Single-molecule devices composed of polyphenyl ether (PPE) chains between two metallic electrodes can achieve ZT >> 4, making them high-efficiency quantum thermoelectric devices [Bergfield et al., ACS Nano, (2010) 4, 9, 5314].
Thermoelectric devices are the ideal solution to energy waste as they can convert between thermal and electrical energy. The efficiency of such devices relies on what is known as the figure of merit ZT, a value that is dependent on the electrical and thermal conductance of the material. The higher the ZT, the more commercially viable the device will be, but current commercially available thermoelectric devices only reach a ZT of 1, mainly as a result of packaging and fabrication.
Justin Bergfield explains why increasing ZT is so important. “Although thermoelectric devices exist, more efficient thermoelectric materials would make it economically feasible to phase out ozone-depleting chlorofluorocarbons and hydrofluorocarbons, and conserve energy by harvesting waste heat in a variety of applications, from roof-top solar panels to automobile and factory exhausts.”
Now he and the rest of the team at the University of Arizona in the US have used theoretical and computational studies to determine the quantum interference effects in single-molecule junctions and they predict that thermoelectric materials with a ZT greater than 4 can be achieved using wires of polyphenyl ether molecules chains self-assembled between two metallic electrodes. At the single-molecule level, electrons act as both particles and waves and as they move along the PPE chain they are split into separate paths across the benzene ring which forms part of the PPE backbone. Once the electrons reunite again, they are out of phase and what results is called quantum wave interference. This leads to a voltage buildup between the two electrodes.
“We predict a giant thermoelectric effect due to quantum wave interference,” says Bergfield. “The key to achieving high-efficiency quantum thermoelectric devices proved to be higher-order interferences (constructive or destructive), which arise in certain types of organic molecules. While our proposed high-efficiency devices have not yet been made, the thermoelectric response of single-molecule junctions has recently been measured. At the moment we are working with several partners in moving the project forward,” he adds.