Lance Wheeler looks at glassy skyscrapers and sees untapped potential. Houses and office buildings, he says, account for 75% of electricity use in the United States, and 40% of its energy use overall. Windows, because they leak energy, are a big part of the problem. "Anything we can do to mitigate that is going to have a very large impact," says Wheeler, a solar power expert at the National Renewable Energy Laboratory in Golden, Colorado.

A series of recent results points to a solution, he says: Turn the windows into solar panels. In the past, materials scientists have embedded light-absorbing films in window glass. But such solar windows tend to have a reddish or brown tint that architects find unappealing. The new solar window technologies, however, absorb almost exclusively invisible ultraviolet (UV) or infrared light. That leaves the glass clear while blocking the UV and infrared radiation that normally leak through it, sometimes delivering unwanted heat. By cutting heat gain while generating power, the windows "have huge prospects," Wheeler says, including the possibility that a large office building could power itself.

Most solar cells, like the standard crystalline silicon cells that dominate the industry, sacrifice transparency to maximize their efficiency, the percentage of the energy in sunlight converted to electricity. The best silicon cells have an efficiency of 25%. Meanwhile, a new class of opaque solar cell materials, called perovskites, are closing in on silicon with top efficiencies of 22%. Not only are the perovskites cheaper than silicon, they can also be tuned to absorb specific frequencies of light by tweaking their chemical recipe.