Smart perovskite windows change colour

Technology News |
By Nick Flaherty

The researchers at NREL in the US have developed a perovskite material that can be used to shade windows and generate power at the same time.

Heat generated by sunlight shining through windows is the single largest contributor to the need for air conditioning and cooling in buildings. Residential and commercial buildings use 74% of all electricity and 39% of all energy in the US, so having a programmableshading effect from tinting windows helps buildings use less energy.

The ‘thermochromic photovoltaic’ technology allows the window to change colour to block glare and reduce unwanted solar heating when the glass gets warm on a hot, sunny day. The material can also be used to generate power.

The newest version provides a range of colours and a broader range of temperatures that drive the colour switch. This increases design flexibility for improving energy efficiency as well as control over building aesthetics that is highly desirable for both architects and end users.

“A prototype window using the technology could be developed within a year,” said Bryan Rosales, a postdoctoral researcher at NREL and lead author of the paper, “Reversible Multicolor Chromism in Layered Formamidinium Metal Halide Perovskites.”

The first-generation solar window was able to switch back and forth between transparent and a reddish-brown colour, requiring temperatures between 65 ºC and 80 ºC, which wasn’t hugely practical. The latest version has a broader choice of colours and works at 35 ºC to 47 ºC.

By using a different chemical composition and materials, the researchers also were able to rapidly speed up the colour transformation. The time was reduced to about seven seconds from the three minutes it took during the proof-of-concept thermochromic photovoltaic window demonstrated in 2017.

The team sandwiched a thin perovskite film between two layers of glass and injected vapour. The vapour triggers a reaction that causes the perovskite to arrange itself into different shapes, from a chain to a sheet to a cube. The different colours emerge with the changing shapes. Lowering the humidity returns the perovskite to its normal transparent state.

Additional research is planned. One area to be explored is the number of times the thermochromic window can be cycled into an electricity-generating operating mode and return to transparent. The efficiency of converting sunlight into electricity will also be explored.

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