Solar cells could someday be as easy to produce as corporate memos. Just ask a team at MIT, whose chemical vapor deposition (CVD) technique allows arrays of photovoltaic (PV) cells to be printed on ordinary untreated paper – including tissue, printer, tracing, even newsprint with the printing still on it. Printing the active components of a photovoltaic system onto flexible substrates means that solar cells could be incorporated onto a number of everyday objects. “Rather than thinking of PVs as rooftop or large-field installations, we envision PVs as being ubiquitously present on everyday surfaces such as paper, fabrics, transparent windows,” said Vladimir Bulovic, a professor of electrical engineering at MIT in Cambridge. “Interweaving PVs in common materials as value-added products would allow a gradual scale-up of PV production to long-term levels that would place solar as a significant part of our energy production portfolio.” Most of the weight of the cell is in the substrate used to support it, which is typically a piece of glass. Reducing this weight by using a flexible and light-weight substrate would reduce installation costs and make cells more portable and robust. “It is my opinion that a very significant metric in evaluating solar technologies is the amount of power they can produce per weight of the solar cell; i.e., watts per kilogram,” Bulovic said. “This metric can be further modified to include the cost so that the figure of merit becomes watts per kilogram per dollar. The weight adds cost in the installation of the PV cells, and since the installation is up to half of the installed PV module cost, reducing the weight would reduce the cost of deployment.” The printing technique is a major departure from the systems currently used to create most solar cells. It uses vapors, not liquids, and temperatures less than 120 °C in a process similar to the one used to make the silvery lining in bags of potato chips. And it has the same potential to become a roll-to-roll thin-film coating process that can produce spools of solar cells. A team at MIT has developed a technique that allows a sheet of paper to be printed with an array of CVD polymer electrodes. Courtesy of Patrick Gillooly/MIT. “The PV cells are made in a series of large-area-coating steps,” Bulovic said. “A paper substrate is first coated in a CVD reactor (developed by [MIT] professor Karen Gleason’s chemical engineering lab) with the anode film. If the intention is to connect multiple PV cells in series on the same piece of paper, the CVD anode film would be patterned by use of the shadow mask.” The paper substrate with the anode film is then transferred to Bulovic’s Organic and Nanostructured Electronics Laboratory, where it is introduced in the vacuum coating system to deposit the organic thin films and the cathode, which can again be patterned with a shadow mask if a series of PV cells are intended to be connected on the same paper substrate. The power conversion efficiency of the resulting cells is on the order of 1 percent because of the choice of chemically robust organic materials selected for this initial study. But by choosing different organic thin films, the team says it can increase this efficiency to more than 5 percent, and in the long term, reach 12 percent or more. Bulovic believes that, with five years of intensive technology development, these PVs can be introduced in consumer products whose lifetimes are expected to be on the order of a few years. And with 10 years of intensive development, the power conversion efficiency will increase and the cell longevity will be able to compete with today’s robust PV technologies.