase started to transform to Cs₂Sn I₆ at 10 h,completely transformed to Cs₂Sn I₆ after 80 h,and Cs IO₃ was generated in Cs₂Sn I₆ after 100 h.It completely turned into an amorphous phase after 200 h of failure treatment.The SEM characterization revealed that the B-γ-CsSnI₃ film,with the increasing time of the failure treatment its grains were gradually broken up and formed a paste.Eventually the grains were precipitated.By TEM characterization,the initial B-γ-CsSnI₃ chalcogenide particles were smooth with no obvious defects.As the sample failed in air,the smooth B-γ-CsSnI₃ chalcogenide particles surface was obviously no longer smooth,and flocculent particles appeared at the edge positions,and a large number of defective pores appeared on the particle surface.Finally,complete degradation fragmentation was observed,and the particles were completely broken and essentially completely flocculent,but new particles precipitated in the middle of the particles,covering the surface of the original particles.（2）By adjusting the structural stability tolerance factor doping with different elements Pb²⁺,Co²⁺,Cu²⁺,Mg²⁺,Sr²⁺,Ge²⁺,Zn²⁺,through XRD,UV-Vis,and characterization,it is found that the doping of Zn and Mn has an effect on The stability of B-γ-CsSnI₃ has been greatly improved.The phase change occurs from 10 hours,the stability increases to greater than 100 hours,the phase change occurs,80 hours is completely transformed into Cs₂Sn I₆.Further through TEM,SEM,XPS,and characterization,it is found that the doping of Zn and Sr for the orthorhombic phase B-γ-CsSnI₃ leads to changes in the unit cell parameters.After doping,Zn and Sr replace the position of Sn and transform from the orthorhombic phase.Through XPS characterization,it was found that in the doped sample,the fine spectrum of Zn clearly showed strong peaks,and the fine spectrum of Sn and I,the peak position shifted to high binding energy,indicating that the binding energy of Sn-I became larger and the structural stability was enhanced.In summary,this thesis successfully prepared the B-γ-CsSnI₃ film,and systematically studied the degradation mechanism of B-γ-CsSnI₃ perovskite in the air.In order to improve the air stability of the B-γ-CsSnI₃ film,we have proposed a scheme for B-site element doping to regulate the stability of the crystal structure to improve the stability of B-γ-CsSnI₃ perovskite in the air.