Outstanding ferroelectric properties in the narrow bandgap cobalt-substituted BiFeO₃ spin-coated films

Thin films of BiFe_1−xCo_xO₃ (BFCO, x = 0–0.05) were prepared using the sol–gel method and deposited on a fluorine-doped tin oxide (FTO)/glass substrate. The crystal structure, surface morphology, dielectric properties, polarization, and optical characteristics of the BFCO thin films were investigated. X-ray diffraction (XRD) and Raman spectroscopy analyses show that Co doping induces lattice distortion. Scanning electron microscopy (SEM) images demonstrate that BFCO films with x = 0.03 possess uniform fine grains, which are crucial for their ferroelectric properties. From XPS pattern, it can be observed that Co doping can inhibit the conversion of Fe³⁺ into Fe²⁺, and BiFe_0.97Co_0.03O₃ films exhibit greatly reduced oxygen vacancy concentration. Therefore, BiFe_0.97Co_0.03O₃ film was found to have the lowest leakage current density (J = 7.18 × 10⁻⁷ A/cm²). The film demonstrates outstanding residual polarization at room temperature, with a value of P_r = 152.1 μC/cm², more than twice the magnitude of that in pure BFO (P_r = 72.33 μC/cm²). Moreover, the dielectric properties of BFCO films show a significant improvement when compared to those of pure BFO samples. This enhancement is attributed to the Co doping-induced structural transition, along with a reduction in grain size and a decrease in the concentration of oxygen vacancies. Additionally, the BiFe_0.97Co_0.03O₃ film exhibits a narrower band gap (E_g = 1.69 eV) in comparison to the BFO film (E_g = 1.87 eV). Consequently, an expansion in the range of photovoltaic applications for BFO films can be achieved.