A spectrally selective approach could let tablets, e-readers, and windows turn light into power.
Imagine a world where any surface could be coated with solar cells, converting sunlight and even the glow of light bulbs into small amounts of usable energy. This is the goal of a new startup called Ubiquitous Energy. The company hopes to develop affordable, transparent coatings and films that could harvest light energy when applied to windows or the screens of e-readers or tablet devices. One way to use the technology might be in electrochromic windows that turn from clear to dark when the sun is brightest.
The trick is the way the company's photovoltaics take up light: they collect wavelengths in the ultraviolet and infrared portion of the spectrum but let visible light pass through. Traditional solar cells, in contrast, collect light in the ultraviolet and visible regions and therefore can't be made completely transparent.
"It's definitely an interesting approach if the cost of such cells can be low enough and the stability of the materials is sufficient," says Zhenan Bao, a professor of chemical engineering at Stanford University, who is not affiliated with the startup. She adds that by collecting infrared and ultraviolet light, the technology is filtering the parts of the spectrum people generally want windows to keep out anyway.
Miles Barr, president and chief technology officer of Ubiquitous Energy, says the transparent solar cells are made of various organic layers, deposited one at a time on top of a glass or film. This process could easily be integrated into thin-film deposition systems found in industrial processing. Many modern windows, for instance, have some sort of coating for solar control or insulation; Barr envisions his company's solar cells being manufactured and added similarly. Ubiquitous Energy, which was spun out of the lab of Vladimir Bulović, a professor of electrical engineering at MIT, hasn't yet announced plans for products or pricing.
A paper published in Applied Physics Letters in 2011 described the company's spectrally selective approach: prototypes made of organic materials yielded slightly less than 2 percent efficiencies and a visible transparency of about 70 percent. (Building windows usually require transparencies from 55 to 90 percent, while mobile electronic displays require 80 to 90 percent.) Barr says his team is nudging both efficiency and transparency numbers upward.
While the company is still in the research and development phase, it is exploring various materials and designs for products. "We're getting a catalogue of device structures and ingredients for higher-efficiency devices that can power more power-hungry devices or offset energy for buildings," says Miles. "Once you hit 10 percent efficiency, a lot of applications open up." The company hopes to achieve efficiencies greater than 10 percent at "high visible transparency."
There are other types of see-through solar cells, but many of them still harvest some light in the visible range and therefore don't have the transparency potential of an approach that ignores visible light. These materials achieve semitransparency when they are sparsely deposited over a surface or when the photovoltaic devices are thinned to allow more visible light to pass through.
"Current photovoltaic technology extensively utilizes the ultraviolet-visible range, but not much in the infrared range," says Shenqiang Ren, a professor of chemistry at the University of Kansas, who is unaffiliated with the company. "Under solar radiation, there is about 45 percent radiant energy from infrared [light]."
As Ubiquitous Energy seeks to improve the efficiency of its solar cells, Barr explains, it is looking at two standard ways to collect more light. The first is optimizing the design of its semiconductor materials. Its current materials include molecular dyes that have selective absorption peaks in the ultraviolet and near-infrared parts of the spectrum; Barr says the company is developing materials that also gather energy deeper into the infrared. The second involves nanoscale engineering and tweaking optical interference within the device to improve light absorption--tricks employed to improve the efficiency of nontransparent solar cells in the past. "There are a lot of knobs you can tune to boost performance," he says.
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