Solar-powered smart window technology takes a big leap forward


Relying on perovskites and single-walled carbon nanotubes, scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a smart window, which converts sunlight into electricity while going from transparent to tinted in order to minimize heat coming into the structure.

The color change comes from molecules (methylamine), which are driven out of the window device when exposed to sunlight (less than 3% visible transmittance), and reversibly absorbed into the device, returning the absorber layer —composed of a metal halide perovskite-methylamine complex— to the transparent state (68% visible transmittance) as the device cools back down.   

The prototypes tested showed a solar power conversion efficiency of 11.3%.

“There is a fundamental trade-off between a good window and a good solar cell. 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,” said Lance Wheeler, the lead author of the paper, Switchable Photovoltaic Windows Enabled by Reversible Photothermal Complex Dissociation from Methylammonium Lead Iodide, published in Nature Communications.

Although, according to Wheeler, the new technology holds the potential to be integrated into vehicles, buildings and beyond, cycle stability, however, still needs improvement. In testing under 1-sun illumination, the 1-square-centimeter demonstration device cycled through repeated transparent-tinted cycles, but the performance declined over the course of 20 cycles due to restructuring of the switchable layer chemical issues, thus leading to degradation.

Meanwhile, the findings from University of Cambridge scientists, currently based at the U.S. Department of Energy’s Argonne National Laboratory, have shed more light on the little known molecular mechanisms between the electrodes and electrolyte that combine to determine how a dye-sensitized solar cell operates.

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“Most previous studies have modeled the molecular function of these working electrodes without considering the electrolyte ingredients,” said Jacqui Cole, research team lead. “Our work shows that these chemical ingredients can clearly influence the performance of solar cells, so we can now use this knowledge to tune the ions to increase photovoltaic efficiency.”

The research findings, published in Nanoscale, indicate how a thin-film electrode containing titanium dioxide, a naturally occurring compound found in paint, sunscreen and food coloring, can have a huge impact on solar cell efficiency.

“Prior research considered the working electrodes outside the device, so there has been no path to determine how the different device components interact,” Cole said. “Our work signifies a huge leap forward as it’s the world's first example of applying in situ neutron reflectometry to dye-sensitized solar cells.”

The cells recently broke a world record with a power conversion efficiency of 14.3% using a dye-sensitized electrode featuring two co-sensitized metal-free organic dyes. These dyes “promise cheaper, more environmentally friendly synthetic routes and greater molecular design flexibility than their metal-containing counterparts,” according to the paper.

“We just need a modest boost in performance to make these solar cells competitive,” Cole said, “since price-to-performance governs the economics of the solar cell industry. And manufacturing dye-sensitized solar cells is very cheap relative to other solar cell technologies.”

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