Please check out the following podcast, made by ‘Power System Design’ where their Alix Paultre interviews Electroscience’s Prof Markys Cain, about the European Union research project ‘Nanostrain’.
From the MRS Spring 2015 meeting in San Fransisco, Electroscience’s Professor Markys Cain gave an invited presentation on “Zinc Oxide Nanorod P-N Junction-Based Piezoelectric Energy Generators: Metrological Considerations of Origin of Charge Generation”.
Self-powered piezoelectric systems are vital components to harvest ambient waste energy for applications such as autonomous self-powered sensors. ZnO nanorod-based devices are gaining wide attention for energy harvesters as they are easily synthesized at low temperature onto a range of substrates – including flexible ones. However, losses related to screening of piezoelectric polarisation charges by free carriers in ZnO nanorods can significantly reduce the output of these devices. The surface chemisorbed and physisorbed species on ZnO reduces the piezoelectric voltage generation and reliability by increasing the carrier concentration and therefore the internal screening. In this presentation we discuss approaches to reduce this internal screening through surface passivation of the ZnO nanorods. The electrical field which can be delivered using a strained ZnO nanorod energy harvester is related to the rate at which the depolarising field is set up and the rate at which piezoelectric polarisation charges are separated in the material. This balance between charge leakage and stored charge is related to the surface and bulk conductivities of the materials system and we show how this may be both modelled and optimised. Though there is still some controversy in the community concerning the origins of the charge measured in many ZnO systems and by many labs, in our work, we demonstrate that the effect is very likely to be piezoelectric in nature. Controlled vibration testing of the devices provides strong evidence that the effect results from a piezoelectric response in the material. The generated voltage increases linearly with the pressure applied to the sample, which is expected for a piezoelectric effect. When the devices are illuminated the voltage output drops significantly. This is attributed to the photo-induced conductivity of the ZnO, which is known to reduce the piezoelectric coefficient due to screening by conduction electrons. The input force to the hybrid device is measured using controlled bending. By simultaneously measuring the output power of the devices the energy conversion efficiency is calculated to be 0.0067 % with slow bending, increasing exponentially with bending rate up to ~8.5 % with much faster bending. This increase agrees with our proposed model of screening-limited energy output. These results not only demonstrate an alternative approach to the design of a ZnO nanorod energy harvesting design, but also contribute to the understanding of the factors that limit the device performance. Finally, we summarise some of the issues surrounding the importance of how measurements are made in these nanosystem energy harvesters.