Flexible piezoelectric nanogenerators

Abstract

Nowadays, miniaturization, functionalization and low power consumption are the developing directions of personal electronic devices because of the portable and implantable requirements. Developing an energy source with a comparable size to power the electronic devices is desirable and of great challenge. Piezoelectric nanowires based nanogenerators (NGs) can harvest tiny and irregular mechanical energy in the surroundings and turn them into electricity via piezoelectric effect. NGs can be used to power the functional devices. Since the invention of NG in 2006, it has developed quickly, until now it has been successfully combined with functional devices such as UV sensor, pH sensor and environmental monitoring sensor to form the self-powered systems. In this review, the piezoelectric NGs’ working principle is described firstly. It focuses on the ferroelectric materials which do not have internal free carriers. The static analysis gives a detailed working mechanism of the NG. The dynamic analysis gives the frequency dependence of the NG’s output. Next, the working mechanism of the ZnO based NGs is discussed. Compared with the NGs fabricated by ferroelectric materials, the ZnO based NGs’ working process is also affected by the ZnO’s internal free carriers. According to whether the direction of the NG’s output current is changed, ZnO NGs have been categorized into DC mode and AC mode. In the section of DC ZnO NG, the electron’s transportation in the circuit is described in detail. The effect of the material’s free carrier density and Schottky contact on the NG’s output is analyzed. The working process of AC NG is described, subsequently. And its merits compared with the DC mode are also analyzed. Then, we review some typical papers about flexible NGs. The NGs have been categorized into fiber based woven NGs and composite based NGs. For each category, typical papers have been reviewed and each work’s improvement and drawbacks has been analyzed. In addition, considering the development of the implantable medical devices, the biocompatible and implantable NGs aiming at providing power supply for the medical devices are also discussed. At last, this review puts forward the bottleneck of the flexible NGs and point out the future developing trend of the flexible NGs.