Polymer materials have attracted increasing interests in the fields of optoelectronics, energy conversion, and energy storage devices due to their low toxicity, lightweight, large-area solution processability, and mechanical flexibility. Among them, organic thermoelectric materials are a kind of energy conversion material that can directly convert thermal energy and electrical energy into each other. To achieve highly efficient thermoelectric modules, both p- and n-type conjugated polymers with comparable performance are required. In the field of organic thermoelectric materials, p-type conjugated polymers (such as PEDOT) have shown a high thermoelectric figure of merit (ZT > 0.4) close to that of inorganic thermoelectric materials, which is mainly due to their high electrical conductivity (> 1000 S/cm) and high power factor (> 300 μW/m K). In contrast, only a few n-type conjugated polymers have an electrical conductivity close to or slightly above 1 S/cm, and the power factor is generally below 10 μW/m K.
In recent years, with the emergence of various building blocks, the carrier mobilities of D-A type conjugated polymers have been significantly improved. Among them, diketopyrrolopyrrole (DPP) is a type of structural unit that has received wide attention. Currently, DPP-based D-A conjugated polymers reported in the literature have shown electron mobilities over 5 cm2/V s. This value is even higher than the hole mobilities of p-type conjugated polymers. However, the conductivity of n-doped DPP conjugated polymers is usually low (0.1 - 1 S/cm), which is mainly caused by the low n-type doping efficiency of such polymers.
Figure 1. Chemical structure and several features of polymer P(PzDPP-CT2)
Based on the above research background, Ting Lei's group designed and synthesized a new DPP building block, namely PzDPP. Among all the DPP building blocks, PzDPP has the lowest LUMO energy level. A new D-A-type conjugated polymer P(PzDPP-CT2) was obtained by polymerizing PzDPP and an electron-deficient 3,3'-dicyano-2,2'-bithiophene. The polymer exhibits a conformationally-locked coplanar skeleton structure and a LUMO energy level as low as −4.03 eV. A low LUMO energy level is beneficial to improve the n-type doping efficiency of the polymer. After being doped with n-type dopants, the polymer P (PzDPP-CT2) showed a conductivity as high as 8.4 S/cm and a power factor of up to 57.3 μW/m K. The electrical conductivity is much higher than other D-A type conjugated polymers, and the power factor is also among the highest values in n-type solution-processable polymer thermoelectric materials.
Figure 2. Schematic diagram of a flexible thermoelectric device based on polymer P(PzDPP-CT2)
Dr. Xinwen Yan and Miss Miao Xiong are the co-first authors of the paper, and Prof. Ting Lei is the corresponding author. Collaborators include Prof. Jie-Yu Wang from the College of Chemistry and Molecular Engineering, Peking University, and Prof. Xiaodan Gu from the University of Southern Mississippi. The work was recently published in the Journal of the American Chemical Society (J. Am. Chem. Soc., https://pubs.acs.org/doi/10.1021/jacs.9b10107).