A new kind of photovoltaic cell based on sheets of flexible graphene coated with a layer of nanowires have been produced for the first time by MIT researchers, but a caveat is lower efficiency in first examples. The approach holds the promise of low-cost, transparent and flexible solar cells that could be deployed on windows, roofs or other surfaces, and make solar power even more attractive and competetive. Building Integrated Photovoltaic (BIPV) power is already set on a steep growth path, since construction elements with integrated solar cells are not much more expensive than for example ordinary glass elements, but this trend will be amplified by even lower-cost and easier and more flexible to integrate solar cell technologies.

Details about the new approach were just published in a report in the journal Nano Letters, co-authored by MIT postdocs Hyesung Park and Sehoon Chang, associate professor of materials science and engineering Silvija Gradečak, and a team of eight other MIT researchers.

Key advantage of the new technique is substituting rather expensive or rare materials like silicon or indium with cheap carbon. Most of today's solar cells are made of silicon, which is abundant but needs to be highly purified and then made into crystals that are sliced thin, an energy and cost intensive process. Most nanostructured or hybrid solar cells use indium tin oxide (ITO) as a transparent electrode, which is also used for touch screens.

"Currently, ITO is the material of choice for transparent electrodes," Gradečak says, such as in the touch screens now used on smartphones. But the indium used in that compound is expensive, while graphene is made from ubiquitous carbon.

Graphene could replace ITO, and even has some more advantages apart from being cheaper: It is flexible, light and chemically robust. To make this possible, the MIT team used a series of polymer coatings to modify the properties of graphene, allowing them to bond a layer of zinc oxide nanowires to it, and then an overlay of a material that responds to light waves.

"We've demonstrated that devices based on graphene have a comparable efficiency to ITO," Silvija Gradečak says - in the case of the quantum-dot overlay, an overall power conversion efficiency of 4.2 percent - less than the efficiency of general purpose silicon cells, and only competitive for specialized applications. "We're the first to demonstrate graphene-nanowire solar cells without sacrificing device performance."

László Forró, a professor at the Ecole Polytechnique Fédérale de Lausanne, in Switzerland, who was not associated with this research, says that the idea of using graphene as a transparent electrode was "in the air already," but had not actually been realized.

"In my opinion this work is a real breakthrough," Forró says. "Excellent work in every respect."

He cautions that "the road is still long to get into real applications, there are many problems to be solved," but adds that "the quality of the research team around this project ... guarantees the success."