Hot Advance for Thermoelectrics

Cheap organic molecules could more efficiently convert waste heat into electricity.

By Kevin Bullis



By trapping organic molecules between a gold surface and the ultrafine gold tip of a scanning tunneling microscope, researchers have shown that the molecules could be used to generate electricity.

Inside fossil-fuel and nuclear-power plants, as well as in cars and trucks, the lion's share of energy in fuel is wasted as heat rather than converted into electricity or mechanical power. But the search for a practical material that can convert at least some of this waste heat into electricity has been long and frustrating.

Researchers have long known that some inorganic semiconductors can do this. Indeed, deep-space probes have been powered by using such materials. But these inorganic materials are costly and difficult to make, and have low efficiencies. Now, new research shows that certain organic molecules produce voltage when exposed to heat. Ultimately, they could be much cheaper and thus more practical to implement.

"This is the first demonstration that you can use organic molecules in this kind of energy generation," says Rachel Segalman, professor of chemical engineering at the University of California, Berkeley, who with her colleagues reported new measurements last week in Science Express. "That's really significant because they are so inexpensive and abundant," she says.

Experts had previously theorized that some organic molecules could have the qualities necessary to generate electricity from heat. But until now, they lacked experimental proof, which the Berkeley researchers were able to provide by isolating and measuring the properties of just a few molecules of organic substances called benzene dithiols at a time.

These were "very difficult experiments," says Brian Sales, a senior research scientist at the Oak Ridge National Laboratory, who was not involved with the work. The researchers trapped a few molecules between a sheet of gold and the ultrafine gold tip of a scanning tunneling microscope, which is so sharp it can end in a single atom. They heated up the gold surface and measured, via the microscope tip, the voltage that was created. "These are the type of difficult experiments that get nanotechnology past the 'picture' stage [and] into the realm of real science," Sales says.

The experiments showed that the organic molecules have the three qualities that make for good thermoelectric materials. The first is the ability to create a voltage. But this works best when the materials have two other qualities: they do not conduct heat, but they do conduct electrons. That way, applying heat, rather than just raising the temperature of the material, actually drives electrons, creating a current.

Organic materials are appealing because they cost much less than thermoelectric inorganic materials: even if they are inefficient, they might still be economical. "These molecules are dirt cheap," Majumdar says. "If the efficiency is low, that's fine. You're throwing that heat away anyway."