Sped-up video shows materials going from low-T to high-T phase. Source: Berkeley LabSped-up video shows materials going from low-T to high-T phase. Source: Berkeley Lab

Scientists from the Lawrence Berkeley National Laboratory have discovered a new form of perovskite that works as a stable and photoactive semiconductor material that can be reversibly switched between a transparent state and a non-transparent state.

This photovoltaic glass could be used to create a new form of smart window that is transparent when it is dark or cool but automatically darkens when the sun is too bright, at the same time producing electricity.

The Berkeley Lab scientists made the discovery while they were investigating the phase transition of inorganic perovskite.

“This class of inorganic halide perovskite has amazing phase transition chemistry,” says Peidong Yang, a professor at UC Berkeley’s department of Chemistry, and Materials Science and Engineering. “It can essentially change from one crystal structure to another when we slightly change the temperature or introduce a little water vapor.”

As the material changes its crystal structure, it goes from transparent to non-transparent so you can manipulate it in a way uncommon to existing conventional semiconductors, researchers say. Halide perovskite materials are compounds that have the crystal structure of mineral perovskite, including high efficiency rates and ease of processing that have made the material promising in solar technology.

The National Renewable Energy Laboratory (NREL) recently made a similar discovery for smart windows using a hybrid perovskite to demonstrate switchable solar windows.

The material developed by Berkeley Lab is triggered to transition from the low-T to high-T phase (transparent to non-transparent) by applying heat. At about 100 degrees Celsius, the material changed in the lab but the researchers are working to bring it down to 60 degrees Celsius.

“The solar cell shows fully reversible performance and excellent device stability over repeated phase transition cycles without any color fade or performance degradation,” says 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.”

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