Electric-Field-Dependent Surface Potentials and Vibrational Energy-Harvesting Characteristics of Bi(Na _0.5 Ti _0.5 )O ₃ -Based Pb-Free Piezoelectric Thin Films

The successful utilization of Pb-free piezoelectric materials is considered as critical since the piezoelectric material-based thin-film cantilever is still the preferred choice for commercial vibrational energy harvesters. Herein, we introduce a highly efficient piezoelectric energy harvester based on a Pb-free representative compound, Bi _0.5 Na _0.5 TiO ₃ , which has not been explored so far. Applying a strong electric field for poling purposes brought unexpectedly huge changes in the dielectric constant and piezoelectric coefficient, which were responsible for the promising power density of 21.2 μW/cm ² /g ² /Hz with 537.7 mV output voltage and 2.22 μW output power for a 2 μm thick 0.94(Bi _0.5 Na _0.5 )TiO ₃ -0.06BaTiO ₃ thin-film cantilever. The power density value is the best so far compared with any reported values for thin-film-based harvesters. As the origin of the effects of poling, the surface potentials across the grain structure are discussed in conjunction with the defect-dipole alignment, as evidenced by the increased oxygen vacancies on the film surface under an external bias field.