Masanori Matoba

From kesterite to stannite photovoltaics: Stability and band gaps of the Cu2(Zn,Fe)SnS4 alloy

Kesterite semiconductors, particularly Cu2ZnSnS4 (CZTS), have attracted attention for thin-film solar cells. We investigate the incorporation of Fe into CZTS to form the Cu2(Zn,Fe)SnS4 solid-solution for tuning the lattice spacing and band gap. First-principles calculations confirm a phase transition from kesterite (Zn-rich) to stannite (Fe-rich) at Fe/Zn ∼ 0.4. The exothermic enthalpy of mixing is consistent with the high solubility of Fe in the lattice. There is a linear band-gap bowing for each phase, which results in a blue-shift of photo-absorption for Fe-rich alloys due to the confinement of the conduction states. We propose compositions optimal for Si tandem cells.

Effective Ex-situ Fabrication of F-Doped SmFeAsO Wire for High Transport Critical Current Density

We demonstrate the fabrication of superconducting SmFeAsO1-xFx (Sm-1111) wires by using the ex-situ powder-in-tube technique. Sm-1111 powder and a binder composed of SmF3, samarium arsenide, and iron arsenide were used to synthesize the superconducting core. Although the F content of Sm-1111 is reduced in the process of ex-situ fabrication, the binder compensates by sufficiently supplementing the F content, thereby preventing a decrease in the superconducting transition temperature and a shrinkage of the superconducting volume fraction. Thus, in the superconducting Sm-1111 wire with the binder, the transport critical current density reaches the highest value of ~4 kA/cm2 at 4.2 K.

Thermal Expansion, Thermoelectric Power, and XPS Studyof the Nonmetal-Metal Transition in Ni1-xS1-ySey

Magnetic susceptibility χ, electrical resistivity ρ, thermoelectric power S , XPS spectra and hexagonal lattice parameters c and a are measured on the NiAs-type Ni 1- x S 1- y Se y ( x ≤0.04; y ≤1.00). We find the followings (1) the nonmetal-metal transition temperature T t , the phonon-drag coefficient C below T t and dχ( T > T t )/d T decrease and they are suppressed at x c and y c with increase in x and y , respectively. (2) The nearest neighboring S 1- y Se y (Z)-Z, Ni-Z and Ni-Ni distances just below T t ( D Z-Z N , D Ni-Z N , D Ni-Ni N , respectively) are depressed with increase in x . D Z-Z N and D Ni-Ni N only slightly depend on y , while D Ni-Z N increases with increasing y . (3) No change in the d 8 L peak ( L : hole in the Z network) in the valence band region is observed among Ni 1- x S 1- y Se y and Ni 0.93 Te. (4) The Ni 2p_3/2 d 9 L core level peak shifts to lower binding energy E B with increasing y , while it hardly changes with x . (5) The E B of S 2 s and S 2 p core levels decrease with...

Transport Properties and Electronic States in the Layered Thermoelectric Rhodate (Bi1-xPbx)1.8Ba2Rh1.9Oy

Resistivity, thermopower and Hall coefficient have been systematically measured for polycrystalline samples of (Bi 1- x Pb x ) 1.8 Ba 2 Rh 1.9 O y . An upturn of the resistivity and the Hall coefficient below 50 K is ascribed to decrease in the density of states due to a pseudogap formation. The thermopower and Hall coefficient are nearly identical to those of the thermoelectric oxide Bi 2 Sr 2 Co 2 O y . This suggests a nearly identical density of states between the two oxides, which is also verified from a first-principle calculation, where a broader 4d orbital and a larger rhodium-rhodium distance along the in-plane direction accidentally give nearly the same density of states as those of the layered cobalt oxides.

Effect of Early Transition-Metal (M=Ti, V and Cr) Doping on the Electronic Structure of Charge-Transfer Type Compound NiS Studied by Thermoelectric Power and X-Ray Photoemission Measurements

The M 2 p core-level XPS peaks of doped elements in Ni 1- x M x S are observed at the binding energies of M in “M 2 S 3 ” rather than those of “MS” below and above the nonmetal-metal transition temperature ( T t ), indicating that the M atom are ionized to M 3+ and acts as shallow donor impuritites. Below T t the T 1/2 contribution with negative sign to the thermoelectric power ( S ) is superimposed on the usual degenerate semiconductive contribution with positive sign. It overcomes the latter contribution above a concentration ( x c ). The value of x c increases in the order of M=Ti, V and Cr. These results indicate that the T 1/2 contribution to S comes from variable range hopping through the localized states formed near E F within the narrow charge-transfer gap of the parent NiS. Above T t the sign of S is negative in the electron-doped Ni 1- x M x S as well as in the hole-doped Ni 1- x S, suggesting that they have the Fermi surface topology.

Enhancement of thermoelectric properties by Se substitution in layered bismuth-chalcogenide LaOBiS2-xSex

We have investigated the thermoelectric properties of the novel layered bismuth chalcogenides LaOBiS2-xSex. The partial substitution of S by Se produced the enhancement of electrical conductivity (metallic characteristics) in LaOBiS2-xSex. The power factor largely increased with increasing Se concentration. The highest power factor was 4.5 μW/cmK2 at around 470 °C for LaOBiS1.2Se0.8. The obtained dimensionless figure-of-merit (ZT) was 0.17 at around 470 °C in LaOBiS1.2Se0.8.

Effect of Sn-Substitution on Thermoelectric Properties of Copper-Based Sulfide, Famatinite Cu3SbS4

Copper-based sulfide is an attractive material for Earth-abundant thermoelectrics. In this study, we demonstrate the effect of Sn-substitution on the electrical and thermal transport properties of fematinite Cu3SbS4 from 300 to 573 K. The carrier concentration is controlled in the range from 4 × 1018 to 8 × 1020 cm−3 by Sn-substitution. The density-of-states effective mass is found to be ∼3.0 me, assuming the single parabolic band model. The direct-type optical band gap is ∼0.9 eV, which is consistent with the density functional theory calculation. The dimensionless figure of merit reaches 0.1 for Sn-doped samples at 573 K.

Effects of the Cu off-stoichiometry on transport properties of wide gap p-type semiconductor, layered oxysulfide LaCuSO

Layered oxysulfide LaCu1−xSO (x = 0–0.03) was prepared to elucidate the effect of Cu off-stoichiometry on their electrical and thermal transport properties. Electrical resistivity drastically decreases down from ∼105 Ω·cm to ∼10−1 Ω·cm as a result of Cu deficiency (x = 0.01) at 300 K. Thermal conductivity of the samples at 300 K, which is dominated by lattice components, is estimated to be 2.3(3) Wm−1K−1. Stoichiometric LaCuSO has an optical band gap of 3.1 eV, while broad optical absorption at photon energies of approximately 2.1 eV was observed for Cu-deficient samples. Density functional theory calculation suggests that these broad absorption structures probably originate from the in-gap states generated by the sulfur vacancies created to compensate the charge imbalance due to Cu off-stoichiometry. These results clearly demonstrate that Cu deficiency plays a crucial role in determining the electrical transport properties of Cu-based p-type transparent semiconductors.

Enhanced Thermoelectric Figure of Merit in Stannite–Kuramite Solid Solutions Cu2+xFe1–xSnS4–y (x = 0–1) with Anisotropy Lowering

In this Article, we elucidate the structural and thermoelectric properties of stannite-kuramite solid solutions, Cu(2+x)Fe(1-x)SnS(4-y) (x = 0-1), with sulfur defects (y) ≤ 0.4. Structural analysis revealed that anisotropy decreases and Cu/Sn disorder increases with an increase in x. The samples with x = 0.8-1 exhibit degenerate conduction, whereas the Seebeck coefficient (S) remains relatively high, S ≈ 100 μV K(-1) for x = 0.8 at 300 K. Thermal conductivities (κ) of the solid solutions are in the range 10(-3)-10(-2) W cm(-1) K(-1), which is close to the κ value of silicon dioxide. The dimensionless figure of merit (ZT) reaches 0.044 for x = 0.8 at 300 K. The ZT is enhanced significantly by an increase in temperature and is doubly larger than that of x = 0 at 300 K. These findings allow us to attain higher ZT values through optimization of chemical composition.

Localized versus itinerant character of d electrons in NiS: Photoemission spectroscopic study

Abstract The electronic structure of NiS on both sides of the metal-nonmetal transition has been studied by photoemission spectroscopy using synchrotron radiation and compared with band structure calculations. Interplay between Ni d-ligand hybridization, electron correlation, exchange interaction, and d-band formation is discussed.