In a study published in Nature, professors Chong’an Di of the Chinese Academy of Sciences Institute of Chemistry and Lidong Zhao of Beihang University, together with others, have presented a polymeric multi-heterojunction (PMHJ) structure with a ZT greater than 1.0.
Organic thermoelectric materials show considerable potential as flexible energy sources for IoT and wearable devices. However, their comparatively low dimensionless figure of merit (ZT) compared to traditional materials has been a significant barrier, restricting their usage in thermoelectric power production and solid-state cooling.
Ideal thermoelectric materials should follow the “phonon-glass electron-crystal” concept. Currently, improving the power factor is the key goal in the development of high-performance organic thermoelectric materials.
Despite efforts to increase thermoelectric efficiency by measuring thermal conductivity, a lack of efficient techniques for phonon scattering in soft material systems has hampered considerable progress toward increasing the ZT value during the last decade.
The PMHJ structure was suggested by the researchers in this study as a way to control thermal conductivity in organic systems. This new design has a regularly structured nanostructure with fewer than 10 nm thick polymer layers. The neighboring interface layers have bulk heterojunction characteristics and are around two molecular layers thick.
The size effect and diffuse dispersion of phonon/phonon-like thermal vibrations inside the PMHJ structure were investigated through precise control of the thickness of the polymer layer and the interfacial structural properties.
They found that interface scattering increased as the layer thickness got closer to the phonon mean free route along the conjugated backbone direction. This resulted in a notable fall in the film's lattice thermal conductivity, which dropped by more than 70 % to 0.1 W m–1 K–1.
They also found that the doped (6,4,4) film outperformed existing organic thermoelectric materials, exhibiting superior electrical transport capabilities with a maximum ZT of 1.28 and a high power factor of 628 μW m–1 K–2.
In addition to these developments, PMHJ films work with large-area solution processing technologies. The thermoelectric integrated devices demonstrated a remarkable 1.12 μW cm–2 K–2 normalized power density, underscoring its potential application in flexible power supply components.
The study emphasizes how crucial nanostructure engineering is to overcome the low ZT constraint for weakly interacting plastics. It presents a fresh approach to developing thermoelectric materials based on plastic.
Journal Reference:
Wang, D., et al. (2024) Multi-heterojunctioned plastics with high thermoelectric figure of merit. Nature. doi:10.1038/s41586-024-07724-2