Researchers Develop Switchable Photovoltaic Window

A team of researchers at the U.S. DoE’s National Renewable Energy Laboratory has developed a thermochromic window capable of converting sunlight into electricity at a high efficiency.

Dr. Wheeler (front) developed a switchable photovoltaic window along with (from left) Nathan Neale, Robert Tenent, Jeffrey Blackburn, Elisa Miller, and David Moore. Image credit: Dennis Schroeder, National Renewable Energy Laboratory.

“Relying on such advanced materials as perovskites and single-walled carbon nanotubes, the new technology responds to heat by transforming from transparent to tinted; as the window darkens, it generates electricity,” said Dr. Lance Wheeler, a researcher in materials science at the National Renewable Energy Laboratory, and colleagues.

“The color change is driven by methylamine molecules that are reversibly absorbed into the device. When solar energy heats up the device, the molecules are driven out, and the device is darkened.”

“When the Sun is not shining, the device is cooled back down, and the molecules re-absorb into the window device, which then appears transparent.”

The demonstration device allows an average of 68% of light in the visible portion of the solar spectrum to pass through when it’s in a transparent, or bleached, state.

When the window changes color — a process that took about 3 min of illumination during testing — only 3% is allowed through the window.

Existing solar window technologies are static, which means they are designed to harness a fraction of the sunlight without sacrificing too much visible light transmission needed for viewing or the comfort of building occupants.

“There is a fundamental tradeoff 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,” Dr. Wheeler said.

The proof-of-concept paper published in the journal Nature Communications established a solar power conversion efficiency of 11.3%.

“There are thermochromic technologies out there but nothing that actually converts that energy into electricity,” Dr. Wheeler said.

In testing under one-sun illumination, the one-cm² 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.

“The technology could be integrated into vehicles, buildings, and beyond,” Dr. Wheeler said.

“The electricity generated by the solar cell window could charge batteries to power smartphones or on-board electronics such as fans, rain sensors, and motors that would open or close the windows as programmed.”

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Lance M. Wheeler et al. 2017. Switchable photovoltaic windows enabled by reversible photothermal complex dissociation from methylammonium lead iodide. Nature Communications 8, article number: 1722; doi: 10.1038/s41467-017-01842-4