Thermoelectrics based on polymers have potential for near-room-temperature energy generation in flexible and wearable devices. Yet, it is challenging to improve their performance due to thermal and electrical transport trade-offs. In conventional inorganic thermoelectrics, these trade-offs are addressed through multiscale hierarchical engineering of the material structure: such a structure lowers the lattice thermal conductivity through phonon scattering while preserving the electrical conductivity. However, the approach is not easily implementable in polymers because they lack a sufficiently ordered crystal lattice and the structure can be partially dissolved by solvents used for the deposition of the upper layers. Now, Chong-an Di, Li-Dong Zhao and team across China, Republic of Korea, UK and Saudi Arabia overcome these limitations by creating a hierarchical heterojunction nanostructure using two polymers and implementing a photocrosslinking process to suppress dissolution in solvents.

The hierarchical heterojunction consists of multilayers of two alternating cross-linked polymers with interpenetrating interfaces. The research team shows that, when the thickness of the individual layers is comparable to or smaller than the mean free path of phonon vibrations, phonons are scattered and the thermal conductivity is lowered. The rough interfaces further contribute to lower the thermal conductivity through phonon scattering. High electron mobility is retained owing to a quasi-two-dimensional charge transport between the thin polymer layers and the intermolecular charge transfer at the interface.