Tatsuya Fukushima

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.

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.

Nanorod growth of copper phthalocyanine on fluorinated phosphonic acid SAM-modified indium tin oxide substrate for organic photovoltaic devices

ABSTRACT CuPc nanorod growth was investigated on ITO substrates with and without 1H,1H,2H,2H-perfluoro-noctylphosphonic acid (FOPA) SAM modification, using vapor phase deposition, through the control of the temperature and surface energy of ITO substrates. At a substrate temperature of 200°C, copper phthalocyanine (CuPc) thin films that composed nanorods were prepared on both substrates. On FOPA-ITO, it was suggested that CuPc molecules were stacked with an edge-on orientation because of the dominant intermolecular π–π interaction through the decrease of surface energy of ITO with FOPA modification. Organic photovoltaic cells containing CuPc nanorods as a p-type semiconductor were fabricated and photovoltaic characteristics were observed.