| It is a key characteristic for ferroelectric material that polarization can be reversed under the external proper electric field.The polarization owns two opposite directions,i.e.upward and downward along vertical direction,which can well match the binary numbers '0' and '1'.Moreover,the polarization can be kept stable after withdrawing the field.So the ferroelectric has been considered as one of promising potential candidates for nonvolatile data storage,such as ferroelectric random access memory(FeRAM).Recently,a novel photovoltaic effect,which shows an abnormal open circuit voltage unlimited by the band gap of the material and a polarization-dependent separating phenomenon of photo-generated carriers,has attracted numerous attentions in physics and application studies on the ferroelectric due to its great potential in electronic devices,such as solar cell.Because of the close relation between the effect and polarization,it is called as ferroelectric photovoltaic effect(FePV effect),which naturally has a programmed and nonvolatile property as same as the polarization and shows great potential in electric-optical memory.Among many ferroelectric materials,BiFeO3(BFO)film has potential beyond others in the electric-optical memory,due to its smaller band gap and sizable remanent polarization which can significantly improve the photovoltaic performance.However,many groups have found that many factors,such as the bulk photovoltaic effect,complicate domain structure,Schottky barrier at the interfaces,depolarization field induced by the polarization and structure defects in the film,could contribute to the photovoltaic effect.From the point of actual application,it is very important to manipulate the FePV process simply.Unfortunately,the complicated FePV mechanisms in the BFO film-based devices make it difficult.Besides,the FePV effect in the BFO film also shows the great potential in energy field,speciously in solar cells.However,compared with semiconductor,BFO has a larger band gap which is extremely limited its light absorption.And its very low carrier mobility can also reduce the quantity of electrons collected at electrodes.As a result,the photocurrent obtained in BFO will be very low and limit its actual application in solar cell.To resolve these problems,here,we have carried out three works by selecting and designing the materials and device structures properly,and innovating the working mechanism of the devices,as described below:(1)To improve the reading/writing reliability of the FePV-based electric-optic memory,we hoped to realize it by reversing the PV effect via the polarization.In this work,we fabricated well-ordered microarrays based on the tetragonal BFO(T-BFO)films with the assistances of pulsed laser deposition and microfabrication method.The microarrays not only improve the storage density but also avoid various structure defects,such as oxygen vacancies and grain boundaries in the films,which have influence on the FePV effect.The T-BFO film has a simple domain structure,which can exclude the influence of the complicated domain structure to a degree compared to the rhombohedral BFO film.What's more,the T-BFO film has a larger remanent polarization,which can provide a larger depolarization field to separate the photo-generated carriers.In this work,based on the epitaxial tetragonal-phase BFO(T-BFO)films,we realize the direct observation of the FePV effect at microscale using a combined system integrating conductive atomic force microscope(CAFM)and ultraviolet light lamp.This result can directly prove the photovoltaic effect occurred in the films is FePV effect.Moreover,the photovoltaic process is well manipulated in such T-BFO-based micro-array by the combined changing of top electrode and ferroelectric polarization direction.We reveal that the manipulation of photovoltaic process in the present micro-array is dominated by the competition between two factors,i.e.,the depolarization field induced by the ferroelectric polarization in T-BFO and the irreversible electric field caused by potential barriers at interfaces.In this study,we successfully realize programmed photovoltaic effect.From the aspects of the selection of the BFO film and the design of device structure,we clearly undestand the FePV mechanism and provide a reasonable memorizer scheme.This work is meaningful for understanding the FePV process in other BFO-based materials,and provides us a feasible avenue for developing high-performance electric-optical integrating devices based on the FePV effect.(2)In the first study,we have found that the polarization-controlled memory has a closed relation with the electrodes.That will limit its actual application to a degree.To avoid that,we propose an electric-optical memory prototype based on the combination of FePV and filament-type resistive switching effects,and realize the coexistence of these two effects in Ti-doped BFO films.We fabricated an electric-optical memory device with a structure of Au/BiFe0.85Ti0.15O3/ITO(Au/BFTO/ITO).In the device,we find that the photovoltaic effect can be well controlled by switching the resistance state in BFTO films.Accordingly,we realize the electric-optical memory effect by electrically switching resistance state and optically reading the photovoltaic open circuit voltage.And this device shows excellent memory performances.Using first-principles calculations based on density functional theory,we further give an insight into the mechanism of the present electric-optical memory prototype,and reveal that the Ti-doping can dramatically change the electronic structure and charge density of the film,which is very important for realizing the filament-type RS effect and improving the photovoltaic property.In the study,from the aspects of design of material and innovation of the working mechanism,we realize the programmed photovoltaic effect,and fabricated an electric-optical memory prototype with good memory performances.This work provides us a feasible way for developing next-generation low-power-dissipation memorizers.(3)Because of the very low photoelectric conversion efficiency,the FePV effect has been greatly limited to be used in energy field.In this work,we fabricate new FePV photovoltaic cells in which the photovoltaic property of the Au/BFTO/ITO heterostructure is enhanced with the assistance of the surface plasmon resonace(SPR)effect of Au nanoparticals(Au NPs).In these devices,the Au NPs enlarge the light absorption of the film by scattering the incident light.At the meantime,the NPs also absorb the energy of photons via SPR effect,then transfer the energy to the electrons and improve the electron mobility which leads electrons to arrive the ITO electrode more easily.As a result,the photovoltaic effect is improved.Additionally,we find that descreasing the distance between Au NPs and ITO can lead more electrons to arrive the ITO electrode,which is good to the photovoltaic performance.Our results indicate that the SPR effect of the noble metal NPs has a great potential in developing the emerging solar cell based on the FePV effect. |
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