Graphene sheets yield cheap, flexible solar cells

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Researchers at MIT have developed a new kind of photovoltaic cell based on sheets of flexible graphene coated with a layer of nanowires.

The advance, they believe, could lead to low cost, transparent and flexible solar cells that can be deployed on windows, roofs or other surfaces. While most of today's solar cells are made of silicon, they remain expensive because the silicon is highly purified and then made into thinly sliced crystals. Many researchers are exploring alternatives, such as nanostructured or hybrid solar cells, which rely on a material called indium tin oxide (ITO). Now, the team from MIT has developed a material which they claim may be a cheaper alternative to ITO, and could provide other advantages such as flexibility, low weight, mechanical strength and chemical robustness. In the past, graphene's stable and inert structure has meant that building semiconducting nanostructures directly on it without impairing its electrical and structural properties has been challenging. To overcome this, lead researcher Silvija Gradecak and 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 the innate properties of graphene remained intact, providing significant advantages in the resulting hybrid material. "We've demonstrated that devices based on graphene have a comparable efficiency to ITO," she noted — in the case of the quantum-dot overlay, an overall power conversion efficiency of 4.2% — less than the efficiency of general purpose silicon cells, but competitive for specialised applications. "We're the first to demonstrate graphene-nanowire solar cells without sacrificing device performance." In addition, Gradecak says the manufacturing process can be carried out at temperatures below 175°C, and is easily scalable. "I believe within a couple of years we could see commercial devices based on this technology," she concluded.