the vibration-based electret generators (eGs) for energy harvesting have been extensively studied because they can obtain electrical energy from ambient vibrations. eGs exhibit a sandwich structure of electrodes surrounding an air gap and an electret, which is a dielectric material with a quasi-permanent electrical charge or dipole polarisation. Various charging processes have been developed because the surface charge density (σ) of the electret determines the output power of the device. However, such processes are considered to constitute a key productivity-limiting factor from the mass production viewpoint, making their simplification or elimination a highly desired objective. Herein, a model EG that does not require any charging process by utilising the spontaneous orientation polarisation of 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi) is demonstrated. The surface potential (V sp ) of an evaporated TPBi film has reached 30.2 V at a film thickness of 500 nm without using a charging process. the estimated σ of 1.7 mC m −2 is comparable with that obtained using a conventional polymerbased electret after charging. Furthermore, V sp is considerably stable in environmental conditions; thus, tpBi can be considered to be "self-assembled" electret (SAe). Application of SAe leads to developing an EG without requiring the charging process.Sensor nodes have become indispensable for obtaining an extensive spectrum of information related to factors such as personal health, human and animal locations and the condition of the natural and built environments, including buildings, bridges and tunnels. However, the problem associated with powering the wireless sensor networks has to be solved for maintaining a society permeated by such devices. Recently, energy harvesting from ambient sources, such as heat, electrical waves, light and vibrations, has attracted considerable attention as a substitute for batteries, which exhibit various problems in terms of, for instance, the necessity for regular replacement and their contribution to toxic waste. Among the ambient-source devices, vibration harvesters are considered to be favourable owing to the presence of vibration in several environments, enabling a potentially extensive range of applications 1 .The vibration-based electret generators (EGs) are of particular interest for energy harvesting because they can provide relatively high output voltages even at low vibration frequencies ranging from few to several tens of hertz without the usage of any external bias source 1-4 . EGs typically exhibit a capacitor structure in which an electret and air gap are sandwiched between the top and bottom electrodes, as depicted in Fig. 1(a). An electret is a dielectric material with a quasi-permanent electrical charge or dipole polarisation 5 . These are indispensable in EGs because charges are induced by the electric field of the electret, eliminating the external bias for charging the capacitor 3,6,7 . An AC current is generated by the variance in air gap due to the out-of-plane (O...

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Electret generators (EGs) for energy harvesting is a device which ambient vibrations convert into electric energy. They are expected as power supplies for low energy applications such as wireless sensors. In EGs, charges are induced by an electric field of electret, and thus output power is determined by a surface charge density (σ) of the electret. Various charging processes were proposed so far to realize the electret with high σ, however, they were one of problems decreasing productivity. The elimination of the processes is extremely desired. In this study, we developed charged film composed of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi) without any charging process. A surface potential (SP) exceeded 46 V for ca. 740 nm thick film due to spontaneous orientation of TPBi, and was sufficiently stable under room light illumination in atmosphere. Finally, we demonstrated EGs utilizing TPBi as electret generated an AC current of the order of nano-ampere in environmental condition, indicating the feasibility of EGs using TPBi film in practical atmosphere. The series of results strongly suggested that application of spontaneous orientation of polar molecules was promising for the realization of EGs with high productivity.

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.

Electret-based vibrational energy generators (E-VEGs) have attracted much attention because they can generate electrical power from ambient vibration. The electret is a key material for the E-VEG because the output power of the device is proportional to the square of the surface charge density of the electret. A challenge is that charging process is indispensable for making the electret from a dielectric material, which involves a factor that limits device productivity.To solve this problem, we developed a novel E-VEG that does not require any charging process by utilizing spontaneous orientation of polar organic molecules which have been widely used for organic light-emitting diodes. In this paper, the property and stability of the device are introduced. We believe that the application of polar molecules opens up a new pathway for the development of electret-based devices such as VEGs, sensors, and microphones, etc.