Atom-thick photovoltaic sheets — 20 to 50 times thinner than the skinniest solar cells made today — are pushing toward the ultimate power conversion possible from a material for solar power, packing hundreds of times more power per weight than conventional silicon solar cells. Most efforts to improve photovoltaics have focused on increasing energy conversion efficiency or lowering manufacturing costs, but what would happen if the size of the cell were shrunk? Engineers at MIT are using computer modeling to help answer that question, aiming to produce the thinnest and most lightweight solar panels possible. Such panels, which have the potential to surpass any substance other than reactor-grade uranium in terms of energy produced per pound of material, could be made from stacked sheets of one-molecule-thick materials such as graphene or molybdenum disulfide. Although considerable attention has been devoted to the potential of 2-D materials such as graphene over the past several years, there has been little study of their potential for solar applications, said Jeffrey Grossman, the Carl Richard Soderberg associate professor of power engineering at MIT. As it turns out, “they’re not only OK, but it’s amazing how well they do.” Two layers of such atom-thick materials could yield solar cells with 1 to 2 percent efficiency in converting sunlight to electricity, Grossman’s team predicts. That’s low compared with the 15 to 20 percent efficiency of standard silicon solar cells, he said, but it is achieved using material that is thousands of times thinner and lighter than tissue paper. The two-layer cell is only 1 nm thick, while typical silicon solar cells can be hundreds of thousands of times that. Stacking several of the 2-D layers could boost the efficiency significantly. The MIT team found that an effective solar cell could be made from a stack of two one-molecule-thick materials: Graphene (a 1-atom-thick sheet of carbon atoms, shown at bottom in blue) and molybdenum disulfide (above, with molybdenum atoms shown in red and sulfur in yellow). The two sheets together are thousands of times thinner than conventional silicon solar cells. Courtesy of Jeffrey Grossman and Marco Bernardi. “Stacking a few layers could allow for higher efficiency, one that competes with other well-established solar cell technologies,” said Marco Bernardi, a postdoc in MIT’s Department of Materials Science. Maurizia Palummo, a senior researcher at the University of Rome who is visiting MIT through the MIT International Science and Technology Initiatives (MISTI) Italy program also contributed to the research. These lightweight cells, Bernardi said, could have great potential for applications where weight is a crucial factor — such as in spacecraft or aviation or for use in remote areas of the developing world where transportation costs are significant. Pound for pound, he said, the new solar cells produce up to 1000 times more power than conventional photovoltaics. At only 1 nm in thickness, Grossman said, “You couldn’t make a solar cell any thinner.” The slender size is not only advantageous in shipping, but also in ease of mounting solar panels. About half the cost of today’s panels is in support structures, installation, wiring and control systems, expenses that could be reduced through the use of lighter structures. The material itself is also much less expensive than the highly purified silicon used for standard cells — and because the sheets are so thin, they require only minuscule amounts of the raw materials. The investigators’ work is “just the start,” Grossman said. For one thing, molybdenum disulfide and molybdenum diselenide — the robust, stable materials used in this work — are just two of many 2-D materials whose potential could be studied, to say nothing of different combinations of materials sandwiched together. “There’s a whole zoo of these materials that can be explored,” he said. “My hope is that this work sets the stage for people to think about these materials in a new way.” Although no large-scale methods for producing molybdenum disulfide and molybdenum diselenide currently exist, this is an active area of research. Manufacturability is “an essential question,” Grossman said, “but I think it’s a solvable problem.” The group is now working to produce an atom-thick device. “I think this is the tip of the iceberg in terms of utilizing 2-D materials for clean energy,” Grossman said. The work, supported by the MIT Energy Initiative, appears in Nano Letters (doi: 10.1021/nl401544y). For more information, visit: www.mit.edu