Tokyo, Japan – Researchers from Tokyo Metropolitan University have used a theoretical framework to derive design rules for creating new thermoelectric materials. The efficiency of converting thermal energy to electricity is often determined by parameters which strongly depend on each other, making it difficult to identify ideal recipes. The team’s findings help lay out goals for optimizing “band structure,” a map showing what states electrons can take, promising a shift towards rational material design.
Around 60% of the energy generated when burning fossil fuels is lost to heat. Efficient ways to convert this “waste heat” to electricity would be an important stepping stone as the world tries to reduce oil and gas consumption. However, “thermoelectric” conversion remains a big challenge. The physical effect which underlies thermoelectric technology, the Seebeck effect, relies on a current being generated when two different materials are brought into contact and heated. While it is possible to improve conversion efficiency by adding small amounts of a different element (“doping”) to modify properties, the factors which determine successful conversion of heat to current often depend on each other in complex ways, making it challenging to nail down quantitative rules for exactly how materials might be tweaked.
In recent work, a team led by Assistant Professor Yuya Hattori from Tokyo Metropolitan University has uncovered design rules which apply across a wide range of materials. They used a theoretical framework called Boltzmann transport theory to show that there are common conditions under which thermoelectric conversion efficiency might be maximized. Their work optimizes the “band structure” of materials, a map of the energies which charge carriers like electrons and “holes” (“lack of electrons”) can take. A key feature of band structure is the band gap, the energy gap separating the highest energy electrons in a material, and the lowest energy states to which it can shift. For example, they discovered that the “bipolar effect,” where electrons and holes contribute in opposite directions to the Seebeck effect, reducing performance, becomes activated when the thermal energy of the environment reaches around five times the band gap.
The team focused on “band converged” materials, where fine-tuning of composition can create electrons in different environments inside a material with the same or similar energy, all of which might contribute to thermoelectric transport. They found that the figure of merit for the Seebeck effect was maximized when the energies were precisely matched. They also investigated the optimal chemical potential, the energy required to add a single electron to the material, and how it informs the best amounts of dopant to use for different materials.
While universal rules are hard to come by, their work shines a light on key principles which might help identify new recipes for better thermoelectric materials. Combined with numerical simulations, the team anticipates a big leap forward for the engineering of thermoelectric materials, away from an empirical approach, and in favor of rational material design.
This work was supported by JSPS KAKENHI Grant Numbers 24KJ0227 and 24K08231.
Materials Today Advances
Ideal band structures for high-performance thermoelectric materials with band convergence
17-Jul-2026