Oxygen annealing induced enhancement in output characteristics of ZnO based flexible piezoelectric nanogenerators

Abstract

Piezoelectric nanogenerators (PENG), which convert random mechanical energy into electrical energy, have attracted considerable attention due to their application potential in self-powered micro and nano electronics devices, wearable electronics, sensors etc. PENGs based on ZnO nanostructures are particularly important due to their low cost, flexibility and bio-compatibility. Flexible PENGs based on ZnO nanostructures dispersed in Polydimethylsiloxane (PDMS) in ITO/ZnO:PDMS/ITO device configuration were fabricated and characterized for vibration energy harvesting applications. Surfactant free ZnO nanostructures were grown using co-precipitation route and ZnO:PDMS nano-composite films with ZnO content varying in the range of 1.5–7.5 wt% were deposited on ITO/PET substrates for PENG fabrication. The maximum open circuit voltage and short circuit current of ~ 2.5 V and ~ 4 µA respectively were obtained in PENG fabricated with 6 wt% ZnO. The output characteristics were further enhanced to 5 V and 6 µA with flowing oxygen ambient annealing of ZnO nanostructures generating an instantaneous power density of ~9.55 µWcm −2 . X-ray photoelectron spectroscopy studies confirmed reduction in oxygen vacancy and enhancement in oxygen interstitial concentrations on oxygen annealing which leads to the suppression of free carrier induced internal screening in ZnO nanostructures. This suppression of internal screening effect is the possible reason for the enhancement in output characteristics of ZnO:PDMS PENG fabricated with oxygen annealed ZnO nanostructures. The electrical energy generated by the ZnO:PDMS PENG was rectified and effectively stored in capacitors and used to power LEDs and LCD display. • ZnO nanostructures were grown by co-precipitation method with air and oxygen annealing. • ZnO:PDMS PENG with variable wt% of ZnO in PDMS were fabricated in ITO/ZnO:PDMS/ITO configuration. • The output performance of ZnO:PDMS PENG is improved significantly by annealing ZnO nanostructures in flowing oxygen ambient. • XPS established a correlation between the oxygen related point defects in ZnO and output characteristics of ZnO based PENG. • The output energy from ZnO:PDMS PENG was rectified and effectively stored in capacitor to flash LEDs and LCD display.