Transparent, color solar cells for energy-harvesting decor3/1/2014
Color, see-through solar cells invented at Michigan Engineering could enable ‘stained’ glass windows, decorations and even shades that make electricity.
The cells, believed to be the first semi-transparent, colored photovoltaics, have the potential to vastly broaden the use of the energy source, said Jay Guo, a professor of electrical engineering and computer science, mechanical engineering, and macromolecular science and engineering at U-M. Guo is lead author of a paper about the work published in Scientific Reports.
“I think this offers a very different way of utilizing solar technology rather than concentrating it in a small area,” he said. “Today, solar panels are black and the only place you can put them on a building is the rooftop. And the rooftop a typical high-rise is so tiny!
“We think we can make solar panels more beautiful – any color a designer wants. And we can vastly deploy these panels, even indoors.”
He envisions them on the sides of buildings, as energy-harvesting billboards and as window shades – a thin layer on top of homes and cities. Such an approach, he says, could be especially attractive in densely populated cities.
In a palm-sized American flag slide, the team demonstrated the technology.
“All the red stripes, the blue background and so on – they are all working solar cells,” Guo said.
The Stars and Stripes achieved 2 percent efficiency. A meter-square panel could generate enough electricity to power fluorescent lightbulbs or small electronic gadgets, Guo said. State-of-the art organic cells in research labs are roughly 10 percent efficient.
The researchers are working to improve their numbers with new materials, but there will always be a trade-off between beauty and utility in this case. Traditional black solar cells absorb all wavelengths of visible light. Guo’s cells are designed to transmit, or in other versions, reflect certain colors. So by nature they’re kicking the energy in those wavelengths out to our eyes rather than converting it to electricity.
Unlike other colored solar cells, Guo’s don’t rely on dyes or microstructures that can blur the image behind them. The cells his team devised are mechanically structured to transmit certain wavelengths of light. To get different colors, the researchers varied the thickness of the semiconductor layer of amorphous silicon in the cells. The blue regions are six nanometers thick while the red is 31. (The team also made green, but that color isn’t included in the flag.)
Amorphous silicon is commonly used in displays of cell phones, laptops and large LCD screen panels. It’s not hard to deposit over large areas for photovoltaic applications, the researchers say. They sandwiched an ultra-thin sheet of it between two semi-transparent electrodes that could let light in and also carry away the electrical current. One of these so-called charge transport layers is made of an organic material. This hybrid structure lets the researchers avoid the thick “doped” region that’s required in tradition amorphous silicon solar cells. Doping involves adding impurities and it’s how researchers control electrical conductivity.
The ultra-thin hybrid structure not only enables the cells to hold their color, but also leads to high quantum efficiency.
The cells’ hues don’t change based on viewing angle, and this is important for several reasons. It means manufacturers could lock in color for precise pictures or patterns. It’s also a sign that the devices are soaking up the same amount of light regardless of where the sun is in the sky. Conventional solar panels pivot across the day to track rays.
“Solar energy is essentially inexhaustible, and it’s the only energy source that can sustain us long-term,” Guo said. “We have to figure out how to use as much of it as we can.”
The paper is titled, “Decorative power generating panels creating angle insensitive transmissive colors.” The work was funded by the National Science Foundation.
About Michigan Engineering: The University of Michigan College of Engineering is one of the top engineering schools in the country. Eight academic departments are ranked in the nation's top 10 -- some twice for different programs. Its research budget is one of the largest of any public university. Its faculty and students are making a difference at the frontiers of fields as diverse as nanotechnology, sustainability, healthcare, national security and robotics. They are involved in spacecraft missions across the solar system, and have developed partnerships with automotive industry leaders to transform transportation. Its entrepreneurial culture encourages faculty and students alike to move their innovations beyond the laboratory and into the real world to benefit society. Its alumni base of more than 75,000 spans the globe.