Thermoelectric materials can be used to turn waste heat into electricity
or to provide refrigeration without any liquid coolants, and a research
team from the University of Michigan has found a way to nearly double
the efficiency of a particular class of them that's made with organic
semiconductors.
- Improved properties of PEDOT:PSS for thermoelectric applications
Organic semiconductors are carbon-rich compounds that are relatively
cheap, abundant, lightweight and tough. But they haven't traditionally
been considered candidate thermoelectric materials because they have
been inefficient in carrying out the essential heat-to-electricity
conversion process.
Most efficient thermoelectric materials are made of relatively rare
inorganic semiconductors such as bismuth, tellurium and selenium that
are expensive, brittle and often toxic. Still, they manage to convert
heat into electricity more than four times as efficiently as the organic
semiconductors created to date.
This greater efficiency is reflected in a metric known by researchers as
the thermoelectric "figure of merit." This metric 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 of the ways scientists and engineers increase a material's capacity
for conducting electricity is to add impurities to it in a process known
as doping. When these added ingredients, called dopants, bond to the
host material, they give 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.
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.