This paper reports an equivalent circuit model of self-assembled electret (SAE) micro-electro-mechanical systems (MEMS) vibrational energy harvesters (VEHs). For the first time, a detailed equivalent circuit model of the SAE-MEMS VEH has been implemented on a circuit simulator, considering the device energy diagram. The implemented equivalent circuit model allows analysis of the relationship between output current and SAE properties, device structures, and electrode materials. The validity of the model is confirmed by comparing the measurement and simulation results. Thus, the proposed model would be useful for the design of SAE-MEMS VEHs. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

This paper reports the first direct measurement results of the surface potential of micro-patterned self-assembled electrets (SAEs) for microelectromechanical systems vibrational energy harvester applications. We have developed an evaluation chip that utilizes a removable shadow mask with through-holes to realize SAE micro-patterning during SAE evaporation. A Kelvin probe is used to measure the surface potential of the micro-patterned SAEs. The surface potential of the micro-patterned SAEs with the average thickness of 65 nm was found to be 2099 mV, which is in the same order of magnitude as the surface potential estimated from the average thickness of the SAEs.

Electret-based vibrational energy harvesters (VEHs) have attracted much attention as power sources for wireless sensors. Generally, a charging process is needed to make electret from dielectric materials. By utilizing spontaneous orientation polarization phenomena, recently, we developed self-assembled electret-based VEHs (SAE-VEHs), which do not require any charging process during preparation. A detailed understanding of the operation mechanism in SAE-VEHs is highly required to improve the device's performance. In this study, we propose a model for SAE-VEHs and discuss the relationship between the surface potential of SAE and the output current of SAE-VEH.

In vacuum-deposited film composed of polar OLED materials such as Alq3 and TPBi, the permanent dipole moment of the molecules spontaneously orders surface normal on average, resulting in the formation of polarization charge on the film surface and reverse sides. These polarized charges attract charge carriers of the opposite sign because of the Coulomb interaction: They are an essential factor determining the dynamic behavior of charge carriers in the device, such as injection from the electrode to organic material and accumulation at the heterointerface. In this talk, we will mainly discuss the role of the polarization charge in OLEDs.

This paper reports the first direct measurement results of the surface potential of micro-patterned self-assembled electrets (SAEs) for microelectromechanical systems vibrational energy harvester applications. We have developed an evaluation chip that utilizes a removable shadow mask with through-holes to realize SAE micro-patterning during SAE evaporation. A Kelvin probe is used to measure the surface potential of the micro-patterned SAEs. The surface potential of the micro-patterned SAEs with the average thickness of 65 nm was found to be 2099 mV, which is in the same order of magnitude as the surface potential estimated from the average thickness of the SAEs.

Electret-based vibrational energy generators (E-VEGs) have been extensively studied as a next-generation power source to replace conventional batteries [1]. The E-VEGs generally have a capacitor structure in which the movable and fixed electrodes sandwich air-gap and electret (Fig. 1(a)). Here, an electret is a dielectric material with a quasipermanent electrical charge or dipolar polarization, and inserted to induce charges on the device's electrodes. Charging process, such as corona discharge, is required to make conventional electret and is one a factor that limits the productivity of E-VEGs. Recently, we demonstrated that polar molecules for organic light-emitting diodes (OLEDs), such as tris-(8-hydroxyquinolinato)aluminium (Alq3) or 1,3,5-tris(1-phenyl-1H-benzimidazol-2yl)benzene (TPBi), can be used as the electret for E-VEGs. Very interestingly, surface potential (VVs) linearly increases with the film thickness and reaches several volts at 100 nm due to spontaneous orientation of the permanent dipole moment (Fig. 1(b)). Using this peculiar property, we realized E-VEG does not require any charging process, suggesting that Alq3 and TPBi can be regarded as selfassembled electret (SAE) [2]. The application of SAE for VEG is promising; however, the VVs of the film decreases under light illumination. As shown in the left figure of Fig. 1(c), excitons are generated due to light absorption and separated by the electric field inside the film. Then generated electrons and holes compensate the polarization charges, resulting in dissipation of VVs (right figure of Fig. 1(c)). Thus, to enhance the VVs stability against light irradiation, application of wider energy gap SAEs is beneficial. In fact, the time for 10% decay of VVs in TPBi is approximately 50 times longer than that in Alq3 [2]. Nevertheless, the lifetime is still not enough for practical realization. For further improvement of photostability of VVs in SAE, it is essential to manage exciton behavior precisely. It can be expected that the exciton quenching leads to a prolonged lifetime because of the suppression of photo-carrier generation, as shown in Fig. 1(d). In this talk, we discuss the effect of dye doping on the stability of SAE-based VEG.

Electret-based vibrational energy generators (E-VEGs) are of particular interest since they can generate electrical power from ambient vibration. A challenge is that the charging process is required for making the electret from a dielectric material, and the process is one of the main factors that determine the manufacturing costs of the device. Recently, by utilizing the spontaneous orientation of polar molecules, which have been used for organic light -emitting diodes, we realized a novel E-VEG that does not require any charging process. In this paper, the operation mechanism and stability of the device are discussed. We believe that the E- VEG using spontaneous orientation enables us to reduce the costs, leading to the device's mass commercialization.

Electret-based microelectromechanical system (MEMS) vibratory energy harvesting is a key technology for converting the mechanical energy of environmental vibrations into electricity. Unfortunately, conventional electret charging methods generally rely on high-voltage and high-temperature processes that present limitations to MEMS design and production. Here, we show a MEMS post-processed self-assembled electret (SAE) that enables the integration of electrets with MEMS vibratory devices via evaporation as a post-MEMS process. Owing to the spontaneous orientation of polar molecules, the surface potential of the SAE can build up at room temperature in a microscopic region without charging processes, which enhances the design and fabrication flexibility of electret-based MEMS energy harvesters. We develop a MEMS vibratory device followed by post-processing the SAE and confirm induced electrical currents caused by the electrical field of the SAE at the vibrational input. This SAE-based MEMS technology is a promising design guideline for highly integrated single-chip MEMS vibratory energy harvesters.