Demonstration of an electret generator for energy harvesting without any charging process: Utilization of spontaneous orientation of polar molecules

Vibration-driven electret generators (EGs) for energy harvesting have been intensively studied since they are expected to be used as electric source, especially, for low power consumption devices. EGs have generally capacitor structure consisting of air gap and electret which is a dielectric material with a quasi-permanent electrical charge or dipole polarization. Because charges are induced on top and bottom electrodes by the electret, higher surface charge density (σ) is needed to achieve sufficient output power in EGs. Various techniques have been proposed to make electret with high σ; however, they are one of the factors reducing the productivity. Thus the elimination of charging process is crucially required. In this study, we develop electret film without any charging process by utilizing spontaneous orientation polarization of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi). A surface potential of TPBi vacuum evaporated film exceeded 46 V at 739 nm, and σ was estimated to be 1.8 mC/m². In addition, the σ was quite stable in dark and vacuum condition. By taking the advantages of TPBi film, we developed a model EG without any charging process. An AC current in nano-ampere order appeared due to the vibration, suggesting the power generation in EG without charging process. (paper)