A new flexible, transparent solar cell developed at MIT is bringing a future of alternative energy one step closer. This is after MIT researchers developed a device that 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.
Researchers had initially 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,” Professor Jing Kong of electrical engineering and computer science (EECS) once said.
The most widely used option today has been 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.
“Building semiconducting nanostructures directly on a pristine grapheme surface without impairing its electrical and structural properties has been challenging due to graphene’s stable and inert structrure”, Gradecak an associate professor of materials science and engineering said. So instead her team used a series of polymer coatings to modify its properties, allowing them to bond a layer of zinc oxide nanowires to it, and then an overlay of a material that responds to light waves- either lead-sulfide quantum dots or a type of polymer called P3HT.
Despite these modifications, Gradecak says, graphene’s properties remain intact, providing significant advantages in the resulting hybrid material.
“We’ve demonstrated that devices based on grapheme have a comparable efficiency to ITO,” she said- in the case of quantum-dot overlay, an overall power conversion efficiency of 4.2 percent- less than the efficiency of general purpose silicon cells, but competitive for specialized applications.
“We’re the first to demonstrate grapheme nanowire solar cells without sacrificing device performance.
While these kind of solar cells only have a power conversion efficiency of 4.2%, much lower than photovoltaic currently on the market which typically have efficiencies of 12% to 20%, the lower cost, transparency, and flexibility give them advantages for special applications. Also, after scaling up to commercial operation, years down the road, they are expected to have certain advantages in the manufacturing process of this alternative energy, because they can be produced at temperatures below 175 degrees Celsius, and applied to a wide variety of substrates such as plastic or glass, and other unconventional surfaces.
Commercial applications are expected in “a couple of years,” but it is worth noting with any “proof-of-concept” stage inventions, it would not be surprising if the ramp up to commercial innovations runs into unexpected challenges, and delays. Still, one aspect of solar energy I enjoy and find inspirational is the steady stream of encouraging and optimistic news we seem to hear almost every day.
This alternative energy research was supported by both National Science Foundation and the Eni-MIT Alliance Solar Frontiers Program, and used facilities provided by the MIT Center for Materials Science Engineering
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