Finally, anti-reflective coatings can be used on both outside surfaces to reduce reflections because any light that reflects-potentially as much as 10% of the total-doesn’t go through the device. Since both electrodes must be transparent-not the usual reflective metal-a layer on the back of the cell can be added to reflect sunlight of selected wavelengths, sending it back for a second pass through the active layers. Sandwiching those layers are electrodes that connect to the external circuit that carries the current out of the device. At the core of the coating are the two active layers-the absorptive semiconductor materials that get excited by sunlight and interact, creating an electric field that causes current to flow. The thickest layer (toward the left) is the glass, plastic, or other transparent substrate being coated the multiple layers of the PV coating are toward the right.
The schematic figure below shows its components and how they work together. Inspired by Lunt’s idea, the team developed a transparent PV cell. With the right materials and design, the light that we can detect would pass through the solar cell to our eyes the rest would be absorbed by the solar cell-and we’d never miss it. But the human eye can detect only part of that spectrum-the so-called visible light.
All light is made up of electromagnetic radiation spanning a spectrum of wavelengths, each containing energy that potentially can be harvested by a solar cell. Richard Lunt, then an MIT postdoc and now an assistant professor at Michigan State University, proposed making a solar cell that would absorb all the energy from the sun except the part that allows us to see. Three years ago, a team in MIT’s Organic and Nanostructured Electronics Laboratory began to tackle the problem using a different approach. A former MIT postdoctoral researcher, Lunt is now an assistant professor in the Department of Chemical Engineering and Materials Science at Michigan State University. Using a prototype cell, Richard Lunt demonstrates the transparency of the novel solar cell that he and his MIT colleagues have developed. “So with existing PV technologies, it’s difficult to optimize for efficiency and aesthetics at the same time.” “When you start with opaque PV materials, you typically have to decrease the amount of active area to increase the transparency,” says Miles Barr PhD ’12, president and CTO of Ubiquitous Energy, Inc. But those approaches involve an inherent tradeoff between transparency and efficiency. Other research groups have previously worked on making “see-through” solar cells, usually by taking conventional opaque PV materials and either making them so thin they are translucent or “segmenting” them-a process Bulović likens to mounting pieces of a solar panel on a window with gaps for seeing out. “They could be on everything around you-including all your windows-and you wouldn’t know it.” “You can have zebra stripes or elephant footprints or whatever you want underneath because the cells that sit on top are invisible,” says Vladimir Bulović, professor of electrical engineering and director of MIT’s Microsystems Technology Laboratories.
It could be deposited on any surface without obscuring the look of the underlying material. But a transparent photovoltaic (PV) cell would change the rules of the game. Inventing a new solar technology that can compete commercially with today’s solar cells is difficult, given existing deployment methods. They’re now beginning to integrate their solar cells into consumer products, including mobile device displays.
#DIGRAM OF THE INSIDE WORKINGS OF A SOLARCELL WINDOWS#
They estimate that using coated windows in a skyscraper could provide more than a quarter of the building’s energy needs without changing its look. Using simple room-temperature methods, the researchers have deposited coatings of their solar cells on various materials and have used them to run electronic displays using ambient light. Visible light passes through the cells unimpeded, so our eyes don’t know they’re there. How? Their new solar cells absorb only infrared and ultraviolet light. MIT researchers are making transparent solar cells that could turn everyday products such as windows and electronic devices into power generators-without altering how they look or function today. This research was supported by the MIT Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy. Vladimir Bulović of electrical engineering and computer science (left), Miles Barr PhD ’12 (right), and Richard Lunt (below) are making transparent solar cells that could one day be deposited on everyday objects from mobile devices to windows, turning surfaces everywhere into low-cost energy-harvesting systems.
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