Imagine a future in which solar cells are all around us — on windows and walls, cell phones, laptops, and more. A new flexible, transparent solar cell developed at MIT is bringing that future one step closer.

The device combines low-cost organic (carbon-containing) materials with electrodes of graphene, a flexible, transparent material made from inexpensive and abundant carbon sources. This advance in solar technology was enabled by a novel method of depositing a one-atom-thick layer of graphene onto the solar cell — without damaging nearby sensitive organic materials. Until now, developers of transparent solar cells have typically relied on expensive, brittle electrodes that tend to crack when the device is flexed. The ability to use graphene instead is making possible truly flexible, low-cost, transparent solar cells that can turn virtually any surface into a source of electric power.

Photovoltaic solar cells made of organic compounds would offer a variety of advantages over today’s inorganic silicon solar cells. They would be cheaper and easier to manufacture. They would be lightweight and flexible rather than heavy, rigid, and fragile, and so would be easier to transport, including to remote regions with no central power grid. And they could be transparent. Many organic materials absorb the ultraviolet and infrared components of sunlight but transmit the visible part that our eyes can detect.

Organic solar cells could therefore be mounted on surfaces all around us and harvest energy without our noticing them.

Researchers have made significant advances over the past decade toward developing transparent organic solar cells. But they’ve encountered one persistent stumbling block: finding suitable materials for the electrodes that carry current out of the cell.

“It’s rare to find materials in nature that are both electrically conductive and optically transparent,” says Professor Jing Kong of the Department of Electrical Engineering and Computer Science (EECS).

The most widely used current option is indium tin oxide (ITO). ITO is conductive and transparent, but it’s also stiff and brittle, so when the organic solar cell bends, the ITO electrode tends to crack and lift off. In addition, indium is expensive and relatively rare.

A promising alternative to ITO is graphene, a form of carbon that occurs in one-atom-thick sheets and has remarkable characteristics. It’s highly conductive, flexible, robust, and transparent; and it’s made from inexpensive and ubiquitous carbon. In addition, a graphene electrode can be just 1 nanometer thick — a fraction as thick as an ITO electrode and a far better match for the thin organic solar cell itself.