In the 70s of the last century, the scientific discovery of doped polyacetylene subverted the traditional cognition that "plastics cannot conduct electricity", set off a research boom of optoelectronic molecular materials, gave birth to the organic light-emitting diode electronics industry, gave birth to cutting-edge research directions such as organic photovoltaics and field-effect transistors, and at the same time drove the start of the field of organic thermoelectricity. Among them, the thermoelectric research of polymer systems can not only deepen or even change people's understanding of the thermoelectric conversion mechanism of soft matter systems, but also hope to meet the urgent needs of the Internet of Things and wearable electronics for attached energy, which is of great scientific significance. However, compared with the existing thermoelectric material system, polymer thermoelectric materials have long faced the bottleneck of low thermoelectric merit value (ZT), which cannot meet the core index requirements of temperature difference power generation and solid-state refrigeration applications, which directly restricts the rapid development of the field.
With the support of the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Beijing Municipality, the research team of Zhu Daoben/Di Chong'an from the Institute of Chemistry of the Chinese Academy of Sciences, in collaboration with Zhang Deqing's group, Zhao Lidong's group at Beihang University, and six other research teams at home and abroad, proposed and constructed polymer multi-period heterojunction (PMHJ) thermoelectric materials. This type of molecular assembly has a periodically ordered nanostructure, in which the thickness of the two polymers is less than 10 nanometers, the adjacent interface is about 2 molecular layers and has the characteristics of bulk heterogeneity. The optimized PMHJ film can not only maintain excellent charge transport characteristics, but also greatly inhibit phonon/phonon-like propagation, thus achieving a significant leap in the thermoelectric performance of polymers, providing a new path for the research and application of high-performance plastic-based thermoelectric materials. The ideal thermoelectric material should have a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity to meet the "phononic glass-electron crystal" model. It is generally accepted in the scientific community that polymers possess the characteristics of phononic glass, which gives them intrinsically low thermal conductivity. Based on this, the existing main research paths of high-performance organic thermoelectric materials are to regulate the Seebeck coefficient, conductivity and their constraints through molecular creation, assembly and doping. Although the thermoelectric merit of organic materials has been evaluated by thermal conductivity characterization, there is a lack of control strategies for the thermal transport properties, and the thermoelectric merit of the corresponding system has not been significantly improved in the past ten years. The research team used two polymers, PDPPSe-12 and PBTTT, combined with molecular cross-linking methods, to construct PMHJ films with different structural characteristics, revealing the size effect of thermal conductivity and the diffuse reflection effect at the interface. It was found that when the thickness of each polymer approached the mean free path of the "phonon" of the conjugated framework, the interfacial scattering was significantly enhanced, and the lattice thermal conductivity of the film was reduced by more than 70% to 0.1 W m-1 K-1. In addition, the doped (6,4,4)PMHJ film exhibits excellent electrical transport properties, with a power factor of up to 628 μW m-1 K-2,368 K and a thermoelectric merit value of 1.28, reaching the room temperature thermoelectric performance level of commercial materials, driving plastic-based thermoelectric materials into the ZT >1.0 era. In addition, the PMHJ structure has excellent universality, and its processing method is compatible with the solution preparation technology, which has important application potential in flexible energy supply devices. The above research breaks the cognitive limitation of existing high-performance polymer thermoelectric materials that do not depend on thermal transport regulation, and provides a new path for the sustainable development of the field of plastic-based thermoelectric materials. The research results were published in the journal Nature (Nature 2024, DOI: 10.1038/s41586-024-07724-2), the co-first authors of the article are Dr. Dongyang Wang, Dr. Jiamin Ding and Dr. Yingqiao Ma, and the corresponding authors are Prof. Chongan Di of the Institute of Chemistry and Prof. Lidong Zhao of Beihang University. The research was supported by the Huairou Research Center of the Institute of Chemistry, Chinese Academy of Sciences.
Fig.1 The design idea of PMHJ structure and the characterization results of time-of-flight secondary ion mass spectrometry
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Source: Institute of Chemistry