Energy-harvesting discovery generates 200 times higher voltage to power wearables, other portable devices

January 13, 2015

Left: conventional zinc-oxide energy-harvester chip (gray: top electrode; gold: zinc oxide layer; blue: bottom electrode; lime: substrate). Right: aluminum nitride insulating interlayer (purple) added, boosting voltage up to 200 times and improving other characteristics. (Credit: Eunju Lee et al./Applied Physics Letters, adapted)

Korea Advanced Institute of Science and Technology (KAIST) researchers have discovered how to radically improve conversion of ambient energy (such as body movement) to electrical energy for powering wearable and portable devices.

As has been noted on KurzweilAI, energy-harvesting devices can convert ambient mechanical energy sources — including body movement, sound, and other forms of vibration — into electricity. The energy-harvesting devices or “nanogenerators” typically use piezoelectric materials such as zinc oxide* (ZnO) to convert mechanical energy to electricity. Uses of such devices include wearables and devices for portable communication, healthcare monitoring, environmental monitoring; and for medical implants.

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).

The researchers report their findings in an open-access paper published today (Jan. 13) in the journal Applied Physics Letters.

*According to Giwan Yoon, a professor in the Department of Electrical Engineering at KAIST, ZnO nanostructures are particularly suitable as nanogenerator functional elements because of their “transparency, lead-free biocompatibility, nanostructural formability, chemical stability, and coupled piezoelectric and semiconductor properties.” They are also easily fabricated, can be fully integrated with conventional complementary metal-oxide-semiconductor (CMOS) fabrication technologies, and provide greater durability than other types of nanogenerators, the authors explain.

Abstract for Characteristics of piezoelectric ZnO/AlN—stacked flexible nanogenerators for energy harvesting applications

Flexible piezoelectric zinc oxide (ZnO)-based nanogenerators (NGs) using an aluminum nitride (AlN) interlayer are proposed for high-efficiency energy harvesting applications. The effects of the AlN interlayer on device performance are studied. Use of the AlN interlayer in ZnO-based vertically integrated NGs (VINGs) results in a significant improvement in terms of the magnitude of the output voltages of up to 200 times when compared with a ZnO-based VING without any AlN interlayer. The improved device energy conversion efficiency is mainly attributed to a high contact potential barrier that the AlN interlayer provides in VINGs, along with the relatively high dielectric constant and large Young’s modulus of the AlN material. In addition, the effects of AlN thickness on the electric potential and device performance of the VINGs are investigated through observation of the output voltages of ZnO-based VINGs with thickness/position-controlled AlN interlayers. Our findings in this work are expected to provide effective and useful approaches for realizing highly energy-efficient ZnO-based NGs and their extended applications, including self-power sources and sensor devices.