A Flexible Poling‐Free Piezoelectric Energy Harvester Based on Gradient Ti‐Doped <scp> BiFeO ₃ </scp> 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.