First, it is introduced how much photovoltaics can contribute to the realization of a carbon-neutral society. Next, the history of the development of Si solar cells is introduced as the progress of passivation technology and the accompanying transition of conversion efficiency. The key to future solar cell development is the realization of high-efficiency tandem solar cells based on Si. The current status and issues of perovskite/Si tandem solar cells will be described. With the development of solar cell technology, the photovoltaic system is also evolving rapidly. In addition to the existing mega-solar and giga-solar, new application fields such as for factory roofs, building walls and vehicles are opening up.

High-performance passivating contacts are crucial to realizing high-efficiency crystalline silicon (c-Si) solar cells. The passivating contacts can efficiently transport photogenerated carriers from c-Si to metal electrodes. A representative passivating contact is the stack of ultra-thin silicon oxide (SiO_x) and doped polycrystalline Si. In the structure, the layer thickness of silicon oxide is limited to 1～2 nm to collect photogenerated carriers. For further improvements, we have developed NAnocrystal Transport path in Ultra-thin dielectrics for REinforced passivating contact（NATURE contact）as a new passivating contact. The NATURE contact consists of Si nanocrystals (NCs) and SiO_x matrix. The Si NCs and the SiO_x matrix function as carrier transport paths and passivation layers for the c-Si surface, respectively. The NATURE contact can attain good passivation performance and conductive property. In this paper, we briefly explain the fundamentals of the passivating contacts and introduce the properties of the NATURE contact.

Ultrathin (∼1 nm) films have been widely used for high-efficiency crystalline silicon (c-Si) solar cells such as tunnel oxide passivated contact (TOPCon) solar cells. In this article, we present our recent results for the applications of ultrathin silicon oxide (SiO_x) and silicon nitride (SiN_x) films to c-Si solar cells. The following topics are reviewed. 1) Ultrathin SiN_x can be utilized for passivating contacts instead of SiO_x in the TOPCon structure. 2) The addition of SiO_x between c-Si and thick catalytic-chemical-vapor-deposited (Cat-CVD) SiN_x significantly improves the quality of surface passivation. 3) Ultrathin Al-doped SiO_x films formed just by dipping in Al(NO₃)₃ solution on c-Si provide strong upward band bending due to negative fixed charges, which can be used for hole-selective contacts.

Development of high-efficiency solar cells is very important for clean energy society based on photovoltaics. Multi-junction solar cells are very attractive as high-efficiency solar cells because 2-junction and 3-junction solar cells have high-efficiency potential of more than 36% and 42%, respectively. This paper overviews present status of multi-junction solar cells. As a results of reducing non-radiative recombination and resistance losses, 39.5% efficiency has been demonstrated with 3-junction solar cell. In addition, understanding and controlling defects, surface recombination and interface recombination are shown to be very important in order to improve efficiency of multi-junction solar cells. Brief overview for Si tandem solar cells composing of III-V, II-VI, chalcopyrite, perovskite top cells and Si bottom cell is also presented.

Organometal halide perovskites have captured wide interest as a promising material for light-weight and high-efficiency solar cells. Through recent studies of the organometal halide perovskite solar cells (PSCs), the composition of organometal halide perovskites is recognized as one of the key factors in the improvement of the PCE. In this study, we investigated mixed cation perovskite absorber. Additionally, we successfully constructed monolithic PSC mini-module without I-V hysteresis. In the case of MA-free PSCs, the 24.9% PCE (0.187 cm²) and 21.6% PCE (2.76 cm² monolithic PSC mini-module) were obtained, respectively. We also developed semitransparent perovskite top cells with higher durability and transmittance for tandem solar cells. The PCEs of 19.5% (certified 19.3%) and 26.2% were achieved for a 1 cm² semi-transparent perovskite top cell and four-terminal PSC/CIGS tandem solar cells, respectively.

We report the measurement results of public road tests of a solar-powered plug-in hybrid electric vehicle that is equipped with photovoltaic (PV) modules that have a rated-output power of approximately 860 W. The measured results show that the number of plug-in charging cycles can be reduced significantly by installing PV modules, which increase the convenience of use of the electric vehicles. Even energy self-sufficiency was achieved for some driving patterns, such as neighborhood driving pattern. Further, the annual PV-powered driving range reached 6211 km in Aichi in the year 2021. As the average annual driving range of passenger cars in Japan is approximately 10000 km, it was experimentally confirmed that CO₂ emission from passenger cars in Japan can be reduced by approximately 62% by installing PV modules in passenger cars.

This report introduces hydrogen sensing with Membrane-type Surface stress Sensor (MSS) coated with two kinds of hydrogen adsorbent materials : Pd and PdCuSi. The equilibrium hydrogen pressure can be measured by detecting surface stress associated with a volume change of the material caused by hydrogen absorption. For Pd on MSS, it takes a long time for the hydrogen absorption reaction to reach saturation. However, it is possible to shorten the determination time by using the initial absorption rate as an index. As for PdCuSi on MSS, it needs a short time to reach saturation. We show that the relationship between the hydrogen concentration and the respective index of the absorption rate or the saturation value of the absorption can be well explained by using a 2-step reaction kinetic model for hydrogen absorption into bulk via adsorption on surface.