Material strategies to enhance the performance of piezoelectric energy harvesters based on lead-free materials

Over the past four decades, energy microsources based on piezoelectric energy harvesting have been intensively studied for applications in autonomous sensor systems. The research is triggered by the request for replacing standard lead-based piezoelectric ceramics with environmentally friendly lead-free materials and potential deployment of energy-harvesting microsystems in internet of things, internet of health, 'place and leave' sensors in infrastructures and agriculture monitoring. Moreover, futher system miniaturization and co-integration of functions are required in line with a desired possibility to increase the harvested power density per material volume. Thus, further research efforts are necessary to develop more sustainable materials/systems with high-performance. This paper gives a comprehensive overview on the processing and functional testing the lead-free bulk materials and thin films and discusses their potential in the applications in the stress- and strain-driven piezoelectric energy harvesting. This includes the methodology of estimation of the substrate clamping and orientation/texture effects in the thin films, and identification of orientations offering high figure of merit. The ability to control film orientation of different lead-free materials is reviewed and the expected piezoelectric performances are compared with the ones reported in literature.