Organic semiconductors have traditionally not been considered candidate thermoelectric materials because they have been inefficient in carrying out the essential heat-to-electricity conversion process. Now, a team of researchers from the University of Michigan has improved upon the state of the art in organic semiconductors by nearly 70%, achieving a figure of merit of 0.42 in the PEDOT:PSS compound.

“That’s about half as efficient as current inorganic semiconductors,” said project leader Kevin Pipe, an associate professor of mechanical engineering as well as electrical engineering and computer science at Michigan.

PEDOT:PSS—or poly(3,4-ethylene-dioxythiophene) poly(styrene sulfonate)—has previously been used as a transparent electrode in devices such as organic light-emitting diodes and solar cells, and as an antistatic agent for materials such as photographic film. In PEDOT doped by PSS, only a small fraction of the PSS molecules actually bond to the host PEDOT; the rest of the PSS molecules do not become ionized and are inactive. The researchers report in the May 5 online edition of Nature Materials (DOI: 10.1038/NMAT3635) that these excess PSS molecules dramatically inhibit both the electrical conductivity and thermoelectric performance of the material.

“The trouble is that the inactive PSS molecules push the PEDOT molecules apart, making it harder for electrons to jump between PEDOT molecules,” Pipe said. “While ionized PSS molecules improve electrical conductivity, non-ionized PSS molecules reduce it.”

To improve its thermoelectric efficiency, the researchers restructured the material at the nanoscale. Pipe and his team used solvents to remove some of these non-ionized PSS dopant molecules from the mixture, leading to large increases in both the electrical conductivity and the thermoelectric energy-conversion efficiency.