Stabilizing a solar material

A promising solar material hampered by stability problems has been fortified by a KAUST team that studied how the material degraded when exposed to air and moisture, and then tweaked its chemical composition to make it more durable^[1]. This is a key step towards using the material in solar panels and other devices.

The material is known as a perovskite, a family of inexpensive compounds that are easily processed into thin films that convert light into electricity. These perovskites generally contain three types of ingredients: metals such as tin or lead; halogen ions, including bromide or iodide; and positively-charged ions such as cesium, methylammonium or formamidinium.

Lead-only perovskites typically absorb visible light, but perovskites containing a mix of tin and lead can also absorb near-infrared light. In principle, a solar cell made from a tin-lead perovskite could be teamed with a second solar cell that absorbs different wavelengths of light, forming a partnership that offers a higher power output than a conventional solar panel.

“In these ‘tandem’ solar cells, each of the layers specializes in absorbing a specific part of the solar spectrum,” explains Luis Lanzetta, one of the leading scientists behind the new research. “It means that a larger portion of the photons in sunlight can be converted into electricity.”

Meanwhile, tin-lead perovskites could also be used to detect near-infrared light in biological imaging or medical monitoring devices.

The big problem is that oxygen and moisture in the air rapidly degrade tin-lead perovskites. This process also turns some of the perovskite’s iodide into iodine, causing further damage that significantly reduces the material’s performance in less than an hour.

The KAUST researchers studied exactly how this degradation happens at the atomic level, and used that knowledge to tackle the problem. They made perovskites containing various mixtures of cesium, methylammonium and formamidinium, and found that cesium-rich blends were far more stable than the others. In contrast, methylammonium-rich formulations generated about four times as much iodine as their cesium-rich counterparts.

Computer simulations suggested that cesium can capture iodine in a way that slows perovskite breakdown. In contrast, methylammonium turns iodine into an even more active form called triiodide. “The bad news is that triiodide forms right on top of the perovskite surface, where it is in close contact with the material and able to oxidize it rapidly,” says Lanzetta.

So the researchers added a thin layer containing cesium or rubidium to the material’s surface, and included a chemical agent into the perovskite that could scavenge iodine. These tactics produced perovskite solar cells with a good efficiency of around 18%, and the scavenger ensured the cell’s performance was virtually unchanged after 2 hours exposure to the air.

The team plans to investigate other types of iodine scavenger and test different ways of incorporating it into the perovskite to improve its performance. “We believe that further optimizing of iodine-scavenging additives for perovskite solar cells will lead to highly durable technologies,” says Derya Baran, who led the team.