• Expanding on its previous research, NREL says it has been able to introduce various colors for the thermochromic photovoltaics
  • Using metal halide perovskites, it injected vapor to trigger a reaction in the former to arrange itself into different shapes and colors
  • When used on windows of a building or a vehicle, it can lower the need to blast the air conditioner thereby reducing energy use from the grid
  • Further research is required to cycle the thermochromic window into an electricity-generating operating mode and return to transparent, as well as the efficiency of converting sunlight into electricity

The US National Renewable Energy Laboratory (NREL) says it has achieved a breakthrough in developing a next-generation thermochromic window calling it thermochromic photovoltaic using metal halide perovskites to be used for windows that can lower the need for air conditioning as well as to generate electricity.
The research builds on a 2017 work of the NREL (termed first-generation) under which the thermochromic window darkened under the heat of the sun, leading the embedded perovskites to generate electricity. Now, the team has furthered the research (calling it next-generation) to enable the perovskite window to assume various colors and deal with a broader range of temperatures that drive the color switch.
To achieve this aim, the team placed a thin perovskite film between 2 layers of glass and injected vapor with the latter triggering a reaction due to which perovskite is forced to arrange itself into different shapes from a chain to a sheet to a cube. With changing shapes, the colors also emerge, and lowering the humidity returns the perovskite to its normal transparent state.
Pointing out the fact that heat generated by sunlight shining through the windows is the single largest contributor to the need for air conditioning in a building or a vehicle, with US residential buildings using 74% of all electricity and commercial buildings using 39% of all energy in the country, the thermochromic photovoltaic can allow the window to change color to block glare and lower the need to blast the air conditioner when the glass gets too hot.
This color change aspect, as the NREL points out, leads to the formation of a functioning solar cell that can generate electricity on receiving the sunlight.
“The first-generation solar window was able to switch back and forth between transparent and a reddish-brown color, requiring temperatures between 150º and 175º Fahrenheit to trigger the transformation. The latest iteration allows a broad choice of colors and works at 95º to 115º Fahrenheit, a glass temperature easily achieved on a hot day,” explain the researchers.
However, there is still scope for improvement as NREL sees the next step of research concentrating on the number of times the thermochromic window can be cycled into an electricity-generating operating mode and return to transparent, as well as the efficiency of converting sunlight into electricity.
The team believes thermochromic photovoltaic windows can help buildings turn into energy generators, increasing their contribution to the broader energy grid’s needs.
The NREL research titled Reversible Multicolor Chromism in Layered Formamidinium Metal Halide Perovskites has found its way to the scientific journal Nature Communications.
Earlier this month, NREL researchers created a new perovskite material called Apex Flex to reach 23.1% power conversion efficiency to be deployed for vehicular applications claiming to cost 200 times less than spaceship grade solar cells (see 23.1% Efficiency For All-Perovskite Tandem Solar Cells).