Shohei Horike

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.

Fabrication and characterization of elastomeric semiconductive thiophene polymers by peroxide crosslinking

AbstractIn this study, semiconductive elastomers composed of homopolythiophene with disiloxane moieties were developed. The crosslinked molecular structure in the polythiophene elastomers was introduced by dicumyl peroxide, one of the typical peroxide crosslinking reagents. The elastomers were produced through a hot-pressing process above the melting point of the polythiophene. Stress–strain curves that included tensile tests and cycle loading–unloading tests defined the crosslinked polythiophenes as elastomers. Their electrical conductivities were evaluated by two-point measurements under nondeformation and uniaxial deformation states. The results indicated that the concentration of crosslinking reagents greatly influenced the mechanical and electrical properties of crosslinking polymers. With the addition of the crosslinking reagent in concentrations from 2.5 phr to 10.0 phr, elongation at break decreased largely from 95% to 51%, while excellent elastic recoveries were observed. In the electrical resistivity measurements, all the crosslinking polymers possessed high stability of electrical properties against elongation.We crosslinked polythiophenes with disiloxane groups in its side chains by dicumyl peroxide. The crosslinked polythiophenes maintained low glass transition temperatures and low Young’s modulus, while they emerged high elastic recoveries. Moreover, as the concentration of dicumyl peroxide increased, the crosslinked polythiophenes possessed higher elastic recoveries. We obtained completely elastic semiconductive polythiophenes.

Orientation Dependence of Power Generation on Piezoelectric Energy Harvesting Using Stretched Ferroelectric Polymer Films

The piezoelectric vibration energy harvesters were fabricated by using uniaxially stretched poly (vinylidene difluoride/trifluoroethylene) copolymer (P(VDF/TrFE)) film, and the relationship between piezoelectric power generation and molecular orientation was investigated. The molecular orientation in the stretched P(VDF/TrFE) films was evaluated with polarized Fourier transfer infrared (FT-IR) spectra measurement. In stretched films, the main-chains of P(VDF/TrFE) were aligned along the stretching direction. The piezoelectric properties and the electric power generation of stretched P(VDF/TrFE) films were strongly depended on their molecular orientation, measuring by cantilever-type energy harvesters. The piezoelectric coefficient(e) and output power observed in the energy harvester with the film stretched in the longitudinal direction of cantilever were 16.9 mC/m2 and 222 nW, respectively. These values were approximately 2.1 and 3.5 times these of the unstretched elements.

Field-effect and chemical charge-type modulations of carbon nanotubes using functional polymers for thermoelectric energy harvesters

As devices continue to shrink in size and grow in abundance, power supply methods using clean and abundant materials become increasingly important. Single-walled carbon nanotubes (SWCNTs) are promising materials as the thermoelectric (TE) energy-harvesting candidate due to their flexibility, nontoxicity, and relatively large TE power factors. This paper reviews our current studies on the charge-type conversion of SWCNTs for readily pairing several p- and n-type SWCNTs to improve the TE voltage output. First, we describe the polarity tuning of SWCNTs by electric dipoles of ferroelectric polymer. The sign of Seebeck coefficient of SWCNTs could be tuned by the direction of the electric dipoles adjacent to nanotubes in the ferroelectric-gate field-effect transistor-like device configuration. Second, we demonstrate the electron doping into SWCNTs by several versatile polymer-dopant charge transfer. The obtained negative Seebeck coefficients of SWCNTs were correlated to the highest occupied molecular orbital energy levels of the dopant polymers. Our findings enables the direct use of low-temperature waste heats around dwelling environments using eco-friendly materials and techniques.

Improving the light-emitting properties of single-layered polyfluorene light-emitting devices by simple ionic liquid blending

This paper describes an evaluation of ionic liquids (ILs) as potential electrolytes for single-layered light-emitting devices with good emission performance. As optoelectronic devices continue to grow in abundance, high-performance light-emitting devices with a single emission layer are becoming increasingly important for low-cost production. We show that a simple technique of osmosing IL into the polymer layer can result in high luminous efficiency and good response times of single-layered light-emitting polymers, even without the additional stacking of charge carrier injection and transport layers. The IL contributions to the light-emission of the polymer are discussed from the perspectives of energy diagrams and of the electric double layers on the electrodes. Our findings enable a faster, cheaper, and lower-in-waste production of light-emitting devices.