Flexible vibrational energy harvesting devices using strain-engineered perovskite piezoelectric thin films

The material properties of Pb(Zr,Ti)O₃ (PZT) thin film with a LaNiO₃ (LNO) buffer layer on an ultra-thin Ni-Cr-based austenitic steel metal foil substrate are systematically investigated for flexible piezoelectric vibrational energy harvesting device applications. The use of the flexible austenitic metal foil substrate with a high thermal expansion coefficient results in the large compressive stress in the deposited PZT thin film, and subsequently produces much enhanced ferroelectric polarization of the film. The measured polarization value of the PZT film on the flexible metal foil substrate is over 50 µC/cm², showing a great improvement in comparison with the film on the conventional Pt/Ti/SiO₂/Si substrate. Simultaneously, the LNO buffer layer prevents any undesirable reactions, provides uniform nucleation sites and promotes easy crystallization of the PZT thin film, achieving the highly dense and uniform microstructure of the film with a smooth surface. Using the strain-engineered PZT thin film, a small-scale and flexible vibrational energy harvester is fabricated using a simple punching process without any complex microelectromechanical system (MEMS) or etching processes. The maximum power and the corresponding peak voltage of the device are 5.6 µW and 690 mV, respectively, which are much higher values than those of MEMS-based vibrational energy harvester. The improved device performance of our flexible small-scale vibrational energy harvester is due to the enhanced PZT film properties by the stress engineering, the increased flexure and deflection of the flexible structure, and the easy vibrational sensitivity at low acceleration of 0.5g, compared to the MEMS-based device integrated with conventional Si-based substrates. The results prove that our strain-tuned PZT thin film-based flexible vibrational piezoelectric energy harvester is a promising candidate for autonomous power systems and future small-scale mobile platforms.