Glass houses, power windows

Transparent semiconductors could enable solar power from windows.

Sun reflecting off of glass office buildings. (Photo by John Dominis//Time Life Pictures/Getty Images)

It is for good reason Vancouver is called the City of Glass. On a clear evening, there’s something magical about the way the sunset illuminates the skyline. The windowed structures seem to come alive, glowing yellow, pink and mauve—a sun-on-glass spectacle replicated to various degrees at dawn and dusk in urban centres around the world. But what if the glittering towers of Vancouver, Toronto and New York were not just pretty, but also a source of green energy? This is the thinking behind a new solar technology, which seeks to use windows to generate electricity without inhibiting our ability to see through them.

Though solar-cell technology has been available for some time, it has generally come from using either opaque materials or ultra-thin cells that absorb all visible light. “When you do that, there’s always a direct trade-off between transparency and power production,” says Richard Lunt, a post-doctoral student at MIT whose research seeks to address this concern. “That is, if you want to make it more transparent, you’re going to get less power.”

Instead, Lunt and electrical engineering professor Vladimir Bulovic, whose findings were recently published in the online journal Applied Physics Letters, built their solar cells from a class of materials called “excitonic organic semiconductors,” transparent molecules that can be applied like a coating to the inside of a windowpane. Rather than capturing all the sunlight, the active layers of these cells absorb only the near infrared, a portion of the light spectrum not visible to the human eye. Meanwhile, says Lunt, “All the visible light passes freely through the active layers and through the glass [of the window].”

A problem with using organic semiconductors in this way is that when near-infrared light is absorbed, it creates excitons, quasi-particles held together by an electrostatic force. But by figuring out how to break up the excitons, the researchers have hit upon a means of creating an electrical current, which can in turn be used “to power something like lighting or everyday electronics,” says Lunt.

At present, these solar cells get less than 2% efficiency, which is about one-fifth of the efficiency offered by the standard solar cells on the market. But he says there’s reason to believe that the efficiency could be increased to as much as 12%. “At this point, we’re only using a small sliver of the near infrared, so if we were to expand our range of absorption, if we were to [stack] multiple cells together that absorb deeper into the infrared, that would boost our efficiency,” he says, pointing out that further optimizing the structure of the cells will also make them more efficient.

When it comes to making solar energy more widespread, the most significant obstacle has always been the price tag. “There’s enough solar energy to power the whole country, but we don’t do it because it costs too much,” says Lunt. So when developing these new solar cells, Lunt says the idea was to create something that could be integrated “in a way where all the infrastructure is already in place.” By designing cells that can be applied to traditional windowpanes, either upon construction or within existing structures, the hope, says Lunt, is that “we can reduce a lot of the added balance-of-systems costs.” In the meantime, applying the coating to the inside layer of double-pane windows would protect it from weather and washing.

According to the researchers, it will be another five to 10 years before their technology is capable of enabling skyscrapers to produce their own power. Aside from increasing the efficiency of the solar cells, Lunt, who will be continuing this work at Michigan State University this fall, says it must also be determined how long they can last. “These are really new materials, and we haven’t really characterized what the lifetime is,” he says. But the successful integration of LEDs, which he says are very similar molecules, in electronics suggests that “we can solve that as well.”

While Lunt stresses that the transparent solar cells are but “one tool in the clean-energy toolbox,” the potential applications are vast. Ultimately, he says the technology could one day be used in homes, and in shopping malls, which often come equipped with football-sized skylights. But the greatest impact will be in tall buildings. “You think about them glistening in the morning and evening,” he says, “and you’re taking about a very considerable amount of energy.”