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Berkeley Lab researchers adapt perovskite for thermochromic windows

Jan. 24, 2018

Perovskite holds promise for breaking down the barriers toward wider solar-cell deployment. In a new twist, Berkeley Lab researchers have made thermochromic windows with perovskite solar cells. Their efforts could lead to smart windows that are transparent when it’s dark but automatically darken—and generate power—when the sun is bright.

“Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy,” the researchers write in a paper published in Nature Materials. “Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide.”

They continue, “The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply colored perovskite phase (35.4% visible transparency) with high power output.” They report efficiencies as high as 7%.

The research is led by Peidong Yang of Berkeley Lab’s Materials Sciences Division. The paper’s 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 the material, an inorganic perovskite. “This class of inorganic halide perovskite has amazing phase transition chemistry,” said Yang, in a press release. “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 nontransparent. “These two states have the exact same composition but very different crystal structures,” he added. “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.”

Researchers at another DOE lab, the National Renewable Energy Laboratory (NREL), recently made a related discovery, demonstrating a device that dynamically responds to sunlight by transforming from transparent to tinted while converting sunlight into electricity.

The NREL-developed demonstration device allows an average of 68% of light in the visible spectrum to pass through when it’s in a transparent state. When the window changes color—a process that took about three minutes of illumination during testing—only 3% is allowed through the window, which achieves a solar-power conversion efficiency of 11.3%. “There is a fundamental tradeoff between a good window and a good solar cell,” said Lance Wheeler, a scientist at NREL. “This technology bypasses that. We have a good solar cell when there’s lots of sunshine and we have a good window when there’s not.”

The NREL researchers described their work in a paper published in Nature Communications.

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 in the prototypical organic-inorganic hybrid perovskite 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. Yang said they are working to bring it down to 60°C.

Lin, a Berkeley Lab postdoctoral fellow, said moisture was 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 longer time to transform from the high-T to low-T state.”

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 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.”

The research was supported by DOE’s Office of Science. Other co-authors of the paper are from UC Berkeley, Stockholm University, and Lawrence Livermore National Laboratory. The Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory and the Advanced Light Source at Berkeley Lab, both DOE Office of Science User Facilities, were used to collect some of the data.

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