A research team from the University of Michigan has nearly doubled the efficiency of certain organic thermoelectric materials. A paper on their work is published in the journal Nature Materials.
The most efficient thermoelectric materials currently are made of relatively rare inorganic semiconductors such as bismuth, tellurium and selenium that are expensive, brittle and often toxic. These convert heat into electricity more than four times as efficiently as the organic semiconductors created to date.
The thermoelectric figure of merit is approximately 1 near room temperature for state-of-the-art inorganic thermoelectric materials, but only 0.25 for organic semiconductors.
U-M researchers improved upon the state-of-the-art in organic semiconductors by nearly 70 percent, achieving a figure-of-merit of 0.42 in a compound known as PEDOT:PSS.
PEDOT:PSS is a mixture of two polymers: the conjugated polymer PEDOT and the polyelectrolyte PSS. It has previously been used as a transparent electrode for devices such as organic LEDs and solar cells, as well as an antistatic agent for materials such as photographic films.
One way to increase a material’s capacity for conducting electricity is through the use of dopants. Dopants bond to the host material, giving it an electrical carrier. Each of these additional carriers enhances the material’s electrical conductivity.
In PEDOT doped by PSS, however, only 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 found 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 further apart, making it harder for electrons to jump between PEDOT molecules. While ionized PSS molecules improve electrical conductivity, non-ionized PSS molecules reduce it.—Kevin Pipe, co-author
To improve its thermoelectric efficiency, the researchers restructured the material at the nanoscale. Pipe and his team figured out how to use certain 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.
This particular organic thermoelectric material would be effective at temperatures up to about 250 degrees Fahrenheit (121 °C).
This work was supported as part of the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the US Department of Energy Office of Science, Office of Basic Energy Sciences.
G-H. Kim, L. Shao, K. Zhang & K. P. Pipe (2013) Engineered doping of organic semiconductors for enhanced thermoelectric efficiency. Nature Materials doi: 10.1038/nmat3635