Mitsumasa Iwamoto

The entropy change causing dipolar depolarization is proposed as an origin of electrical power generation in triboelectric generators using polar materials. Rubbing mechanically forces permanent dipoles in materials to become orientationally ordered, establishing a low entropy state with an initial dipolar polarization P0. In this state, the electric field within the materials is zero, but a nonzero Helmholtz free energy F=F0=P022(Cs−C∞) arises from the entropy contribution (Cs, static capacitance; C∞, capacitance at high frequency). Consequently, this state is energetically unstable and undergoes a spontaneous transition into a disordered high entropy state with F<F0, resulting in the establishment of a non-zero electric field. Through this process, the energy bounded in materials as the entropy component of free energy is converted, and the electrical energy becomes F0η with η=1−C∞Cs(0<η<1). Electrical circuit analysis shows that this energy conversion process can be well represented by introducing a virtual resistance Rp=τCs−C∞ (τ, dipolar relaxation time). This suggests that the entropy change in finite time τ serves as the origin of electrical power generators. Under the matching condition τ=CsR, the power spectrum at the load is best aligned with that at the generator, maximizing the power transmission. The results presented here serve as a basis for understanding the principle of electric power transmission in triboelectric generators through dipolar depolarization.

Glycolipids are components of cellular membranes comprising a hydrophobic lipid tail and one or more hydrophilic sugar heads, and are widely associated with the fields of life science and biochemistry. Due to the hygroscopic nature of sugar, the dry thermotropic phases of glycolipids have fewer studies. We report on the electric charge generation in anhydrous glucosides excited by the external high electric field (∼2 MV/m). This causes a large current in the smectic A phase, but not in the isotropic phase. Its intensity is about 100 times larger than the steady state current. The generation of the current was found to be irreversible by the repetition of the field application. The large electric carrier generation is originated in the smectic A phase, possibly due to an electron avalanche breakdown mechanism caused by the collisions of electrons through the impact ionization.

We investigated the I-V hysteresis of tin perovskite solar cells using capacitance-voltage (C-V) measurement coupled with charge modulation spectroscopy (CMS). The C-V characteristics and CMS of the solar cell device (ITO/PEDOT:PSS/CH₃NH₃SnI₃ (MASnI₃)/C₆₀/Al) showed that electrons inject from a C₆₀ layer into an MASnI₃ layer at positive bias voltage, while the injected electrons return to a C₆₀ layer at negative bias voltage. Results suggested that I-V hysteresis of tin perovskite solar cells is originated from the carrier injection into an MASnI₃ layer. This study provides a method for evaluating the carrier motion in tin perovskite solar cells.

Organic photoconductors (OPC) have been an important research and development topics for high quality electrophotography. By using electric field induced optical second harmonic generation (EFISHG) measurement, we can probe carrier processes in electrophotographic processes such as photo carrier generation, carrier separation, and carrier transportation for copier image production. We here selectively probe charge generation and accumulation in charge generation layer and charge transport layer in multilayer structure OPCs. We studied charge accumulation in OPC under illumination (wavelength 635nm) of double-layer-type OPC with structure of hole transport layer coated on charge generation layer. The result showed that light absorption efficiently produces free holes and electrons in the charge generation layer, followed by excessive hole accumulation at the CG/CT interface due to photo-conducting effect of CG layer. The short-wavelength irradiation at 405nm induced photovoltaic effect. These results demonstrated that the EFISHG measurement is useful to selectively probe carrier process in one layer of the multilayer OPC and to the discussion of carrier process for electrophotographic image productions.

We analyzed the I-V hysteresis behaviors of tin perovskite (MASnI₃, MA: CH₃NH₃) thin films using impedance spectroscopy coupled with charge modulation spectroscopy (CMS). The capacitance-voltage (C-V) characteristics of the ITO/MASnI₃/Al device showed hysteresis behaviors, in accordance with the trap filling process suggested by the I-V characteristics. The CMS measurement indicated the enlargement of the energy bandgap of the MASnI₃. On the basis of these results, we proposed a model that trap states in the vicinity of the bottom of conduction band are filled, and concluded that the trap filling process occurring at Sn vacancies makes a significant contribution to the electrical properties of tin perovskite film and the hysteresis of the I-V and C-V characteristics of our device. The I-V hysteresis is suppressed with the decrease of defects such as Sn vacancies in MASnI₃ films.

Thermally stimulated current (TSC) measurement is widely used in a variety of research fields, i.e., physics, electronics, electrical engineering, chemistry, ceramics, and biology. TSC is short-circuit current that flows owing to the displacement of charges in samples during heating. TSC measurement is very simple, but TSC curves give very important information on charge behaviors. In the 1970s, TSC measurement contributed greatly to the development of electrical insulation engineering, semiconductor device technology, and so forth. Accordingly, the TSC experimental technique and its analytical method advanced. Over the past decades, many new molecules and advanced functional materials have been discovered and developed. Along with this, TSC measurement has attracted much attention in industries and academic laboratories as a way of characterizing newly discovered materials and devices. In this review, we report the latest research trend in the TSC method for the development of materials and devices in Japan.

Trapped charges underneath the field-plate (FP) in a p-gallium nitride (GaN) gate AlGaN/ GaN high electron mobility transistor device were visualized by using electric field-induced optical second-harmonic generation imaging. Second-harmonic (SH) signals in the off-state of the device with FP indicated that the electric field decreased at the p-GaN gate edge and concentrated at the FP edge. Nevertheless, SH signals originating from trapped charges were slightly observed at the p-GaN gate edge and were not observed at the FP edge in the on-state. Compared with the device without FP, reduction of trapped charges at the p-GaN gate edge of the device with FP is attributed to attenuation of the electric field with the aid of the FP. Negligible trapped charges at the FP edge is owing to lower trap density of the SiO₂/AlGaN interface at the FP edge compared with that of the SiO₂/p-GaN sidewall interface at the p-GaN gate edge and attenuated electric field by the thickness of the SiO₂ passivation layer on the AlGaN surface.