Investigation On K_0.5Na_0.5NbO₃-based Films For Energy Storage And Ferroelectric Photovoltaic Effect

Ferroelectric materials have attracted much attention due to their unique characteristics of residual polarization.It is currently used in many fields,such as new information storage devices,sensors and photoelectric materials.Potassium sodium niobate(K_0.5Na_0.5NbO₃)based materials are typical ferroelectric.Because of its excellent properties in many aspects and even compared with lead-based materials,it is considered as a substitute for lead-based materials.In recent years,people have paid great attention to the research of KNN based materials,but most of the research is about the expansion of ceramics and less research on films.Moreover,because of its poor ferroelectric properties,KNN based materials have limited its application in many aspects.In this paper,the KNN based films were prepared by the chemical sol-gel method.In order to improve electrical properties of KNN based films,the properties of the films were improved by doping,and their related properties were studied.The specific work is described as follows.we prepared KNN and KNN based solid-solution films by adding similar structure BiMnO₃?BMO?into KNN to improve the ferroelectric characteristics.The structure characterization and electrical properties of KNN and 0.85KNN-0.15BMO films were characterized.The results show that the 0.85KNN-0.15BMO film shows obvious relaxation characteristics,which is suitable for the application in energy storage.The energy storage density and energy storage efficiency are calculated by ferroelectric loops.Electrical properties under variable temperature and bipolar fatigue at room temperature were tested to analyze the thermal stability and anti-fatigue activity.The high energy storage efficiency with excellent thermal and electric cycling stability were obtained,and the origins were discussed detailedly.0.9KNN-0.1BMO film is successfully prepared by the same method.The effects of BMO addition on the electric,band-gap and photovoltaic properties were investigated systematically.It is shown that,compared with traditional KNN film,BMO modification obviously improves the ferroelectric properties due to the depression of the leakage current and the enhancement of domain switching.More importantly,ferroelectric photovoltaic effect is observed with a large short-circuit current and open-circuit voltage in the lead-free solution film,which benefits from the excellent ferroelectric properties and photovoltaic response in visible region derived from the narrow band gap modified by BMO.Mn doped KNN film is selected to improve its electrical properties.The crystal structure,ferroelectric properties,and photovoltaic effect of Mn doped K_0.5Na_0.5NbO₃film were investigated systematically.A fascinating phenomenon is observed that the ferroelectric photovoltaic effect exhibits high short circuit current and open circuit voltage,which is originated from the enhanced ferroelectric properties and narrower band gap.The transition element Nb partially substituted by Mn results in the lattice distortion and further destroys the symmetry space structure,thus enhances ferroelectric polarization.And the narrower band gap effectively decreases the internal potential barrier to separate the carriers.All these contribute to the enhancement of photovoltaic effect.