Smart windows that are transparent when it's dark or cool but automatically darken when the sun is too bright are increasingly popular energy-saving devices. But imagine a window that simultaneously produces electricity when it darkens. Such a material – a photovoltaic glass that is also reversibly thermochromic – is a green technology that researchers have long worked toward, and now scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated a way to make it work.
The researchers at Berkeley Lab, a US Department of Energy (DOE) national laboratory, discovered that a form of perovskite works surprisingly well as a stable and photoactive semiconductor material that can be reversibly switched between a transparent state and a non-transparent state, without degrading its electronic properties. Perovskite is one of the hottest current materials in solar research due to its high conversion efficiencies.
The research, led by Peidong Yang of Berkeley Lab's Materials Sciences Division, is reported in a paper in Nature Materials. The lead authors were Jia Lin, Minliang Lai and Letian Dou, all in Yang's research group.
The scientists made the discovery while investigating the phase transition of an inorganic perovskite. "This class of inorganic halide perovskite has amazing phase transition chemistry," said Yang, who is also a professor in the University of California, Berkeley's departments 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."
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," he said. "That was very interesting to us. So you can easily manipulate it in such a way that is not readily available in existing conventional semiconductors."
"The solar cell shows fully reversible performance and excellent device stability over repeated phase transition cycles without any color fade or performance degradation. With a device like this, a building or car can harvest solar energy through the smart photovoltaic window."Minliang Lai, University of California, Berkeley
Halide perovskite materials are compounds that have the crystal structure of the mineral perovskite. Their unique properties, high efficiency rates and ease of processing have made halide perovskite materials one of the most promising developments in solar technology in recent years. Researchers at another DOE lab, the National Renewable Energy Laboratory (NREL), recently made a related discovery, using a chemical reaction in a hybrid perovskite to demonstrate a switchable solar window.
The Berkeley Lab researchers did not originally set out to develop a thermochromic solar window. They were investigating phase transitions in perovskite solar cells and trying to improve the stability of the prototypical organic-inorganic hybrid perovskite known as methylammonium lead iodide. So they tried using cesium to replace the methylammonium.
"The chemical stability improved dramatically, but unfortunately the phase was not stable," said Dou, who was a postdoctoral research fellow and is now an assistant professor at Purdue University. "It transformed into the low-T [temperature] phase. It was a drawback, but then we turned it into something that's unique and useful."
The material is triggered to transition from the low-T to high-T phase (or from transparent to non-transparent) by applying heat. In the lab, the temperature required was about 100°C, but Yang said they are working to bring that down to 60°C.
Lin, a Berkeley Lab postdoctoral fellow, said moisture, or humidity, was then used in the lab to trigger the reverse transition. "The amount of moisture needed depends on the composition and the transition time desired," he said. "For example, more bromide makes the material more stable, so the same humidity would require a longer time to transform from the high-T to low-T state."
The researchers will continue to work on developing alternative ways to trigger the reverse transition, such as by applying a 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 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."
This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.