Modified ferroelectric and photovoltaic properties of BiFe0.91Zr0.09O3 thin films via altered annealing atmospheres

In this work, BiFe0.91Zr0.09O3 (BFZrO) films are successfully prepared on ITO/glass substrates via sol–gel synthesis. The effects of different annealing atmospheres (air, O2, and N2) on the crystal structure; defect concentration; and ferroelectric, photovoltaic, and aging performances of the BFZrO samples are studied, and related mechanisms are analyzed in detail. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses reveal that all samples exhibit a two-phase structure composed of rhombohedral R3c and orthorhombic Pnma phases with different contents. X-ray spectroscopy (XPS) and scanning electron microscopy (SEM) analyses indicate that annealing under an O2 atmosphere significantly reduced the concentrations of oxygen vacancy and Fe2+ defects in the films, and is more beneficial to grain growth. Samples annealed in O2 demonstrate lower leakage currents (4.83 × 10−6 A cm−2) and higher remanent polarization strengths (2Pr = 70.26 μC cm−2) than those prepared in air or N2. Furthermore, annealing in O2 improves the optical absorption and photovoltaic performance of BFZrO films, which reduced the band gap (Eg) to 2.65 eV and a doubled short-circuit current density (JSC). Energy band diagrams of Au/BFZrO/ITO structures are constructed from analysis of ultraviolet photoelectron spectra (UPS) and ultraviolet-visible (UV-vis) spectra to explain the mechanisms by which the annealing atmosphere influences the photovoltaic performance. Aging of BFZrO films is found to closely relate to the oxygen vacancy content and mechanisms of domain pinning, so the O2-rich annealing treatment provides an effective means to enhance the film stability.