A new "smart" window can darken to save air conditioning costs on hot days and return to clear in the winter to capture heat from the sun, U.S. researchers say.

While such windows that reflect sunlight away from buildings in summer and switch back to full transparency in winter already are available, current examples are expensive and their performance deteriorates rapidly, researchers write in the American Chemical Society journal Nano.

Their manufacturing processes also involve potentially toxic substances, they said.

Ho Sun Lim of the Massachusetts Institute of Technology and colleagues say using a polymer, so-called "counter-ions" and a solvent such as methanol is an inexpensive and less harsh way to make a stable, robust smart window.

It's extremely "tunable," the researchers say, quickly and easily switching from 100 percent opaque to almost completely clear in seconds.

"To our knowledge, such extreme optical switching behavior is unprecedented among established smart windows," the researchers say in the Nano article. "This type of light control system may provide a new option for saving on heating, cooling and lighting costs through managing the light transmitted into the interior of a house."