Pressure Dependence Of Electron/ion Mixed Conduction And Photo Responsiveness In Organolead Tribromide Perovskites

Ion migration in organometal trihalide perovskites(OTPs)has been attracting extraordinary attention after the report of current-voltage hysteresis in OTPs based solar cells.Ionic transport is suggested to be an important factor that contributes to the stability and performance of OTPs based devices.Recent scientific studies on organic–inorganic hybrid perovskites mainly focus on the ion migration behaviors at ambient conditions.However,how pressure affect the electrical transport properties of OPTs is seldom studied.In this thesis,CH3NH3 Pb Br3(MAPb Br3)polycrystals were selected as the research object.The appearance and morphology of MAPb Br3 were characterized by X-ray diffraction(XRD)and scanning electron microscopy(SEM).Using diamond anvil cell(DAC)device,the electrical transport properties and photoelectric properties of MAPb Br3 polycrystals were in situ investigated systematically by alternating-current impedance spectroscopy and photoconductive measurements under high pressures.The specific research results are as follows:1.The appearance and morphology of MAPb Br3 were characterized by XRD and SEM.It was found that MAPb Br3 is a cubic polycrystalline particle with high purity and a size of 100-150 ?m.2.By high-pressure alternating-current impedance spectroscopy measurements,it was found that both ionic and electronic conductions coexist within MAPb Br3.We used the magnitude of the resistance value to quantify the ease of electron and ion migration and found that there was a discontinuous change in resistance at 0.4 and 1.7 GPa.Ionic conduction dominated before 3.3 GPa,and an obvious ionicelectronic transition can be observed at 3.3 GPa.The experimental results shown that the pressure can change the band gap of MAPb Br3 and the internal ion migration channels,thereby regulating the internal electron/ion mixed conduction of MAPb Br3.3.The effect of pressure on the photoelectric properties of MAPb Br3 was studied by in situ high-pressure photoconductive measurements.It was found that the visible light response of MAPb Br3 in the second phase was significantly better than the other phases.When the pressure exceeds 2.4 GPa,the visible light response is suppressed due to the amorphization of the sample,and as the pressure continues to increase,no significant light response can be observed.In addition,we also conducted an in-depth analysis of the causes of abrupt changes in the current values when switching on and off,and gave reasonable explanations.4.In this thesis,we also compared the high-pressure research results of MAPb Br3 with Zhao's group and discussed the differences between the resistance value and the photoconductivity measurement results.The resistance value of our experimentally measured MAPb Br3 is five to six orders of magnitude higher than that of Zhao et al.'s measurement of the resistance value.After sufficient analysis,we believe that the difference in the resistance value is mainly due to the different experimental conditions in the two sets of experiments,resulting in different internal conductive mechanisms within MAPb Br3.Moreover,the smaller resistance value indicates that electron conduction predominates in Zhao et al.'s test,and the obscure ion conduction behavior is obscured.This also explains very well that a significant light response can still be observed when the pressure reaches 30 GPa.