A Flexible Poling-Free Piezoelectric Energy Harvester Based on Gradient Ti-Doped BiFeO3 Films

Flexible piezoelectric materials that combine high performance and outstanding flexibility are imperative for advancing next-generation wearable electronics, self-powered sensors, and energy harvesters. The creation of useful piezoelectricity in piezoelectrics requires a poling process via the temporary application of a strong electric field, which introduces risks including electrical breakdown and thermal depolarization. Here, a self-aligned polarization state is realized in compositional graded Ti-doped BiFeO 3 films via the formation of defect dipoles and the spatial modulation of strain distributions. The down-graded film, among those with different configurations, manifests a notable upward initial polarization ( P 0 = −15.7 μC cm −2 ) and a substantial internal bias field ( E bias = 167.2 kV cm −1 ). The prototype piezoelectric harvester based on the flexible down-graded film can deliver a maximum output power of 2.26 μW at a load resistance of 3.52 MΩ, a high sensitivity of 5.92 V N −1 without the need for external poling treatment, and robust mechanical durability of 8000 cycles. These advances in self-poling enable the derived films to offer significant freedom to design flexible piezoelectric devices that are well suited for microscale energy harvesters and sensors.