Jay Guo, a professor of electrical engineering at the University of Michigan as developed a method to improve the fabrication technology of polymer solar cells. He used a simplified process for printing them, and demonstrated it on a large-area, roll-to-roll printing system. By eliminating steps in the old fabrication technology, plastic solar cells could be manufactured easier and faster.
The process Guo innovated consists in applying a small amount of force during the printing process with a permeable membrane, to allow the printing solvents to evaporate and create ordered polymer layers from the beginning. Older methods needed further post-processing to line up the polymers at nanoscale, because otherwise the electrons couldn’t get out of the cell.
To harvest the incident light, the semiconductor that the solar cell is made of needs to have electrons and positively charged “holes”. In order to have current flowing, the cell has to separate the electrons from the holes, so they can flow further into the circuit. Inorganic materials like silicon have this intrinsic property, while polymers don’t. The scientists had to combine two polymer materials: one conducting electrons and one conducting holes, and have them placed one on top of the other, with the electron accepting polymer on top of the electron-donating polymer.
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.
Guo and his team didn’t work with high-efficiency polymers to date, and only got a 3.5 percent efficiency so far. Still, they hope to realize efficiencies somewhere in the 12-15 percent range, to compete with the other types of solar cells (thin film and silicon).