Metal nanoclusters (NCs), which are composed of about 250 or fewer metal atoms, possess great potential as novel functional materials. Fundamental research on metal NCs gradually started in the 1960s, and since 2000, thiolate (SR)‐protected metal NCs have been the main metal NCs actively studied. The precise and systematic isolation of SR‐protected metal NCs has been achieved in 2005. Since then, research on SR‐protected metal NCs for both basic science and practical application has rapidly expanded. This review describes this recent progress in the field of SR‐protected metal NCs in three areas: synthesis, understanding, and application. Specifically, the recent study of alloy NCs and connected structures composed of NCs is highlighted in the “synthesis” section, recent knowledge on the reactivity of NCs in solution is highlighted in the “understanding” section, and the applications of NCs in the energy and environmental field are highlighted in the “application” section. This review provides insight on the current state of research on SR‐protected metal NCs and discusses the challenges to be overcome for further development in this field as well as the possibilities that these materials can contribute to solving the problems facing modern society.

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A very stable amorphous SiC/Si (a-SiC/a-Si) heterojunction solar cell has been developed. The cell structure is of multijunction type with blocking barriers for the dopants and metal diffusion. This new tandem cell exhibits excellent stability for both thermal and sunlight conditions. After 2000 h of light exposure, an observable photodegradation is not seen in the cell performance. The improvements are achieved by the low-temperature deposition of first a p a-SiC layer, and by the blocking barriers for dopants and metal diffusion.

Basic technologies for solving problems in thermal and light-induced degradation have been developed. The tandem type a-SiC/a-Si solar cell with blocking layers exhibits excellent stability for both thermal and sun light conditions. An observable light-induced degradation is not seen in the cell performance after light exposure test of 2000 hours. Two instability modes, that is, thermal and light-induced degradation have been investigated. For thermal degradation, a blocking layer for preventing diffusion has been inserted between the back side metal electrode and a n-layer and another blocking layer has been introduced between the np tunnel junction. To prevent light-induced degradation, p-type a-SiC layer of the pin structure on the side of a glass substrate/SnO2 has been deposited at the temperature of 70°C. The highest efficiency is 9.0% at the present stage, but it is expected to be improved to more than 10%.

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