A simplified process for printing polymer solar cells could further reduce the costs of making the plastic photovoltaics. The method, which has been demonstrated on a large-area, roll-to-roll printing system, eliminates steps in the manufacturing process. If it can be applied to a wide range of polymer materials, it could lead to a fast and cheap way to make plastic solar cells for such applications as portable electronics, photovoltaics integrated into building materials, and smart fabrics.

Polymer solar cells aren't as efficient as silicon ones in converting sunlight into electricity, but they're lightweight and cheap, a trade-off that could make them practical for some applications. And they're compatible with large-area printing techniques such as roll-to-roll processing. But manufacturing the solar cells is challenging, because if the polymers aren't lined up well at the nanoscale, electrons can't get out of the cell. Researchers now use post-printing processing steps to achieve this alignment. Eliminating these extra steps will, University of Michigan researchers hope, bring down manufacturing costs and complexity.

"Our strategy solves a number of issues at the same time," says L. Jay Guo, professor of electrical engineering at the University of Michigan. Their process involves applying a small amount of force during the printing process with a permeable membrane. The process allows the printing solvents to evaporate and leads to well-ordered polymer layers--without any need for post-processing. These improvements in the structure of the cell's active layer have an additional benefit: cells made using this technique require one fewer layer of materials than polymer solar cells made using other methods. This work is described online in the journal Advanced Materials.

When light of a certain wavelength strikes the semiconducting material in a solar cell, it creates electrons and positively charged holes. To generate an external current, the cell must separate the electrons from the holes so that they can exit. This separation doesn't happen as readily in polymers as it does in inorganic materials like silicon, says Guo. The active layers in polymer solar cells combine two materials, one that conducts holes and one that conducts electrons. Ideally the electron-accepting polymer would be on top of the electron-donating polymer, so that it's near the cathode, allowing as many electrons to exit as possible.

Guo's group found that spreading the polymer mix onto a plastic substrate, then pressing it against a roller coated with silicone, facilitates the formation of this desirable structure. And the pressure from the roller encourages the polymers to crystallize in a matter of seconds, without the need for the time-consuming chemical or thermal treatments. The structure of the polymers is so good, says Guo, that the Michigan researchers could eliminate a layer from the cells without any change in power-conversion efficiency.

So far, Guo has used a common but relatively low-efficiency polymers to fabricate the solar cells, but he says the method should be compatible with higher efficiency polymer materials. The Michigan cells have an efficiency of only about 3.5 percent. Researchers are working on materials sets that should bring the efficiencies of polymer solar cells up to 12 to 15 percent, a boost that's necessary if polymer solar cells are to reach a broad market and more fully compete with conventional silicon and thin-film cells.

"I think this process has very strong potential," says Yang Yang, professor of materials science and engineering at the University of California, Los Angeles. "It's uncertain whether this method also works for other polymer systems, but there is no reason why it won't." Yang is collaborating with plastic solar-cell company Solarmer of El Monte, CA, which is on track to reach 10 percent efficiency with its devices by the end of this year.