Study On The Controllable Preparation Of Bismuth-based Perovskite Oxide Thin Films And Their Ferroelectric Photovoltaic Effects

Ferroelectrics have the unique ferroelectric photovoltaic effect,which is mainly manifested by the ability of the photovoltage to exceed the bandgap,and the direction of the photocurrent can be reversed and regulated by an external electric field.Furthermore,the unique physical mechanism of the ferroelectric photovoltaic effect makes it possible for the theoretical conversion efficiency of ferroelectric materials to exceed the Shockley-Queisser efficiency.These advantages make ferroelectrics promising for new photovoltaic devices such as new-concept photovoltaic cells,information storage,photovoltaic synaptic devices,and ferroelectric photovoltaic sensors.Bismuth-based perovskite oxides thin films are a significant class of ferroelectric photovoltaic materials.Among them,BiFeO₃（BFO）thin films,characterized by exceptional ferroelectricity and relatively small bandgap,have been widely studied.However,the ferroelectric photovoltaic effect of BFO thin films still faces some significant issues that require further investigation.Firstly,the ferroelectric domain walls of BFO exhibit the anomalous photovoltaic effect,where the presence of domain walls leads to the photovoltage significantly larger than the bandgap.The physical mechanism behind this phenomenon,however,is still a matter of debate.Secondly,freestanding BFO thin films exhibit novel properties due to their freedom from substrate constraints,yet there is a lack of research on the bulk photovoltaic effect of freestanding BFO thin films.Finally,the bandgap of BFO（～2.7 e V）is still relatively large,resulting in a limited visible light response.Reducing the bandgap of BFO while maintaining its ferroelectricity is the key to developing high-efficiency ferroelectric photovoltaic devices.To address the above issues,this paper focuses on the study of BFO and Bi₂FeMnO₆（BFMO）single crystal epitaxial thin films,with the implementation of domain engineering,freestanding film processing,and bandgap tuning techniques.Through these methods,the controllable preparation of BFO and BFMO thin films has been achieved.Based on this,the ferroelectric photovoltaic effect of these materials is systematically investigated.The main research findings are as follows:1.The domain structures of the BFO films are controlled by utilizing substrate anisotropy.By optimizing the growth parameters of the BFO films using pulsed laser deposition technique on Sr Ti O₃（STO）（001）substrates,the high-quality BFO single crystal thin films with multi-domains are obtained.BFO films with non-periodic and periodic striped 71°domain walls are obtained using STO substrates with the miscut angle of 4°along with the[100]direction and Dy Sc O3（110）substrates,respectively.The single-domain BFO thin films are epitaxially grown on STO substrates with the miscut angle of 4°along with the[110]direction(STO_4°[110]).The achievements provide the sample basis for the study of the ferroelectric photovoltaic effect of BFO films.2.The physical mechanism on the anomalous photovoltaic effect at domain walls of the BFO film is revealed.By quantitatively analyzing the polarization angle-dependent photovoltaic effect of non-periodic and periodic 71°domain walls,the anomalous photovoltaic effect is found to arise from the combined effect of the effective electric field and the localized bulk photovoltaic component in a coplanar electrode structure.The photovoltaic measurement under white light illumination directly reveals a significantly enhanced electric field at stripe domain walls in comparison with the domains.Furthermore,the defect states at the domain walls may recombine the photogenerated carriers and drastically weaken the electric field of the domain walls.In addition,the bulk photovoltaic effect of the striped domain wall is approximately 25times as large as that of the domains,as the degree of broken central inversion symmetry at the domain wall is greater.The findings contribute to deeper understanding of the mechanism of the anomalous photovoltaic effect at domain walls.3.Freestanding single-domain BFO films are prepared by the wet transfer method and are found to have enhanced bulk photovoltaic effects.The freestanding BFO films are obtained by dissolving the Sr₃Al₂O₆ sacrificial layer and then using lift-off technology.The stress of the freestanding BFO films are completely released,while still maintaining single-domain structures.In the coplanar electrode structure,the freestanding BFO films exhibit greater in-plane non-centrosymmetric components compared to the strained films on the STO_4°[110]substrates,resulting in increased shift and ballistic currents,thus displaying enhanced bulk photovoltaic effects.Furthermore,the growth rate of the bulk photovoltaic effect remained basically the same as the thickness of the film increases.The above results provide a new approach to enhance the bulk photovoltaic effect of BFO films.4.The bandgap tuning is achieved by using the double perovskite BFMO thin film to dramatically improve the ferroelectric photovoltaic response and achieve electric field modulation.BFMO films exhibit relatively strong ferroelectricity comparable to that of BFO.The Mn element enhances the absorption of BFMO in the near-infrared and visible light,resulting in a BFMO device with the photocurrent two orders of magnitude higher than that of BFO,and the photovoltaic efficiency approximately 430 times larger than that of BFO.The Schottky barrier at the interface of BFMO/Sr Ru O₃ is responsible for separating photogenerated carriers.The applied external electric field can modulate the height of the Schottky barrier,thereby achieving an electric field-modulated photovoltaic response.The redistribution of oxygen vacancies under the electric field is the underlying cause of the modulation of Schottky barrier height.The results provide the theoretical reference for the design and development of high-efficient ferroelectric photovoltaic devices.