Piezoelectric materials such as ZnO, as well as several others, have the ability to convert mechanical energy to electrical energy, and vice versa. “ZnO nanostructures are particularly suitable as nanogenerator functional elements, thanks to their numerous virtues including transparency, lead-free biocompatibility, nanostructural formability, chemical stability, and coupled piezoelectric and semiconductor properties,” noted Yoon.
The key concept behind the group’s work? Flexible ZnO-based micro energy harvesting devices, aka “nanogenerators,” can essentially be comprised of piezoelectric ZnO nanorod or nanowire arrays sandwiched between two electrodes formed on the flexible substrates. In brief, the working mechanisms involved can be explained as a transient flow of electrons driven by the piezoelectric potential.
“When flexible devices can be easily mechanically deformed by various external excitations, strained ZnO nanorods or nanowires tend to generate polarized charges, which, in turn, generate piezoelectronic fields,” said Yoon. “This allows charges to accumulate on electrodes and it generates an external current flow, which leads to electronic signals. Either we can use the electrical output signals directly or store them in energy storage devices.”
The KAIST researchers proposed, for the first time, new piezoelectric ZnO/aluminum nitride (AlN) stacked layers for use in nanogenerators.
The researchers explored ways to improve “vertically integrated nanogenerator” energy-harvesting chips based on ZnO. They inserted an aluminum-nitride insulating layer into a conventional energy-harvesting chip based on ZnO and found that the added layer increased the output voltage a whopping 140 to 200 times (from 7 millivolts to 1 volt, in one configuration). This increase was the result of the high dielectric constant (increasing the electric field) and large Young’s modulus (stiffness).
This illustration shows stacked flexible nanogenerators (left), and a cross-sectional transmission electron microscopy image of the ZnO/AlN-stacked structure. The scale bar on the right represents 200 nm.
CREDIT: Giwan Yoon/Korea Advanced Institute of Science and Technology
In summary, researchers have studied performance improvement in flexible piezoelectric ZnO-based VINGs through the design and selection of insulating interlayer materials. In particular, an AlN insulating interlayer with a relatively high dielectric constant and a large Young’s modulus was newly adopted as an electron-blocking layer in the device design. It was also confirmed that the use of AlN thin interlayers in ZnO-based VINGs resulted in a major improvement in terms of output voltage magnitude of up to 200 times, when compared with a ZnO-based VING without an AlN interlayer. It is believed that the AlN interlayer can protect the piezoelectric potentials generated over ZnO NRs from being reduced by short-circuit or leakage currents across the interface from the electrode. In addition, the AlN thickness effects on the electric potential and the VING device performance were investigated by observing the output voltages of ZnO-based VINGs with thickness/position-controlled AlN interlayers. Our findings in this work are expected to provide effective approaches for realizing highly energy-efficient ZnO-based NGs and extended applications of NGs, such as self-power sources and sensor devices.
SOURCES – Applied Physics Letters, Korea Advanced Institute of Science and Technology (KAIST)