Transparent flexible stretchable piezoelectric and triboelectric nanogenerators for powering portable electronics

Transparent flexible and stretchable nanogenerators (NGs) that harvest various types of mechanical energy exhibit a great potential for powering low-power portable devices and self-powered electronic systems. Integration of transparency, flexibility and stretchability to NGs has gained a lot of interest for realizing the energy harvesting systems in practical life. Flexible piezoelectric nanostructures, which can generate electrical signal when mechanically deformed, are the most promising candidates for piezoelectric NGs, which offer sufficient power to drive portable electronics and cardiac pacemakers. Moreover, triboelectric NGs enlighten a new technique to harvest mechanical energies with high conversion efficiency. This review highlights the recent research progress of transparent and flexible ZnO nanorods/nanowires, two-dimensional ZnO nanosheets, stretchable micro-patterned P(VDF-TrFE) polymer, ZnSnO₃ nanocubes-based piezoelectric NGs along with graphene and hydrophobic sponge structure-based triboelectric NGs, and their potential applications in powering portable electronics are summarized and presented. Finally, the power generation under different modes of pressure/friction such as vertical compressive, bending, contact-separation and stretching are collected and the involved future challenges are discussed in terms of device configuration and efficiency.