Masakazu Mukaida

Morphological Change and Mobility Enhancement in PEDOT:PSS by Adding Co-solvents

Adding ethylene glycol (EG) to a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution improves the crystallinity of the PEDOT and the ordering of the PEDOT nanocrystals in solid films. The carrier-mobility enhancement is confirmed by using ion-gel transistors combined with in situ UV-vis-NIR spectroscopy.

Electronic structures of semiconducting alkaline-earth metal silicides

Electronic structures and densities of states of the following alkaline-earth metal silicides have been calculated using the first-principle pseudopotential method, Mg/sub 2/Si, BaSi/sub 2/, Ca/sub 2/Si and Sr/sub 2/Si. Energetics of these in their equilibrium structures in comparison with other hypothetical structures (i.e. Ca/sub 2/Si-type Mg/sub 2/Si and Mg/sub 2/Si-type Ca/sub 2/Si) are also considered to clarify the structural change of alkaline-earth metal silicides with the promotion to the heavier elements. The semiconducting behaviors of these could be predicted though the energy band gaps calculated were about 40% of the actual measured values. Energetics of the non-stoichiometry and the atomic site of dopants for Mg/sub 2/Si are also discussed.

Recent Progress on PEDOT-Based Thermoelectric Materials

The thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials have attracted attention recently because of their remarkable electrical conductivity, power factor, and figure of merit. In this review, we summarize recent efforts toward improving the thermoelectric properties of PEDOT-based materials. We also discuss thermoelectric measurement techniques and several unsolved problems with the PEDOT system such as the effect of water absorption from the air and the anisotropic thermoelectric properties. In the last part, we describe our work on improving the power output of thermoelectric modules by using PEDOT, and we outline the potential applications of polymer thermoelectric generators.

Polymer thermoelectric modules screen-printed on paper

We report organic thermoelectric modules screen-printed on paper by using conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and silver paste. Our large-area devices provided sufficient power to illuminate light-emitting diodes. This is the first example of thermoelectric modules containing conducting polymers being used to power practical devices. The stability of this proof-of-concept module was tested at 100 °C for over 100 h without any encapsulation. We showed that the decrease in device performance was caused not by the deterioration of the materials but by degradation of the interface between the conducting polymers and silver paste. These results suggest that organic thermoelectric modules could be used to harvest heat energy at low temperature, although the stability of the interface must be improved.

Thermoelectric power enhancement of PEDOT:PSS in high-humidity conditions

We report an increase in the thermoelectric power factor of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) from 23 ± 5 to 225 ± 130 µW/(m·K2) in high-humidity conditions. This enhancement was caused mainly by an increase in the apparent Seebeck coefficient, which could be related to morphological change after water absorption or electrochemical reaction of PEDOT in air. Our results demonstrate a positive effect of water in the PEDOT:PSS system and indicate the need for well-controlled measurement conditions, particularly humidity, in evaluating the performance of conducting organic materials.

Comparison of density of states of transition metal disilicides and their related compounds systematically calculated by a first-principle pseudopotential method using plane-wave basis

Density of states of transition metal disilicides and their related compounds of the following types of structures have been calculated by a first-principle pseudopotential method using plane-wave basis, CASTEP; CaF2, α- and β-FeSi2, TiSi2, ZrSi2, CrSi2, MoSi2, Mn4Si7, Ru2Si3, and AlB2. The principle of “DOS at Fermi level would be hopefully smaller in energetically-favored structure” is valid in most of silicides except for IIIa elements. The broadening of the energy difference between the bonding and anti-bonding states by promotion to the heavier element in the same group in periodic table is dependent on the crystal structure and this may cause the transition of the structure in the same group. Semiconducting property of ReSi2 is probably due to the vacancies located at silicon sites because DOS at Fermi level has relatively large value for stoichiometric ReSi2. Also, it is pointed out that the formula of Mn11Si19 is not consistent with the intrinsic semiconducting nature.

Stoichiometry of tantalum oxide films prepared by KrF excimer laser-induced chemical vapor deposition

Abstract Tantalum oxide films have been prepared by photolysis of Ta(OCH 3 ) 5 vapor under KrF excimer laser irradiation at various laser fluences (50–450 J m −2 ). The composition and Stoichiometry of the films were determined by X-ray photoelectron spectroscopy (XPS). The variation of the shape of the Ta 4f XPS peak for surface erosion of samples by argon-ion etching indicated that films were deficient in oxygen atoms compared with the composition of the stoichiometric Ta 2 O 5 , though the X-ray diffraction pattern of films obtained at higher laser fluences (more than 300 J m −2 ) corresponded to that of the β-Ta 2 O 5 phase. The deficiency in oxygen increased with increasing laser fluence to reach the maximum value of about 20% in the laser fluence range 150 J m −2 to 250 J m −2 and then decreased. The carbon content in the films was less than the background contamination level (a few %) under the experimental conditions investigated. Various possible reasons for the laser fluence effect on the stoichiometry of films are discussed.

Screening of the possible boron-based n-type thermoelectric conversion materials on the basis of the calculated densities of states of metal borides and doped β-boron

Abstract In order to screen candidates for n-type boride-based thermoelectric materials, the energetics of solid solutions of metallic atoms (Zr, Cr, and V) in β-rhombohedral boron (β-boron) and the densities of states of metal-borides of the CrB-, FeB-, MoB-, AlB2-, ReB2-, CaB6-, and UB12- types of structures and some tetraborides (YB4, CrB4, WB4, and MgB4) have been calculated using a first-principle pseudopotential method within the local (spin) density approximation. It was properly concluded that Zr occupies the E site of β-boron while V and Cr occupy the A1 site. As for metal-monoborides and diborides, the ‘rigid band approach’ seems to be valid. The large negative Seebeck coefficient of FexCo1−xB is hopeful if Motts explanation for the trend of the Seebeck coefficient for transition metal elements is valid for these borides. Y- or La- doped Ca(Sr, and Ba)B6 can also be expected to be useful.

Calculation of electronic energy and density of state of iron-disilicides using a total-energy pseudopotential method, CASTEP

Abstract Electronic energies of α-, β-, and γ-FeSi 2 were calculated so as to elucidate the possibility of the prediction of phase stability by a quantum-mechanical calculation using a total-energy pseudopotential code, CASTEP. It was properly predicted that the β-phase is more stable than α- and γ-FeSi 2 . The effect of the non-stoichiometry of β-FeSi 2 and doping elements (Mn, Cr, Co, and Ni) on the Fermi energy was also discussed.

SiC thin film preparation by ArF excimer laser chemical vapor deposition Part 1 : Rate of photolysis of alkylsilanes by ArF excimer laser and their decomposition products

Abstract The photolysis rate of tetramethylsilane (TMS), hexamethyldisilane (HMDS), and trimethylsilane (TrMS) by ArF excimer laser was studied by measuring gas phase IR absorption spectra before and after the irradiation. Decomposition of TMS and HMDS obeyed first-order reaction kinetics and the reaction rate constants of both were proportional to 1.6 power of the laser fluence. Detected gaseous decomposition products by IR spectroscopy of TMS were TrMS and CH4, and those of HMDS were TMS, TrMS, and CH4. Decomposition rate constant of HMDS were two order of magnitude larger than that of TMS. TrMS, which has little absorption at the wavelength of ArF laser radiation, were also decomposed by ArF laser especially in case TMS coexists in the ambient and it was implied that sensitized reaction was involved in decomposition process of TrMS. SiC film was obtained by decomposition of HMDS.