Large reduction in band gap and conduction mechanisms in Bi0.5Na0.5TiO3-based ceramics with a sizable polarization

Low band gap ferroelectrics that exhibit high polarization at ambient conditions are highly desirable for achieving efficient charge carrier separation in photovoltaic applications. In this work, a well-designed Co/Nb-modified Bi _0.5Na _0.5TiO ₃ system (1- x)Bi _0.5Na _0.5TiO ₃- xCaCo _0.5Nb _0.5O ₃ with x =0.00–0.20 offers a platform with the characteristics of low band gap and high ferroelectric polarization simultaneously. A structural phase transition from rhombohedral to monoclinic structure as the x content increases, which is verified by XRD analysis. Specifically, the x=0.10 ceramic exhibited a considerable high remanent polarization of 20.2 μC/cm ² and a low band gap of 2.18 eV, approximately 31% lower than that for a pure BNT sample (∼3.13 eV). However, the leakage current significantly increased with an increase in the x content, where it exceeded by more than three orders of magnitude for the x=0.20 sample in comparison to the pure BNT sample. Current-voltage characteristics revealed that space-charge-limited conduction was the primary conduction mechanism within the ceramics with x <0.20, which can be attributed to the free carriers confined by oxygen vacancies. On the other hand, field-assisted ionic conduction was revealed to be the primary conduction mechanism in the sample corresponding to x =0.20 at room temperature. We ascribe these mechanisms to oxygen vacancies and multivalent cobalt ions based on the XPS analysis. An improved understanding of conduction mechanism sheds light on leakage current problems and facilitates the design of BNT-based ferroelectric devices with excellent insulating character for high-performance photoelectric and photovoltaic applications.