C&I, Solar, Storage

Transparent organic PV window forecast at 12% efficiency

Researchers at Massachusetts Institute of Technology (MIT) created an early prototype organic photovoltaic cell that harnesses the energy of infrared light while allowing visible light to pass through. Coated onto a pane of standard window glass, it could provide power for lights and other devices, and would lower installation costs by taking advantage of existing window structures.

April 18, 2011 — Researchers at Massachusetts Institute of Technology (MIT) created an early prototype organic photovoltaic cell that harnesses the energy of infrared light while allowing visible light to pass through. Coated onto a pane of standard window glass, it could provide power for lights and other devices, and would lower installation costs by taking advantage of existing window structures.

Up to half of the cost of solar panels is glass and structural parts, said Vladimir Bulovi?, professor of electrical engineering in the Department of Electrical Engineering and Computer Science. MIT’s team estimates that 50-66% of the cost of a thin-film solar-power system comes from installation costs. The transparent photovoltaic system, developed with Richard Lunt, a postdoctoral researcher in the Research Laboratory of Electronics, could eliminate many of those associated costs, they say.

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Photo. Richard Lunt, one of the researchers who developed the new transparent solar cell, demonstrates its transparency using a prototype cell. Courtesy of Geoffrey Supran.

In a new building, or one where windows are being replaced anyway, adding the transparent solar cell material to the glass would be a relatively small incremental cost, since the cost of the glass, frames, and installation would all be the same with or without the solar component, the researchers say, although it is too early in the process to be able to estimate actual costs. And with modern double-pane windows, the photovoltaic material could be coated on one of the inner surfaces, where it would be completely protected from weather or window washing. Only wiring connections to the window and a voltage controller would be needed to complete the system in a home.

Previous attempts to create transparent solar cells have either had extremely low efficiency (less than 1% of incoming solar radiation is converted to electricity), or have blocked too much light to be practical for use in windows. But the MIT researchers were able to find a specific chemical formulation for their cells that, when combined with partially infrared-reflective coatings, gives both high visible-light transparency and better efficiency than earlier versions.

“It’s attractive, because it can be added to [buildings] already being deployed,” rather than requiring land and infrastructure for a whole new system, Bulovi? says. Using the window surfaces of existing buildings could provide much more surface area for solar power than traditional solar panels, Bulovi? says. In mornings and evenings, with the sun low in the sky, the sides of big-city buildings are brightly illuminated, he says, and that vertical “footprint” of potential light-harvesting area could produce a significant amount of power. Installations of the new system would also block much of the heating effect of sunlight streaming through the windows, potentially cutting down on air conditioning needs within a building.

MIT’s team has achieved an efficiency of 1.7% in the early-prototype solar cells, but they expect that with further development they should be able to reach 12%, making it comparable to existing commercial solar panels. “It’s a question of excitonic engineering,” noted Lunt, requiring optimization of the composition and configuration of the photovoltaic materials.

The researchers expect that after further development in the lab followed by work on manufacturability, the technology could become a practical commercial product within a decade. In addition to being suitable for coating directly on glass in the manufacture of new windows, the material might also be coated onto flexible material that could then be rolled onto existing windows, Lunt says.

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Image. A prototype of the MIT researchers’ transparent solar cell is seen on top of a promotional item for MIT’s 150th anniversary celebrations. Courtesy of Geoffrey Supran.

The manufacturing process for the MIT researchers’ solar cells could be more environmentally friendly than those used to grow silicon. The MIT process of fabricating solar cells keeps the glass panes at ordinary room temperature, Bulovi? noted.

A paper by Bulovi? and Lunt describing their new system has been published online in the journal Applied Physics Letters, and will appear in a forthcoming issue of the print edition.

The research was funded by the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy.

Learn more at http://web.mit.edu/.

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