Electron Beam Irradiation Stability And Decomposition Mechanism Of MAPbI₃ And MAPbBr₃ Perovskite Crystals

In recent years,solar cells based on organic-inorganic hybrid perovskites(OIHPs)as photoelectric conversion layer have been developed rapidly due to the low synthesis price and high photoelectric conversion efficiency.However,OIHPs are easy to decompose because of their unstable structure,thus affecting the device efficiency and cycle stability which greatly hinders the commercial development of this technology.Studying the decomposition mechanism of the material is helpful to guide the optimization of device performance.Among the techniques,transmission electron microscope(TEM)with high spatial and energy resolution combined with in-situ technology allows us to directly study structure and composition evolution during the decomposition.However,OIHPs are extremely sensitive to the electron beam and can be easily decomposed.Yet,many researchers ignored such electron beam sensitivity and used a large dose when studied OIHPs.Large doses are easy to cause structural damage or even degradation.Thus the research needs to clarify the effect of electron beam irradiation on OIHPs materials and further deepens the understanding of the decomposition mechanism of the material.Based on this,by using low dose imaging techniques,this paper determins the threshold conditions to avoid damage and reveals the decomposition pathway of perovskite materials.According to the characteristics of the decomposition pathway,the carbon coating strategy for restraining the perovskite structure is put forward.Different factors affecting the electron beam irradiation sensitivity are further explored,revealing the optimal conditions for TEM characterizing OIHPs and the electron beam damage mechanism.Accoringly,the atomic-scale structure decomposition mechanism is revealed,which provides experimental guidance for analyzing the decomposition mechanism of OIHPs.Firstly,the structural instability and the decomposition pathway of MAPbI₃were investigated.It is found that PbI₂structure is always mistaken as MAPbI₃structure in conventional TEM analysis.The essential reason is that electron beam irradiation can cause MAPbI₃ to decompose into PbI₂ and their diffraction spots are highly similar.Further results show that the ED pattern of pristine MAPbI₃can be obtained at 1 e^-2(?)^-2s^-1,and the threshold condition to avoid MAPbI₃decomposition is determined.The continuous ED patterns show that the superstructure intermediate phase can be formed during the decomposition process,which is a kind of ion vacancy(MA or I)ordered superstructure.The proposed decomposition pathway of MAPbI₃ under electron beam irradiation provides a new idea for analyzing the decomposition mechanism of OIHPs.The decomposition mechanism of CH₃NH₃PbBr₃(MAPbBr₃)under electron beam irradiation was further studied.Firstly,by controlling the dose rate of electron beam,the ED pattern of single crystal MAPbBr₃ is obtained at 1 e^-2(?)^-2 s^-1,and the threshold condition to avoid MAPbBr₃ damage is determined.The superstructure intermediate phase and the decomposition pathway under electron beam irradiation are revealed.It is found that electron beam irradiation leads to formation of a superstructure phase,further losing MA⁺and Br^-.Finally,the perovskite structure collapses and PbBr₂ is formed.To further explore the factors affecting the electron beam irradiation sensitivity of MAPbI₃ and MAPbBr₃ perovskite crystals,the effects of TEM acceleration voltage,crystal plane and temperature on the electron beam sensitivity were studied and the effective strategies to restrain the decomposition of perovskite were proposed.The results show that low temperature can not reduce the electron-beam-induced damage of MAPbI₃ and MAPbBr₃,on the contrary,it causes rapid amorphization.It is found that MAPbI₃ is more resistant to electron beam irradiation at 300 k V than at 80 k V,so the electron beam damage mechanism of MAPbI₃ is radiolysis.In addition,the(100)plane is more resistant than the(001)plane.Even under the optimal characterization conditions(room temperature,high voltage),OIHPs can be damaged or even decomposed at low doses.In order to further prevent the decomposition of perovskite,thin layer of amorphous carbon was coated on the upper and lower surface of MAPbI₃ as an obstacle layer of ion diffusion.Thus,the structure of perovskite was stabilized and the decomposition of perovskite was inhibited,likely providing experimental evidence for understanding the mechanism of carbon coating to improve the performance of the device.Based on the above research,the atomic structure of MAPbI₃ was obtained by the low dose imaging technique of direct electron detection camera(K2),and its decomposition path was analyzed at an atomic scale.Furthermore,the bonding changes during the decomposition were revealed by TEM spectroscopic techniques.The results show that the electron beam first destroys the chemical bonds of methylamine(MA),resulting in MA vacancies,followed by the iodine or lead diffusing.Then PbI₆ octahedron slips along the oblique corrugated,accompanying with the connection mode changing from vertex connection to oblique corrugated connection,resulting in the transformation into PbI₂ structure.TEM vibrational spectroscopy results show that under electron beam irradiation,the organic part of MAPbI₃ including C-N,C-H and N-H bonds can be damaged,wherein C-N bond is completely destroyed first,followed by C-H and N-H bond,finally leaving-CH₂-and the structure similar to amorphous carbon.The atomic-scale analysis of the decomposition mechanism and evolution in the degradation process can deepen the understanding of the decomposition mechanism of MAPbI₃ and provide experimental support for the study of the decomposition in practical devices.