| Energy shortage and environmental pollution are important problems to be solved urgently on the road of sustainable development.Utilizing clean and renewable energy is an important solution to these problems.The development of new photoelectric functional materials is an important way to realize the efficient utilization of solar energy.Ferroelectric photovoltaic effect has attracted much attention due to its large open-circuit voltage,which is not limited by the bandgap of materials.Moreover,the photovoltaic performance of ferroelectric photovoltaic materials is obviously dependent on the polarization state of the materials,which provides a better choice for the design of controllable devices.Common Perovskite Ferroelectric Materials generally have the characteristics of large band gap(> 3 eV),which greatly limits the research of ferroelectric photovoltaic materials.Therefore,reducing the band gap of ferroelectric photovoltaic materials is the primary problem in the research of ferroelectric photovoltaic materials.For perovskite ferroelectric materials,doping transition metal ions is the most common scheme for reducing the bandgap.However,the doping of transition metal ions with mismatched valence states will lead to structural defects such as oxygen vacancies.Defects have a significant impact on the ferroelectric properties of materials and play an important role in the regulation of properties.Ferroelectricity is the basis of ferroelectric photovoltaic effect,and the polarization state of materials is the key to control the photovoltaic performance of ferroelectrics.Therefore,it is necessary to study the effect of transition metal ion doping on ferroelectric properties of perovskite materials.The common way to realize the photovoltaic performance of ferroelectric materials is to construct a "sandwich" structure of ferroelectric photovoltaic devices.In such devices,the interaction between electrodes and ferroelectrics will obviously affect photovoltaic performance of ferroelectrics,especially in the polarization process,the interaction between electrodes and ferroelectrics will change further.Therefore,the study of photovoltaic devices is also of great significance to the performance realization of ferroelectric photovoltaic materials.In this study,Fe-doped potassium tantalum niobate crystal(Fe-KTN)was selected as the research object,and the following four scientific issues were studied: 1.The effect of Fe-doping on the ferroelectric properties of KTN crystal and its regulation;2.The effect of Fe-doping on the domains orientation of Fe-KTN crystal;3.The effect of Fedoping on the bandgap modulation of KTN crystal;4.Ferroelectrics photovoltaic devices based on Fe-KTN crystals are constructed.The photovoltaic performance of the devices and the physical mechanism of the interaction between the electrodes and the Fe-KTN interface are analyzed.In this paper,the ferroelectric properties of Fe-KTN tetragonal ferroelectric single crystals are systematically studied.The generation mechanism of special double hysteresis loops in Fe-KTN crystal is analyzed,and the binding and recovery mechanism of defect dipole to domain structure is proposed.It is explained that the difference of intrinsic macropolarization between the original state and annealed Fe-KTN crystals is due to the different orientation states of defect dipoles.The controllable evolution of double hysteresis loops in Fe-KTN crystal is realized by changing the electric field conditions in ferroelectric measurement,and the response characteristics of defect dipoles to external electric field are found.The effect of Fe doping on ferroelectric properties and domain structure reversal of Fe-KTN crystals was studied.Firstly,the binding effect,restoring force and response to external electric field of defect dipoles in Fe-KTN crystals with different doping amount were analyzed by ferroelectric measurements.Then,the response of domain structure to external electric field in Fe-KTN0.15 and Fe-KTN1 crystals was analyzed by polarizing microscope imaging.Finally,a theoretical model for the coexistence of polar defect dipoles and non-polar defect structures in crystals and the transition of defect structure types with doping amount is proposed.The ferroelectric properties and domain reversal processes of Fe-KTN crystals with different doping amount are explained.The intrinsic orientation characteristics of domain structure in Fe-KTN crystals and the mechanism of the effect of defect dipoles on the ordering of crystals were studied.Firstly,the ordering of Fe-KTN crystals in the process of heating and cooling was analyzed by means of the temperature dependence of dielectric coefficient test and Fr?ohlich entropy.It was found that the binding effect of defect dipoles on domain structure was the reason for maintaining the ordering of crystals,and the change of binding effect with temperature was analyzed.Meanwhile,the intrinsic orientation of FeKTN crystal was analyzed by ferroelectric measurement and polarizing microscope imaging.Finally,the ferroelectric properties of aged polarized Fe-KTN crystals were tested in variable temperature conditions,and it was found that the defect dipoles could restore the orientation effect of domain structure under the action of electric field.The effect of Fe-doping on the bandgap of KTN crystals was studied.The mechanism of bandgap reduction was analyzed from the viewpoint of band structure.Then,based on Fe-KTN crystals,Fe-KTN crystal photovoltaic devices with ITO/FeKTN/Ag structure were constructed,and the photovoltaic performance of the devices were tested to realize the polarization-reversible ferroelectric photovoltaic performance.Finally,the coexistence model of depolarization field and interface barrier in Fe-KTN crystal photovoltaic devices is proposed.The polarization-reversible photovoltaic performance of Fe-KTN crystal photovoltaic devices is the result of the coupling of two built-in electric fields. |
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