Flexible Piezoelectric Energy-Harvesting Exploiting Biocompatible AIN Thin Films Grown onto Spin-Coated Polyimide Layers

The increasing demand of piezoelectric energy harvesters for wearable and implantable applications requires biocompatible materials and careful structural device design, paying special attention to the conformability characteristics, properly tailored to scavenge continuously electrical energy even from the tiniest body movements. This paper provides a comprehensive study on a flexible and biocompatible aluminum nitride (AIN) energy harvester based on a new alternative fabrication approach, exploiting a thin polyimide (PI) substrate, prepared by spin coating of precursors solution. This strategy allows manufacturing substrates with adjustable thickness to meet conformability requirements. The device is based on a piezoelectric A1N thin film, sputtered directly onto the soft PI substrate, without poling/annealing processes and patterned by simple and low cost microfabrication technologies. AIN active layer, grown on soft substrate, exhibits good morphological and structural properties with roughness root mean squared (R-rms) of 6.35 nm, columnar texture and (002) c-axis orientation. Additionally, piezoelectric characterization has been performed and the extracted piezoelectric coefficient value of AIN thin film resulted to be 4.93 +/- 0.09 pm/V. The fabricated flexible A1N energy harvester generates an output peak-to-peak voltage of similar to 1.4 V and a peak-to-peak current up to 1.6 itA, under periodical deformation, corresponding to a current density of 2.1 mu A/cm(2), and providing a maximum generated power of 1.57 mu W under optimal resistive load. Furthermore, the AN energy harvester exhibits high elasticity and resistance to mechanical fatigue. High quality AIN piezoelectric layers on elastic substrates with tunable thicknesses pave the way for the development of a straightforward technological platform for wearable/implantable energy harvesters and biomechanical sensors.