Structural Refinement Of Cs₂AgBiBr₆ Using Single Crystal X-ray Diffraction And Study Of Its Degradation Under Electron Beam Irradiation

As emerging novel functional materials,perovskite solar cell materials have attracted substantial attention in recent years due to their excellent photovoltaic and photoelectric properties and their promising applications in energy devices.Perovskite solar cell materials could be divided into two categories,i.e.organic-inorganic hybrid perovskite,and pure inorganic perovskite.In the category of inorganic perovskites,the toxic Pb²⁺cations could be replaced by a combination of a monovalent cation and a trivalent metal cation so to obtain a lead-free double perovskite structure.Cs₂AgBiBr₆as a representative lead-free double perovskite material,is both environmentally friendly and highly stable,and therefore has promising applications in the fields of photovoltaics and optoelectronics.Annealing can further improve the performance of Cs₂AgBiBr₆,but how annealing modifies the structure and defects of this double perovskite materials remains unclear.In this thesis,single crystal X-ray diffractometer（SCXRD）is used to analyze the structural changes and the defect evolution of the single crystal Cs₂AgBiBr₆ under different annealing conditions through in-situ and quasi-in-situ investigations.Transmission electron microscopy（TEM）is further used to study the structural change of single crystal Cs₂AgBiBr_6.Due to its electron-beam-sensitivity,the degradation mechanism of single crystal Cs₂AgBiBr₆ under electron beam irradiation is first studied to establish“safe condition”in characterizing this material.Electron diffraction is further used to look into the structural change of single crystal Cs₂AgBiBr₆ during annealing using in-situ TEM.The main research contents of this thesis are as follows:1.The single crystal diffraction data of single crystal Cs₂AgBiBr₆ before and after different annealing conditions,is obtained by SCXRD and refined by Shlex97 and Olex2 software.The occupancy of each ion and their changes at different annealing conditions is studied.The results show that the annealing of single crystal Cs₂AgBiBr₆ at low temperature 373K and 423K can effectively reduce defects, while annealing at temperatures of 473K and above can lead to defects increase.Annealing at 523K lead to irreversible decomposition.2.The SCXRD study shows that the Bi³⁺in the single crystal Cs₂AgBiBr₆ was most stable and the occupancy is as high as 0.9998,i.e.close to full occupancy.It was generally accepted that the annealing led to the exchange of Ag⁺and Bi³⁺,but our study showed it is unlikely due to stable and nearly full-occupancy of Bi³⁺. Meanwhile,Br^-occupancy at room temperature is relatively low at 0.9909,but reaches a maximum of 0.9996 after 1 hour of annealing at 373K,which concludes that a relatively high ratio of Br^-vacancy at room temperature is not due to Br-poor composition but more likely to be an off-site positioning of Br^-.3.TEM characterization shows that the rapid degradation of Cs₂AgBiBr₆ by electron beam irradiation is initially induced by the K-F effect as radiolysis.One of the initial degradation products is Cs₃Bi₂Br₉ after minimal lattice adjustment.Since K-F effect is a surface reaction,the structure of Cs₂AgBiBr₆ could be maintained at a low electron beam dose.Electron beam irradiation study shows that single crystal Cs₂AgBiBr₆ is easily degraded by irradiation,and the degradation does not follow a single path,but rather follows two possible reactions:2Cs₂AgBiBr₆→CsAgBr₂ +AgBr+Cs₃Bi₂Br₉ and Cs₂AgBiBr₆→2CsBr+AgBr+BiBr₃.The decomposition products are mainly Cs₃Bi₂Br₉,BiBr₃,CsAgBr₂ and AgBr.Using an electron beam dose less than 2 e/?s,the single crystal diffraction patterns of Cs₂AgBiBr₆ showed no obvious change within 4 min,and therefore is considered as a“safe condition” to study this beam sensitive material.4.In-situ TEM is performed to simulate annealing of Cs₂AgBiBr₆.Analysis of electron diffraction patterns shows that intensity of（333）diffraction related to Bi³⁺/Ag⁺ordering changes little during annealing,but the intensity of（440） diffraction related to Br^-decreases dramatically,confirming the structural change of Cs₂AgBiBr₆ during annealing is dominated by Br^-instability.This thesis proposes a new mechanism of Cs₂AgBiBr₆ structural evolution during annealing and offers a new way of tuning its structure and properties by annealing.The decomposition of Cs₂AgBiBr₆ when exposed to electron beam is also studied in detail providing a safe dose for the study of Cs₂AgBiBr₆ materials using TEM.Since Cs₂AgBiBr₆ is a representative inorganic double-perovskite structure,this study can help the development of the novel family of double-perovskite halide materials.