Yasuko Koshiba

Crystal growth of rubrene in ionic liquids by vacuum vapor deposition

The crystal growth and unique morphological changes of organic semiconductor rubrene, fabricated by ionic liquid (IL)-assisted vacuum vapor deposition, were investigated. The texture and structure of rubrene films strongly depended on the thickness of IL films on substrates, namely, three-dimensional dendrites in 3D-ILs, two-dimensional microfibrils in 2D-ILs, and two-dimensional spherulites in 0D-ILs. The growth of two-dimensional spherulites would be promoted by the supersaturation of ILs, minutely controlling the rate and total amount of rubrene deposition. The growth mechanisms of rubrene fabricated by IL-assisted vacuum vapor deposition were different from that of the conventional one in terms of the nucleation and growth modes.

Enhanced thermoelectric power of single-wall carbon nanotube film blended with ionic liquid

We have investigated the thermoelectric power of single-wall carbon nanotubes (SWCNTs) with an ionic liquid (IL). The SWCNT/IL films showed simultaneous increase in electrical conductivity and the Seebeck coefficient compared with the pristine SWCNT. No thermoelectric power was observed for the IL. The X-ray diffraction pattern and impedance diagram showed a unique behavior with the concentration of IL, which implies that the interaction between the SWCNTs and IL enhances the thermoelectric power of the SWCNTs. As a result of the simultaneous increase in these parameters, the power factor exhibited a 10-fold increase.

Polarity tuning of single-walled carbon nanotube by dipole field of ferroelectric polymer for thermoelectric conversion

The tuning of the Seebeck coefficient of a single-walled carbon nanotube (SWCNT) film was achieved by using the dipole field of a ferroelectric polymer. The Seebeck coefficient was positive under an up-poling dipole field, but negative under a down-poling dipole field, whereas the control remained positive. This tunable behavior can be explained by selective carrier injection and accumulation, which was confirmed by the temperature dependence of electrical conductivity. Connecting p- and n-type SWCNT films tuned by dipole fields to create a π module resulted in a significant improvement in output voltage owing to the temperature difference between the two.

Highly stable n-type thermoelectric materials fabricated via electron doping into inkjet-printed carbon nanotubes using oxygen-abundant simple polymers

Single-walled carbon nanotubes (SWCNTs) are important candidates for flexible and non-toxic thermoelectric (TE) energy-harvesting devices because they have large Seebeck coefficients, good flexibility, and inkjet printability onto plastic substrates. Here we describe the successful n-type conversion of intrinsic p-type SWCNTs by polymer–dopant charge transfer. The negative Seebeck coefficients of the polymer-doped SWCNTs were strongly related to the highest occupied molecular orbital levels of the polymer, demonstrating that the polymers were electron donors for the nanotubes and that the doping level could be controlled by modifying the functional groups. The n-type SWCNTs obtained using oxygen-abundant polymers, such as poly(vinyl alcohol) and poly(vinyl acetate), exhibited the largest negative Seebeck coefficients and high stability under ambient conditions lasting for at least 3 weeks. Printed and folded p- and n-type SWCNTs on flexible substrates showed efficient TE voltage improvements. Our findings enable the easy, low-cost preparation of air-stable n-type SWCNTs, permitting the exploitation of SWCNTs as flexible and eco-friendly TE materials.

Polyurea spin-coated thin films: Pyro- and piezoelectric properties and application to infrared sensors

We have investigated the pyro- and piezoelectric properties of polyurea spin-coated films with thermal and chemical stability and their performance as infrared sensors. The piezoelectric coefficient d33 was measured by a laser Doppler vibrometer to be 23.5 pC/N. This coefficient increased with the poling electric field up to around 75 MV/m, suggesting that the polyurea dipole is aligned by applying an electric field greater than 75 MV/m. When a triangular thermal wave was applied, a square-wave pyroelectric current was observed and the pyroelectric coefficient measured to be 5.11 µC/(m2K). The infrared sensor performance of the polyurea thin film was examined by measuring the voltage sensitivity to infrared irradiation. The obtained result of 70.4 V/W at 1 Hz is consistent with the results calculated from the measured pyroelectric coefficient. Our findings suggest that the performance of the sensor may be improved by increasing the pyroelectric coefficient of the polyurea films.

Molecular Rearrangement and Optical Property Changes of p-Sexiphenyl Vacuum-Deposited Film Induced by Rubbing

Structural and optical property changes of p-sexiphenyl (6P) films induced by rubbing were investigated. 6P molecules vapor-deposited on a fused silica glass adsorbed obliquely but had no in-plane orientation. After rubbing, the film surface became smooth and the electron microscopic analysis revealed that the longitudinal direction of the 6P molecules is parallel to the substrate surface and oriented uniaxially along the rubbing direction. The absorption spectrum changed corresponding to the orientation change of the 6P molecules. The 6P molecules grew homoepitaxially on the rubbed film, and the film exhibited a high anisotropic optical property (dichroic ratio: 9.6) when deposition was performed at a low substrate temperature (RT) and a high deposition rate (5 nm/min).

Piezoelectric vibration energy harvesters with stretched and multistacked organic ferroelectric films

We investigated piezoelectric vibration energy harvesters with poly(vinylidene fluoride/trifluoroethylene) films and the improved power generation from using multistacked and stretched ferroelectric films on the cantilevers. The energy harvesters generated electric power with a resonant frequency of approximately 25 Hz, which corresponded to the ambient vibration. The power density of four-layered harvesters was estimated to be 2.5 µW/m3, which was larger than the power density of previous harvesters. The output power of stretched-film harvesters was 3.6 times the output obtained from unstretched films. In addition, because organic ferroelectric films are flexible, the resonant frequency of each harvester was practically constant even when using the techniques of multistacking and stretching.

Thermal stability of piezoelectric properties and infrared sensor performance of spin-coated polyurea thin films

We have investigated the temperature dependence of the piezoelectric coefficients and infrared sensor performance of spin-coated thin films of polyundecylurea (PUA11). The piezoelectric coefficients of the PUA11 films remained constant at temperatures above 180 °C and these films demonstrated thermal resistance superior to those of poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)] films. The infrared sensor performance of the PUA11 films was measured after annealing at 125 °C for 500 h and was found to have retained 84% of its preannealing level. The thermal stability of the PUA11 films was higher than that of the P(VDF/TrFE) films; moreover, PUA11 is also expected to have superior electrothermal stability.

Current-voltage characteristics of organic photovoltaic cells following deposition of cathode electrode

The current-voltage characteristics of benzoporphine-fullerene solar cells were measured subsequent to the deposition of Al as a cathode material. Even in vacuum, a shift in the open circuit voltage was observed at 20 min after Al deposition. Moreover, the displacement of inert gases (N(2)or Ar) in the evaporation chamber enhanced the photovoltaic parameters. The power conversion efficiency was increased by 24% over the initial characteristics (from 1.04% to 1.29%), which indicates that the structure of the organic-metal interface changed rapidly after Al deposition, even if the process was performed in an air-free glovebox.

Vibration energy harvester with piezoelectric properties using polyurea thin films

ABSTRACT We investigated the piezoelectric properties of a vibration energy harvester using large-area, flexible cantilevers composed of polyurea thin films. The piezoelectric constant d31 of the polyurea films was measured based on the relationship between the displacement and applied voltage of large-aspect-ratio cantilevers. Centimeter-scale unimorph cantilevers were fabricated, and their energy harvester performance was evaluated by vibrating them. The power generation of the cantilevers with controlled forms was 2.2 mV at 50.5 Hz (i.e., vibration generation could be efficiently realized at environmental vibrations under 100 Hz).