Powering Up With A Smart Window
Window material repeatedly switches from being see-through to blocking the heat and converting sunlight into electricity
In the future, smart windows will not only be used for thermal management, but they will also be used to provide solar energy for cars, buildings, and displays. Researchers recently discovered a window material that can be reproducible switched by temperature cycling. The image shows the low-temperature (Low-T) material on the left. This transparent material has low electrical power output. In contrast, the high-temperature material (High-T) is darker but exhibits a high electrical power output when illuminated with sunlight.
Smart windows have traditionally been made using materials that let sunlight in but keep the heat out. Now, using mixed halide perovskite CsPbI3−xBrx materials, researchers have added the ability to convert sunlight into electricity. These window materials reversibly change their atomic crystal structure with heating and cooling. The transparent material darkens when it is exposed to direct sunlight. When it changes, it is converted into a new material that can convert sunlight into electricity. In the absence of sunlight, the material cools and is converted back to a transparent material. These new materials can be reversibly heated and cooled with no change in their properties.
These new materials produce power, a new feature compared to today's smart windows. The new windows could keep our future cars, buildings, and displays cool, while simultaneously generating electricity.
Smart windows were developed in the 1970s. They are used to reduce building energy consumption by minimizing unwanted solar heating. Typically, smart windows are made out of materials that undergo a light-induced change that causes the transparent material to be converted into one that either reflects or absorbs the sunlight. However, under these conditions, much of the incident solar energy is being wasted. It would, therefore, be advantageous to have a window material that could, at the same time it is managing the thermal consequences of the sunlight, convert the sunlight into electricity. Silicon is the most common material used today for solar to electric conversion. New materials (mixed-halide perovskites) have been discovered that can convert sunlight into electricity. Recently, a team led by scientists at Lawrence Berkeley National Laboratory have incorporated mixed halide-perovskite solar cells, based on CsPbI3−xBrx, into smart windows. When the material is heated, it changes from being transparent to coloured. In the coloured state, the material is a semiconductor that can efficiently convert sunlight into electricity. The new perovskite-based smart window material has shown that it can repeatedly be switched from transparent to coloured state with no degradation in its properties. The low-temperature transparent state is found to produce little electricity, while the dark-coloured, high-temperature phase effectively converts sunlight into electricity. These materials could be used for a broad range of applications in cars, buildings, and displays.
This work was supported by the Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, including use of the Stanford Synchrotron Radiation Light Source (grazing-incidence wide-angle X-ray data) and Advanced Light Source (X-ray photoelectron spectroscopy data), each is a DOE Office of Science user facility. This work was also supported by the Swedish Research Council (rotation electron diffraction data), Shanghai University of Electric Power and Suzhou Industrial Park (fellowships), Alexander von Humboldt Foundation, Wallenberg Foundation (postdoctoral scholarship), Camille and Henry Dreyfus Foundation, and the DOE Office of Energy Efficiency and Renewable Energy (postdoctoral research award).
J. Lin, M. Lai, L. Dou, C.S. Kley, H. Chen, F. Peng, J. Sun, D. Lu, S.A. Hawks, C. Xie, F. Cui, A.P. Alivisatos, D.T. Limmer, and P. Yang, "Thermochromic halide perovskite solar cells." Nature Materials 17, 261 (2018). [DOI: 10.1038/s41563-017-0006-0]