There's probably not much call for printing solar cells on toilet paper, but a method developed at MIT can do just that, if it's ever needed.
More to the point, oxidative chemical vapor deposition (oCVD) could allow low-cost production of solar cells and other electronic devices on thin, flexible materials that other processes can't easily handle. Miles Barr, a graduate student in the lab of MIT chemical engineering professor Karen Gleason, described the process at the fall meeting of the Materials Research Society, in Boston.
The technique deposits conjugated polymers, plastics with good conductivity and semiconductor properties that are also flexible, stretchable, and even foldable. "We're particularly interested in polymers because of their good mechanical properties," Barr says.
The process sprays a vapor of a monomer and an oxidizing agent onto a substrate. When they meet on the surface, they polymerize, joining into long chains to form a plastic popularly known as PEDOT. Varying the surface temperature of the substrate between 20 ªC and 100 ªC dictates how the surface of the film forms; it can range from smooth to studded with nanopores. The polymer is conductive on its own, but lacing the nanopores with silver particles can increase conductivity up to a thousandfold. Barr says the process allows users to synthesize, deposit, and pattern the conjugated polymer all in one step.
To show off oCVD's abilities, Barr and his colleagues used the process on a number of extremely delicate materials. Rice paper, used to make spring rolls in restaurants, would dissolve in most processes, but because this one is free of solvents, it remained intact. A plastic film, such as Saran wrap—hard to coat because it repels water—could be coated with this dry process. The researchers even constructed a solar cell printed on toilet paper.
"This is kind of just to illustrate the versatility, not that these are substrates we necessarily want to process with electronics," Barr says. "You don't typically think of paper as a good substrate for photovoltaics, because it's not very transparent."
There may, however, be applications where the ability to build electronics, such as flexible displays, on fabrics or paper will come in handy. And engineers are increasingly looking to roll-to-roll printing—in which inks are printed onto plastic or another flexible material as it unspools from one machine and is wound up on another—as a faster, less costly method for producing some electronics, including photovoltaics.
The team built solar cells on a commonly used plastic and bent them to a radius of less than 5 millimeters more than 1000 times, then tested them to see if they still worked. Their efficiency was still greater than 99 percent of what it had been before bending, Barr said. Electrodes were bent to a radius of less than 1 mm, creased more than 100 times, and stretched to approximately 200 percent and still maintained high conductivity. A solar cell built on Saran wrap performed well even while it was stretched to about 180 percent, at which point the wrap pulled apart, destroying the cell.
To illustrate the point, Barr printed a solar cell on a piece of paper. In a video he showed at the conference, a student folded the paper into the shape of an airplane, attached leads, and shone a light on the folded device. It still generated current.
"I don't know if paper airplanes are the future of solar cells," Barr concedes. But in case they are, he's got it covered.