Chalcogenide perovskites BaTi(OS)3 with band gap in the optimal Shockley-Queisser range

For the last half-century, scientists have been searching for a better way to capture the sun's energy. While traditional solar panels made from silicon have served us well, researchers are constantly looking for materials that are cheaper to make, easier to produce, and capable of converting sunlight into electricity more efficiently. One promising family of materials for this job is called perovskites. These are crystals with a specific, repeating structure that can be tuned to absorb light very effectively. However, the most famous perovskites used today contain lead, a toxic metal that poses health and environmental risks, and they often break down when exposed to moisture or heat. To solve this, scientists are exploring a different kind of perovskite made from elements that are abundant and non-toxic, specifically looking at compounds that mix oxygen with sulfur. The goal is to find a material that is stable, safe, and has just the right ability to absorb light to power the next generation of solar cells.

In a recent study, researchers at the Autonomous University of Zacatecas in Mexico set out to create and test one of these new materials, a crystal called barium titanium oxysulfide. They started with a common, stable powder known as barium titanium oxide and subjected it to a high-temperature treatment in a special furnace. Instead of just heating the powder, they introduced a vapor of carbon disulfide, a chemical that acts as a source of sulfur. By heating the mixture to three different temperatures—600, 700, and 800 degrees Celsius—they aimed to replace some of the oxygen atoms in the crystal with sulfur atoms. The researchers wanted to see how changing the heat would alter the material's internal structure and its ability to absorb light, hoping to find the perfect balance for solar energy applications.

The results showed that temperature was the key to controlling the material's composition. As the heat increased, the amount of sulfur in the final crystal grew significantly. At the lowest temperature of 600 degrees Celsius, the material contained about 14.6 percent sulfur by mass. When the temperature was raised to 700 degrees, the sulfur content rose to 18.2 percent, and at the highest temperature of 800 degrees, it reached 20.2 percent. This shift was not just a matter of adding more sulfur; it fundamentally changed the shape of the crystal itself. The starting material had a tetragonal structure, which is a specific geometric arrangement of atoms. As more sulfur replaced oxygen, the crystal slowly transformed into a hexagonal structure, a different geometric shape that is more stable for this type of compound. The samples treated at the highest temperatures were the most successful, showing a clean, well-ordered hexagonal structure with no traces of the original shape remaining.

Beyond just changing the shape, the heat treatment improved the quality of the crystals. The researchers measured the size of the tiny individual crystals, known as crystallites, and found that they grew larger as the temperature increased. The sample made at 600 degrees had smaller, less organized crystals, while the one made at 800 degrees had larger, more perfectly formed crystals. This suggests that higher heat helps the atoms arrange themselves more neatly, reducing internal stress and creating a stronger, more uniform material. The team also analyzed the chemical bonds within the material using light-based techniques and confirmed that the sulfur and oxygen atoms were successfully integrated into the crystal lattice, forming the specific molecular bonds they were looking for.

The most critical discovery, however, concerned how the material interacts with light. For a solar cell to work efficiently, the material it is made of must have a "band gap," which is the specific amount of energy required to knock an electron loose and create an electric current. If this gap is too wide, the material misses out on much of the sun's energy; if it is too narrow, it wastes energy as heat. The ideal range for solar cells is a narrow window between 1.1 and 1.7 electron volts. The researchers found that by simply increasing the temperature and the sulfur content, they could precisely tune this band gap. The material made at 600 degrees had a band gap of 1.53 electron volts. As they increased the heat to 700 and then 800 degrees, the band gap narrowed to 1.51 and finally to 1.47 electron volts. This final value sits squarely in the optimal range for solar energy conversion.

This study demonstrates that it is possible to engineer a new, non-toxic solar material by carefully controlling the temperature during synthesis. The researchers proved that they could transform a common oxide into a sulfur-rich perovskite with a crystal structure and light-absorbing properties that are highly suitable for solar cells. While the material is not yet a finished product ready for a factory floor, the work provides a clear roadmap for how to create these crystals. By showing that higher temperatures lead to more sulfur, better crystal order, and a band gap that is perfectly tuned for capturing sunlight, the study offers a viable path forward for developing the next generation of clean, efficient, and safe solar power technology.