A Simple Quinoid Building Block for Polymer Semiconductors with Tunable Polarity and High n‐Type Thermoelectric Performance

π-Conjugated polymers with deep-positioned lowest unoccupied molecular orbital (LUMO) level and large electron mobility are highly pursued as n-type organic thermoelectric materials. Herein, we synthesized a simple structured electron-deficient quinoid building block, thieno[3,2-b]thiophene-2,5-dione (TTD), via one step from a low-cost starting material. Based on TTD, polymers PQD and PQTD were successfully developed, featuring remarkably low-lying LUMO levels (-3.91 eV for PQD and -3.73 eV for PQTD), which greatly facilitate n-doping process. Unexpectedly, it was found that charge carrier polarity of PQD and PQTD can be tuned from p-type to ambipolar and finally to n-type in organic field-effect transistors after thermal treatment, an unprecedented phenomenon in polymer semiconductors. Organic thermoelectric devices based on n-doped PQD films showed an excellent electrical conductivity up to 19.1 S cm^-1 and a power factor of 36.7 µW m^-1 K^-2. To the best of our knowledge, this PF value ranks the highest reported to date for n-doped quinoid-based polymers. This work underscores the great potential of structurally simple and readily accessible electron-deficient quinoid TTD for the development of high-performing n-type polymer semiconductors.