Charge-generating mode control in high-performance transparent flexible piezoelectric nanogenerators

In this work, we demonstrate the mode transition of charge generation between direct-current (DC) and alternating-current (AC) from transparent flexible (TF) piezoelectric nanogenerators (NGs), which is dependent solely on the morphology of zinc oxide (ZnO) nanorods without any use of an AC/DC converter. Tilted ZnO nanorods grown on a relatively low-density seed layer generate DC-type piezoelectric charges under a pushing load, whereas vertically aligned ZnO nanorods on a relatively high-density seed layer create AC-type charge generation. The mechanism for the geometry-induced mode transition is proposed and characterized. We also examine the output performance of TF-NGs which employ an indium zinc tin oxide (IZTO) film as a TF electrode. It is demonstrated that an IZTO film has improved electrical, optical, and mechanical properties, in comparison with an indium tin oxide (ITO) film. Enhanced output charge generation is observed from IZTO-based TF-NGs when TF-NGs composed of only ITO electrodes are compared. This is attributed to the higher Schottky barrier and the lower series resistance of the IZTO-based TF-NGs. Thus, by using IZTO, we can expect TF-NGs with superior mechanical durability and power generating performance.