Spinel-type sulfide semiconductors to operate the next-generation LEDs and solar cells For solar-cell absorbers and green-LED sources

A spinel-type sulfide semiconductor that can emit light from violet to orange at room temperature has been developed by researchers at Science Tokyo, overcoming the efficiency limitations of current LED and solar cell materials. The material, (Zn,Mg)Sc₂S₄, can be chemically tuned to switch between n-type and p-type conduction, leading to future pn homojunction devices. This versatile semiconductor offers a practical path toward the development of more efficient LEDs and solar cells.

LEDs, solar cells, and semiconductor lasers rely on pn junctions for their operation, where an electron-rich n-type region meets a hole-rich p-type region. At this junction, electrons and holes either recombine to produce light, as in LEDs, or are separated to generate current, as in solar cells. The efficiency of these processes depends on the material itself. Gallium arsenide (GaAs) recombines carriers efficiently and emits light, making it ideal for LEDs, while silicon excels at capturing sunlight and generating current but is a poor light emitter.

Researchers are constantly seeking new materials that could enable more efficient LEDs, solar cells, and lasers. In a recent study, researchers at Institute of Science Tokyo (Science Tokyo) report that the spinel-type sulfide (Zn,Mg)Sc₂S₄, previously overlooked for optoelectronic applications, is a versatile semiconductor capable of light emission from violet to orange at room temperature. More importantly, it can be tuned to behave as either an n-type or p-type semiconductor, making it suitable for pn homojunction devices for next-generation LEDs and solar cells.

The research team was led by Professor Hidenori Hiramatsu and Associate Professor Kota Hanzawa of the Materials and Structures Laboratory, Science Tokyo, Japan, together with Distinguished Professor Hideo Hosono of the MDX Research Center for Element Strategy (also Honorary Professor at Science Tokyo). The findings were published in the Journal of the American Chemical Society on September 17, 2025.

Hiramatsu focuses on the “green gap” problem, a long-standing limitation in LEDs, where materials such as InGaN and AlGaInP lose efficiency in the green region. “Our semiconductor material is suitable for both green emission and photovoltaic applications,” he says, offering a promising path for next-generation LEDs and solar cells.

In spinel-type sulfides with the formula AB₂S₄, the A site is occupied by heavy cations with empty outer s orbital such as zinc, the B site by cations with anisotropic d⁰ orbitals such as scandium, and the X site by sulfur atoms carrying 3p orbitals. This arrangement results in a conduction band minimum at the Γ point, derived from the s orbital of the A-site cations, while the valence band maximum arises from shallow nonbonding sulfur p orbitals. The presence of anisotropic d⁰ orbitals at the B site further stabilizes the band structure by suppressing the valence band at other k-points, thereby ensuring a direct bandgap.

Undoped ZnSc₂S₄ produced a strong orange emission at room temperature. When magnesium was introduced in place of zinc, the emission could be shifted from orange to green and even to blue, depending on the degree of substitution. The team also demonstrated that by introducing small amounts of titanium at the Sc³⁺ site, or by slightly reducing the zinc content, the material could be switched to n-type or p-type conduction, respectively.

This chemical flexibility allowed the conductivity to be modulated over nine orders of magnitude, from the insulating state of undoped ZnSc₂S₄ (2.5 × 10^-11 S/cm) to semiconducting levels in ZnSc_1.84Ti_0.16S₄ (3.7 × 10^-5 S/cm) and Zn_0.9Sc₂S₄ (1.8 × 10^-2 S/cm), enabling its use both as an absorption layer in solar cells and as a green-emission layer in LEDs.

“The sulfide semiconductor identified in this study meets the requirements for both highly efficient light absorbers in solar cells and green light emitters in LEDs, making it a strong candidate for next-generation optoelectronic devices,” says Hiramatsu.

Learn more about this study and other research from Science Tokyo: https://www.isct.ac.jp/en/017/research.

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About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of “Advancing science and human wellbeing to create value for and with society.”