Gen-next smart solar windows could produce electricity

Scientists have discovered a new material for next-generation smart windows that not only darken automatically when the Sun is too bright but also convert solar energy into electricity. Researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) in the US found a form of perovskite that works well as a stable and photoactive semiconductor material that can reversibly switch between transparent and non-transparent state, without degrading its electronic properties.

The scientists made the discovery while investigating the phase transition of the material, an inorganic perovskite. "This class of inorganic halide perovskite has amazing phase transition chemistry," said Peidong Yang from Berkeley Lab, who led the study published in the journal Nature Materials. "It can essentially change from one crystal structure to another when we slightly change the temperature or introduce a little water vapour," said Yang.

When the material changes its crystal structure, it changes from transparent to non-transparent. "These two states have the exact same composition but very different crystal structures. So you can easily manipulate it in such a way that is not readily available in existing conventional semiconductors," he said.

Halide perovskite materials are compounds that have the crystal structure of the mineral perovskite. Its unique properties, high efficiency rates, and ease of processing have made it one of the most promising developments in solar technology in recent years. The researchers were investigating phase transitions in perovskite solar cells and trying to improve the stability in the prototypical organic-inorganic hybrid perovskite methylammonium lead iodide. They tried using cesium to replace the methylammonium.

"The chemical stability improved dramatically, but unfortunately the phase was not stable. It was a drawback, but then we turned it into something that's unique and useful," said Letian Dou, assistant professor at Purdue University. The material is triggered to transition from transparent to non-transparent by applying heat. In the lab, the temperature required was about 100 degrees Celsius. Yang said they are working to bring it down to 60 degrees.

Moisture, or humidity, was used in the lab to trigger the reverse transition. The researchers will also continue to work on developing alternative ways to trigger the reverse transition, such as by applying voltage, or engineering the source of the moisture.

"The solar cell shows fully reversible performance and excellent device stability over repeated phase transition cycles without any color fade or performance degradation," said Minliang Lai, a graduate student in Yang's group. "With a device like this, a building or car can harvest solar energy through the smart photovoltaic window," Lai said.