Issei Suzuki

Herein, a pn homojunction SnS solar cell is fabricated for the first time by the deposition of p‐type SnS polycrystalline thin films on the recently reported large n‐type SnS single crystals. The p‐type thin films consist of columnar grains that grow along the <100> direction, which is the same orientation as the n‐type single crystal. In addition, the interface of the pn homojunctions is void‐free and compositionally sharp. The SnS homojunction solar cell achieves an open‐circuit voltage

The electronic structures of delafossite α-CuGaO2 and wurtzite β-CuGaO2 were calculated based on density functional theory using the local density approximation functional including the Hubbard correction (LDA+U). The differences in the electronic structure and physical properties between the two polymorphs were investigated in terms of their crystal structures. Three major structural features were found to influence the electronic structure. The first feature is the atomic arrangements of catio

Abstract Orthorhombic SnS is a promising thin-film solar-cell material composed of safe and abundant elements with suitable optical properties for photovoltaic application. For approximately two decades, SnS solar cells have employed heterojunction structures with p-type SnS and other n-type semiconductors because undoped SnS typically exhibits p-type electrical conduction. However, their conversion efficiency has remained stagnant at 4%–5% for a long time. A breakthrough is required to signific

The electronic structure of β-CuGaO2 was studied by first principles calculations and X-ray photoelectron spectroscopy (XPS), and the expected electrical and optical properties of this material were discussed. Density functional theory calculations using the local density approximation with corrections for on-site Coulomb interactions (LDA + U) with U = 5–7 eV reproduced well the experimentally obtained crystal structure and valence-band XPS spectrum. The calculated electronic structure indicate

We synthesized solid solutions of β-CuGaO₂ and β-LiGaO₂ (i.e., β-(Cu_{1- x}Li _x)GaO₂) by partial ion exchange of Cu⁺ in β-CuGaO₂ with Li⁺ from LiCl in the composition range of 0 ≤ x ≤ 0.89. The energy band gap of β-CuGaO₂ (1.47 eV) increased linearly up to 3.0 eV with increasing Li content, covering the full visible range. The crystal structures of the solid solutions were analyzed using the Rietveld

An approach to determine the defect energy levels of the Fe impurities in BaTiO3 and SrTiO3 single crystals using electrical conductance measurements is presented. The defect levels are obtained from the dependence of the activation energy of electrical transport on the oxygen vacancy concentration, which is varied by stepwise re-oxidation of a reduced sample. An energy level at 0.7-0.8 eV below the conduction band minimum ECB is identified for BaTiO3, which can be assigned to the Fe2+/3+-transi

Pseudo-binary oxide semiconductor alloy films of (1-x)ZnO-x(AgGaO2)1/2 were fabricated using a conventional rf-magnetron sputtering. The wurtzite-type single phases were obtained in the wide composition range of x ≤ 0.33 because the terminal β-AgGaO2 that corresponds to the composition with x = 1 possesses a wurtzite-derived β-NaFeO2 structure. The energy band gap of ZnO decreased with increasing AgGaO2 concentration, falling to 2.55 eV at x = 0.33. This alloy system enables to use ZnO-based sem

Tin sulfide (SnS) is a semiconductor composed of abundant and non-toxic elements and has potential applications as a light absorbing layer in thin-film solar cells and a thermoelectric material. While controlling the carrier type (n- or p-type conduction) and carrier concentration of SnS by impurity doping has been intensively studied both experimentally and theoretically since 2010s, no comprehensive discussion of them has been made. This review is motivated to provide researchers with an overv

Oxide-based optoelectronic devices have been limited in applicable wavelength to the near-UV region because there are few viable binary wurtzite-type oxides, but ternary wurtzite-type (β-NaFeO2-type) oxides are promising materials to expand the applicable wavelengths of these devices. In the past decade, many attractive properties of β-NaFeO2-type oxide semiconductors have been revealed, such as the band-engineering of ZnO by alloying with β-LiGaO2 and β-AgGaO2, the photocatalytic activities of

Tin monosulfide (SnS) usually exhibits $p$-type conduction due to the low formation enthalpy of acceptor-type defects, and as a result, $n$-type SnS thin films have never been obtained. In this paper, we realize $n$-type conduction in SnS thin films by using radiofrequency-magnetron sputtering with Cl doping and a sulfur plasma source during deposition. Here, $n$-type SnS thin films are obtained at all the substrate temperatures employed in this paper (221--341 \ifmmode^\circ\else\textdegree\fi{

Promising new, abundant, and environmentally friendly absorber materials for thin-film solar cells often suffer from low photovoltages, which are limited by Fermi-level pinning due to bulk or interface defects. If it is difficult to avoid Fermi-level pinning, low photovoltage cannot be overcome by optimizing the contact material and device processing. Therefore, it is essential to understand in the early stages of material development whether such Fermi-level pinning can be avoided and how. Usin

Abstract Tin sulfide (SnS) is a compound semiconductor that has been studied for a wide range of applications, including solar cells and thermoelectric materials. In this study, the electronic structure of the SnS valence band, which is important for such applications, was investigated via angle-resolved photoelectron spectroscopy with two different excitation energies in the extreme ultraviolet region (21 and 60 eV). The contribution of the Sn 5s state to the SnS valence band was determined in

Valence band dispersions of single-crystalline SnS_1-<i>x</i>Se_<i>x</i> solid solutions were observed by angle-resolved photoemission spectroscopy (ARPES). The hole effective masses, crucial factors in determining thermoelectric properties, were directly evaluated. They decrease slightly with increasing Se content in the low Se composition range but sharply in the high Se composition range.