The Microstructure, Electric, Optical and Photovoltaic Properties of BiFeO3 Thin Films Prepared by Low Temperature Sol-Gel Method

Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices due to their abnormal photovoltaic effect. However, the current reported efficiency is still low. Hence, it is urgent to develop narrow-band gap ferroelectric materials with strong ferroelectricity by low-temperature synthesis. In this paper, the perovskite bismuth ferrite BiFeO3 (BFO) thin films were fabricated on SnO2: F (FTO) substrates by the sol-gel method and they were rapidly annealed at 450, 500 and 550 degrees C, respectively. The microstructure and the chemical state's evolution with annealing temperature were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), and the relationship between the microstructure and electric, optical and photovoltaic properties were studied. The XRD, SEM and Raman results show that a pure phase BFO film with good crystallinity is obtained at a low annealing temperature of 450 degrees C. As the annealing temperature increases, the film becomes more uniform and has an improved crystallinity. The XPS results show that the Fe3+/Fe2+ ratio increases and the ratio of oxygen vacancies/lattice oxygen decreases with increasing annealing temperature, which results in the leakage current gradually being reduced. The band gap is reduced from 2.68 to 2.51 eV due to better crystallinity. An enhanced photovoltaic effect is observed in a 550 degrees C annealed BFO film with a short circuit current of 4.58 mA/cm(2) and an open circuit voltage of 0.15 V, respectively.