Research Topics

Solid-state ion exchange is a synthetic method that allows for the replacement of ions in a crystal structure without disrupting the overall lattice framework. This approach enables the formation of new compounds that are difficult to obtain through conventional high-temperature synthesis, including metastable phases and materials with unique functionalities. I consider this method a powerful tool for discovering new materials. My research combines experimental work with first-principles calculations, and I have reported the following key achievements:

• Discovery of the oxide semiconductor β-CuGaO2 with a direct band gap of 1.5 eV, suitable for solar cell applications
  Synthesis paper (2014) Computational paper (2016) Thin-film paper(2017) 

• Development of a method to predict the feasibility of ion-exchange reactions using first-principles calculations
  Press release (2024) Paper (2024)

• Demonstration that new reaction pathways can be discovered by exploring novel ion sources
  Paper (2025)

Tin sulfide (SnS) is a compound semiconductor composed of earth-abundant and non-toxic elements, making it a promising candidate for next-generation, environmentally friendly solar cells. Since SnS typically shows p-type conductivity, research so far has focused on heterojunction solar cells combining p-type SnS with n-type materials. However, I aim to realize higher-efficiency homojunction SnS solar cells by joining p-type and n-type SnS layers. To that end, I have been working on the development of n-type SnS and related investigations, as outlined below:

• Pioneered the realization of n-type SnS thin films by controlling sulfur deficiency
  Press release (2021)　Paper (2021)　Review (2022)

• Reported the world's first homojunction SnS solar cell using n-type and p-type single crystals of SnS
  Press release (2021)　Paper (2021)

• Elucidated in detail how slight deviations in the Sn/S composition ratio affect the electrical properties and morphology of SnS
  Press release (2025)　Paper (2025)

Sulfide compounds are promising for a wide range of applications, including solar cells, transistors, and solid electrolytes. However, the fabrication of high-quality sulfide thin films has traditionally required the use of highly toxic hydrogen sulfide (H2S) gas. In this research, I am developing a new deposition technique that enables the safe and versatile synthesis of various sulfide thin films using sulfur plasma, which supplies highly reactive sulfur species without relying on H2S at all

• Demonstrated that a variety of sulfide thin films can be universally synthesized using an H₂S-free process based on sulfur plasma
  Youtube PV

• Reported that the high energy of sulfur plasma enables the formation of highly crystalline thin films even on low-temperature substrates
  Paper (2025)

To apply a material to a functional device, it is essential to understand its electronic structure, both in the bulk and at interfaces. I have employed experimental techniques such as X-ray photoelectron spectroscopy (XPS) and angle-resolved photoelectron spectroscopy (ARPES) to directly probe these electronic structures. The following are some of the major results:

• Direct evaluation of the band structure and dispersion of sulfide-based thermoelectric materials using synchrotron-based ARPES
  Paper (2022)　Paper (2022)　Paper (2023)

• In-situ XPS analysis of interface electronic structures involving single-crystalline SnS, demonstrating a wide tunability of the Fermi level
  Press release (2022)   Paper (2022)

• XPS analysis of the β-CuGaO₂/ZnO interface, confirming that it forms a functional junction suitable for photovoltaic applications
  Paper (2021)