Solar radiation transmitted through windows is the single largest contributor to the need for air conditioning and cooling in buildings. Researchers at the U.S. National Renewable Energy Laboratory (NREL), Colorado School of Mines Laboratory and SLAC National Accelerator Laboratory are advancing a fenestration system based on photovoltaic technology to help keep interior spaces cooler while generating electricity.

Thermochromic photovoltaics based on metal halide perovskites allow a window to change color to block glare and reduce unwanted solar heating when the glass gets warm on a hot, sunny day. This color change also leads Thermochromic photovoltaic windows switch colors in response to heat and generate electricity. Source: Dennis Schroeder, NRELto the formation of a functioning solar cell that generates on-board power. These window systems can turn buildings into energy generators, increasing their contribution to the energy grid.

The research builds upon earlier work at NREL into a thermochromic window that darkened as the sun heated its surface to reach 150° F to 175° F. As the window shifted from transparent to tinted, perovskites embedded within the material generated electricity. A new breakthrough now enables the display of more colors and a broader range of temperatures that drive the color switch, increasing design flexibility for improving energy efficiency and control over building aesthetics. The thermochromic mechanism is now triggered when the glass temperature reaches the 95° F to 115° F range.

A thin perovskite film is sandwiched between two layers of glass and injected vapor. When the target temperature is reached, the vapor causes the perovskite crystals to form different shapes. Morphing into a chain, then a sheet and then a cube, each new shape changes the color, blocking light and cooling the interior.

The research is published in Nature Communications.