In the forthcoming low-carbon society, the demand for renewable energy will increase and humankind will have to produce a large amount of PV modules to keep our terrestrial environment clean and safe. When we consider the mass production of PV modules on the terawatt scale, we have to face the resource problem, which is the lack of raw materials. However, this problem can be avoided if we can make a new type of thin film solar cells practical using Cu₂ZnSnS₄ (CZTS) as an absorber, because the constituents of this semiconductor are all earth-abundant. This paper provides the history of improving the efficiency of CZTS thin-film solar cells developed in a small laboratory in Japan.

The emergence of graphene has helped the research on two-dimensional atomic layer materials become one of the hottest fields in material science. From both the fundamental and application points of view, an interesting and important question is whether it is possible to form silicene, a honeycomb sheet consisting of Si atoms. Here we present our research results on the geometric and electronic structures of silicene on an Ag substrate. We elucidate that monolayer silicene is a honeycomb without Dirac fermions, and that multilayer silicene is not a stack of silicene layers but it is identical to the reconstructed structure of Ag atoms on diamond-crystalline Si(111).

Magnetic tunnel junction (MTJ) devices have potential advantages such as a fast read/write, and high endurance together with back-end-of-the-line compatibility, which offers the possibility of constructing not only stand-alone RAMs and embedded RAMs that can be used in conventional VLSI circuits and systems, but also standby-power-free high-performance nonvolatile CMOS logic employing a logic-in-memory architecture, called the “MTJ/MOS-hybrid logic-LSI architecture.” The advantage of employing MTJ devices combined with CMOS circuits are discussed and some concrete examples of the novel circuit style are presented along with their prospects and remaining challenges.

Since the publication of a paper in Applied Physics Express in 2008, we have investigated various physical properties of high-purity SWCNTs with a selected electronic structure. We have clarified the optical, electric and thermoelectric properties of high-purity SWCNTs with selected chirality and controlled the properties using electro-chemical doping techniques. Since then we have investigated the relationships between the electronic structure, Fermi level, and physical properties of SWCNTs. In this letter, I present the background of the publication in 2008 and what we have done since the publication.

Solution-processed organic thin-film transistors can be fabricated on flexible materials such as plastic and paper substrates owing to their low temperature processability. Also, the large area processability utilizing printing technologies is advantageous for low cost manufacturing as compared to conventional inorganic semiconductor technologies. In this research introduction, a novel solution process for growing large crystalline films with small organic semiconducting molecules is described. The presented simple solution method, which was reported in Applied Physics Express in 2009, has been used for demonstrating high-mobility organic transistors in many studies. In this article, recent approaches for application development, such as active matrix devices for liquid crystalline displays, are also discussed.

To realize high performance germanium (Ge) CMOS, a low Schottky barrier height at the metal/Ge interface is required in order to reduce parasitic resistance. Therefore, we have to overcome the strong Fermi-level pining (FLP) close to valence band edge of Ge at the metal/Ge interface especially for Ge n-MOSFETs. 10 years ago, we successfully alleviated the strong FLP by inserting an ultra-thin insulator between metal and Ge based on the intrinsic metal-induced gap states model, which was reported in Applied Physics Express (APEX). In this paper, we review the background of this work and recent progress with this approach. Furthermore, we also report another new approach to alleviate the FLP recently published in APEX.

The present status for research and development in regard to high-efficiency cuprous oxide (Cu₂O)-based solar cells fabricated using oxide semiconductor materials is presented. Although it is difficult to obtain a suitable n-type Cu₂O, there are many reports concerning significant improvements of the photovoltaic properties that were achieved in heterojunction solar cells fabricated using a polycrystalline p-type Cu₂O as the active layer. Recently, high-energy conversion efficiencies over 8% were achieved in heterojunction solar cells fabricated by depositing an n-type Zn_1-X-Ge_X-O multicomponent oxide thin film (window layer) on a p-type Cu₂O sheet.

Organic-inorganic hybrid lead-halide perovskites have received quite a bit of attention as a promising class of low-cost and high-efficiency photonic device materials. The fundamental optoelectronic properties of CH₃NH₃PbI₃ films and diode devices are discussed.

The highest efficiency (24.4) for solar-to-hydrogen (STH) energy conversion was obtained in an outdoor field test by combining concentrator photovoltaic (CPV) modules with InGaP/GaAs/Ge three-junction cells and polymer-electrolyte electrochemical (EC) cells. The high efficiency was obtained by using the high-efficiency CPV modules (>31% under the outdoor condition) and a direct connection between the CPV modules and the EC cells with a nearly optimized number of elements in series. The STH efficiency bottleneck was clarified to be the efficiency of the CPV modules, the over-potential of the EC cells, and the matching of the operation point to the maximal-power point of the CPV modules.

This article explains the basics of quantum chemical (QC) calculations, which solve the Schrödinger equation numerically by using computers. The QC method is partitioned into two categories: the molecular orbital (MO) method and the density functional theory (DFT). MO, which means a one-electron wave function in a molecule, is commonly expressed by a linear combination of atomic orbitals or basis sets. The Hartree-Fock (HF) method, which is the most fundamental technique in the MO method, constructs many-electron wave functions by an anti-symmetric product of MOs. The MO method can systematically raise the accuracy from the HF method to higher levels of electron correlation theories such as configuration interaction, coupled cluster, and many-body perturbation theories, along with better basis sets. Exchange-correlation functionals, which play a key role in DFT, have a hierarchy: local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, and hybrid-GGA.