Role of an Oxygen Vacancy Nanostructure on the Switchable Photovoltaic Effect in BiFeO3

In all oxide compounds, oxygen vacancies intrinsically exist and their role and impact on materials’ properties have been studied for several decades. Mostly they have been considered as defects that disturb a ‘perfect world’. Nowadays, however, researchers consider them as new parameters for controlling functionalities of oxide compounds such as quantum and energy materials; here, the multiferroic ferrite, BiFeO3, is presented as a remarkable example. In this context, BiFeO3 has been extensively studied for its electromagnetic properties, in which magnetic order (antiferromagnetism) and electric polarization (ferroelectricity) are coupled and inter-controllable. The combination of two or more controllable parameters is considered a route to realizing new kinds of devices. Moreover, a switchable photovoltaic effect and its diode effect have been demonstrated. In BiFeO3, the mechanism of the photovoltaic effect has been found to be a migration of positively charged oxygen vacancies manipulated via an external electric field. Therefore, it has been simply assumed that the photovoltaic and diode effects are changed by the oxygen vacancy content and supposed to be enhanced by an increasing oxygen content.