Probing Of Multifield Coupling Effects Of Electroactive Materials Based On Scanning Probe Microscopy Methods

Carrier transport phenomenon is observed under the excitation of force,electric,optical,temperature fields,etc.in electroactive materials,exhibiting complex multifield coupling effects,which has a wide range of applications in our daily life.At the same time,there is a close relationship between macroscopic physical properties and intrinsic multifield coupling effects in electroactive materials.Thus,it is essential to characterize and analyze multifield coupling effects of electroactive materials.Scanning probe microscopy has become an important characterization and manipulation method for probing local multifield effects of materials since its inception.It has been regarded as researchers' eyes and hands,providing opportunities to understand the microscopic mechanisms of multifield coupling effects in electroactive materials and offering some guidance for improvement in its macroscopic properties.Although atomic force microscopy(AFM)has a wide range of applications in the characterization and manipulation of local multifield coupling responses,there are still some challenges in terms of understanding the intrinsic electromechanical responses and ionic dynamics mechanisms of electroactive materials.Specifically:(1)It's difficult to differentiate the electromechanical responses due to the various contributions to the surface displacement when the AFM conductive tip in mechanical contact with the surface is biased by an oscillating voltage.(2)The electromechanical responses are highly susceptible to topographic crosstalk by sample structures,making it difficult to acquire intrinsic responses.(3)AFM is mainly used in single-ion systems,and it is difficult to analyze ionic dynamics processes of dual-ion systems.To this end,this dissertation develops AFM new methods and study the complex multifield coupling effects of perovskite solar cell and lithium-ion solid-state electrolyte.The detailed discussion is as follows:(1)The first and second harmonic methods were developed to distinguish electromechanical mechanisms.Based on electrostrictive effects,it is suggested that the system will exhibit an apparent electromechanical response that is linear to the applied electric field when it possesses large spontaneous polarization.However,for the system which has an induced polarization,it will exhibit an apparent electromechanical response that is quadratic to the applied electric field.The results suggest that it's helpful for distinguishing electromechanical responses by comparing the first and second harmonic responses.By combining the switch spectroscopy and the STIM method,the intrinsic mechanisms of first and second harmonic electromechanical responses have been proved.(2)The high-throughout sequential excitation(SE)method is developed to acquire intrinsic electromechanical responses of grain and grain boundary of LATP.The highthroughout SE method can acquire the intrinsic electromechanical responses of LATP without tracking the resonance frequency,and eliminate the topographic crosstalk due to its high-efficiency working principles.The local electromechanical responses of LATP at grain and grain boundary directly correlate with its macroscopic conductivity through the high-throughout SE method,and it's suggested that ionic conductivity of grain boundary plays an important role in the overall ionic conductivity.(3)A novel dynamic mechanism of the dual-ion system is proposed to lay the foundation for understanding the ionic dynamic processes of complex multi-ion systems.Using soda-line float glass,a typical dual-ion system,as a model system,and according to its characteristics of ion transport,the dynamic mechanism of the dual-ion system was proposed.By combining first and second harmonic methods and relaxation curves,the dominant mechanism of electromechanical responses and ionic dynamic processes in the dual-ion system are explored.Finally,we resolve the microscopic dynamic mechanism and acquire the local diffusivity and activation energy through temperature-dependent relaxation curves,which lays the foundation for understanding complex multi-ion systems.(4)Through the newly developed methods mentioned above,the intrinsic multifield coupling effects and ionic dynamic processes of perovskite solar cells are studied to understand its microscopic working mechanisms.Using triple-cation mixed-halide perovskite solar cell with efficiency over 20 % as the model system,the local photoelectric coupling responses are analyzed and a possible mechanism associated with interfacial ionic defects is proposed.By combining first and second harmonic methods and high-throughout SE method,the correlation between force-electric-optical coupling effects is analyzed,and microscopic working mechanism associated interfacial defects is proposed.Furthermore,using dynamic mechanism of the dual-ion system,we analyze the ionic dynamic processes of perovskite solar cell.Finally,the ion transport is manipulated by high temperature and photocurrent degradation pathway is analyzed,providing guidance for device solar cell fabrication.