The article reviews recent evolution of ultra-high-speed imaging technologies. The temporal resolution limit of silicon image sensors is 11.1 ps. Imaging with higher speed than the limit is named super-temporal-resolution (STR) imaging. Various technologies previously developed for STR are briefly explained. To achieve the STR with image sensors, germanium is selected as a photodiode material. The penetration depth of visual light to germanium is much shorter than that to silicon, making the thickness of the germanium photodiode less than 100 nm. Furthermore, saturation drift velocity of electrons in germanium is about 2/3 of that in silicon. The ratio suggests that an image sensor with the germanium photodiode achieves the STR. The technical feasibility of the prediction is confirmed.

Various combined loads are given to photovoltaic modules exposed outdoors, leading to a decrease in output power. Although degradation phenomena in photovoltaic modules are very complicated, degradation factors are roughly classified into three categories; shading, less carrier collection ability, and less photovoltaic ability. In this paper various degradation phenomena will be introduced based on these three categories. Potential-induced degradation will be introduced in detail with recent findings. The influence of ultraviolet irradiation, which is an inevitable load for photovoltaic modules exposed outdoors, on degradation phenomena, will also be discussed.

Carrier selective passivating contacts, which provide the dual functions of surface defect passivation and extracting one type of photogenerated carrier from the light absorber, have attracted attention for the realization of high efficiency crystalline silicon (c-Si) solar cells. In particular, low-cost and transparent materials that can replace the existing silicon-based materials have been desired. In this study, a tunable carrier-selectivity behavior was found in titanium oxide (TiO_x) nanolayers grown by atomic layer deposition. It is demonstrated that the TiO_x acts as an efficient hole-selective passivating contact in c-Si solar cells with >20 % efficiency, which is antithetic to the previous understanding that TiO_x acts solely as an electron contact and hole blocker. This new functionality opens opportunities for replacing the widely used heterocontacts and dielectric-passivation layers in various solar cells.

Single photon sources (SPSs) in silicon carbide (SiC), such as a silicon vacancy (V_Si), a divacancy (V_SiV_C), carbon antisite-carbon vacancy pairs (C_SiV_C), nitrogen-vacancy pairs (N_CV_Si) and a surface SPS, are regarded as candidates for quantum bits (Qubits). To apply these SPSs to quantum applications, it is important to understand their optical and spin properties. Spins in single V_Si, V_SiV_C and N_CV_Si can be manipulated using optically detected magnetic resonance techniques. C_SiV_C and a surface SPS show luminescence with extreme brightness even at room temperature. To realize quantum devices, methodologies for the introduction of SPSs into devices are also important. The creation of a V_Si in certain locations of SiC diodes without their degradation was demonstrated using a proton beam writing technique.

Terahertz frequencies ranging from 0.1 THz to 10 THz have considerable potential for applications in numerous fields, including imaging, spectroscopy, and high-speed communication. However, in this frequency range, many key components for widespread commercialization have not been fully established. Terahertz sources based on difference frequency generation in mid-infrared quantum cascade lasers are currently the only electrically pumped monolithic semiconductor laser sources at room temperature in the THz spectral range. Recently, ultra-broadband comb generation and sub-THz quantum cascade laser sources have been demonstrated. In addition, an imaging experiment using THz sources has been successfully performed for the first time. In this article, we report the recent progress of room temperature quantum cascade laser sources.

Gallium oxide (Ga₂O₃) has a large bandgap (approx. 5.0 eV) and exhibits an excellent potential for power-switching and deep-ultraviolet optoelectronic devices. Ga₂O₃ possesses five polymorphs, namely α-, β-, γ-, δ-, and κ-phases. In this article, among the five polymorphs, we introduce the epitaxial growth of γ- and κ-phase Ga₂O₃. The κ-Ga₂O₃ exhibits a ferroelectric property and a large spontaneous polarization. The γ-Ga₂O₃ with the incorporation of Al₂O₃ is lattice-matched to a spinel substrate and exhibits a large bandgap of 5.8 eV. The results of the epitaxial growth of κ- and γ-Ga₂O₃ thin films by mist CVD are discussed, focusing on lattice matching and alloying.

Fuel cells are clean and efficient energy converters via electrochemical reactions. Their operation principles, component materials, and applications have been reviewed, together with the calculation method of the efficiency. Some research topics for electrocatalysts for polymer electrolyte fuel cells (PEFCs) are also reported. It is expected that PEFCs, polymer electrolyte water electrolyzers (PEWEs), and solid oxide fuel cells (SOFCs), which can be operated inversely as solid oxide electrolysis cells (SOECs), play important roles in a hydrogen society, closely coupled with renewable electric energies.