Solar cells convert sunlight into clean energy - but if the solar cells themselves are made of toxic materials, it almost defeats the purpose. That's where tin sulfide (SnS) comes in. SnS is an environmentally friendly, naturally abundant, and relatively inexpensive semiconductor material that is a promising candidate for use in solar cells and thermoelectric conversion devices.

In order to improve its performance in these applications, researchers from Tohoku University systematically investigated how deviations in the 1:1 ratio of Sn to S influence the electrical properties and morphology of SnS thin films. Until now, achieving precise compositional control of this ratio during thin-film deposition has been a major challenge due to the high volatility of sulfur.

The research team, led by Issei Suzuki (senior assistant professor) and Taichi Nogami (PhD candidate), developed a novel sulfur plasma-assisted sputtering method to precisely control the sulfur content in SnS thin films. In conventional sputtering, a SnS sintered target is atomized and deposited onto a substrate. In this study, the researchers introduced plasma-activated sulfur into this process, enabling precise compositional control of SnS. Using this approach, they fabricated p-type SnS thin films with Sn:S ratios of 1:0.81, 1:0.96, 1:1, and 1:1.04 and analyzed their structural and electrical properties.

"We found that even slightly changing the composition of Sn and S significantly affected the morphology," explains Suzuki. Specifically, the study found that sulfur-rich compositions (S > 50%) lead to a drastic increase in carrier density, while sulfur-deficient compositions (S < 50%) exhibit almost no change in carrier density. Additionally, non-stoichiometric films exhibited rough and porous morphologies, whereas stoichiometric SnS thin films (1:1) displayed a dense structure with high hole mobility, making them highly suitable for solar cell applications.

This research highlights the critical importance of precise sulfur content control in SnS thin films and offers valuable insights for improving their electrical performance and structural integrity. This builds on previous findings by Suzuki and Nogami et al. that examined a different type of SnS thin film. These findings are expected to contribute to the practical application of SnS in next-generation energy conversion devices.

"The next step will be to integrate these optimized SnS thin films into high-efficiency solar cells," says Nogami, "We want to fine-tune their performance and scalability so it can potentially be used to generate clean energy and help fight climate change."

These findings were published in APL Materials on March 21, 2025.