Tailored Side-Chain Engineering and Extended π-Delocalization for Expediting Polarity Switching of Indaceno[1,2-b:5,6-b′]dithiophene-Based Conjugated Polymers

Endowing a single conjugated polymer possessing high bipolar (both P- and N-types) electrical conductivities offers a considerable strategy to boost thermoelectric module performances. However, the effect of polymer structures on expediting the polarity switching process is rarely reported. Six indaceno[1,2-b:5,6-b ']dithiophene(IDT)-based polymers (PIDT-3T, PBIDT-3T, PBIDTT-3T, PIDT-DPP, PBIDT-DPP, and PBIDTT-DPP) were synthesized by modifying the IDT unit through embedding phenyl groups in the side chain (BIDT) and extending the pi-delocalization of backbones (BIDTT). After doping with FeCl3, all the six IDT-based CPs display a switch in the sign of the Seebeck coefficient. According to ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy, the polarity switching can be attributed to both the crossing of the Femi level from above transporting level to below transporting level and transport gap diminishment. The UV-vis absorption spectra and X-ray diffraction patterns demonstrated that the conjugated phenyl groups endow the BIDTT-based CPs with sufficient space along the polymer backbones for FeCl3 distribution, and the extended backbones can promote the doping efficiency, accelerating the polarity switching process. Consequently, PBIDTT-DPP required the shortest polarity switching time of 1 min among the six polymers. This work provides an in-depth understanding of the polarity switching structure-function relationships and sheds light on the design of efficient n-type thermoelectric polymers.