Cyano-Functionalized Fused Bithiophene Imide Dimer-Based n-Type Polymers for High-Performance Organic Thermoelectrics

Doped n-type polymers usually exhibit low electrical conductivities and thermoelectric power factors (PFs), restricting the development of high-performance p-n-junction-based organic thermoelectrics (OTEs). Herein, the design and synthesis of a new cyano-functionalized fused bithiophene imide dimer (f-BTI2), CNI2, is reported, which synergistically combines the advantages of both cyano and imide functionalities, thus leading to substantially higher electron deficiency than the parent f-BTI2. On the basis of this novel building block, a series of n-type donor-acceptor and acceptor-acceptor polymers are successfully synthesized, all of which show good solubility, deep-lying frontier molecular orbital levels, and favorable polymer chain orientation. Among them, the acceptor-acceptor polymer PCNI2-BTI delivers an excellent electrical conductivity up to 150.2 S cm^-1 and a highest PF of 110.3 µW m^-1  K^-2 in n-type OTEs, attributed to the optimized polymer electronic properties and film morphology with improved molecular packing and higher crystallinity assisted by solution-shearing technology. The PF value is the record of n-type polymers for OTEs to date. This work demonstrates a facile approach to designing high-performance n-type polymers and fabricating high-quality films for OTE applications.