Selenium Substitution in Bithiophene Imide Polymer Semiconductors Enables High‐Performance n‐Type Organic Thermoelectric

Abstract Designing n ‐type polymers with high electrical conductivity remains a major challenge for organic thermoelectrics (OTEs). Herein, by devising a novel selenophene‐based electron‐deficient building block, the pronounced advantages of selenium substitution in simultaneously enabling advanced n ‐type polymers is demonstrated with high mobility (≈2 orders of magnitude higher versus their sulfur‐based analogues due to both intensified intra‐ and inter‐chain interactions) and much improved n ‐doping efficiency (enabled by the largely lowered LUMO level with a ≈0.2 eV margin) of the resulting polymers. Via side chain optimization and donor engineering, the selenium‐substituted polymer, f‐BSeI2TEG‐FT, achieves a highest conductivity of 103.5 S cm −1 and power factor of 70.1 µW m −1 K −2 , which are among the highest values reported in literature for n ‐type polymers, and f‐BSeI2TEG‐FT greatly outperformed the sulfur‐based analogue polymer by 40% conductivity increase. These results demonstrate that selenium substitution is a very effective strategy for improving n ‐type performance and provide important structure‐property correlations for developing high‐performing n ‐type OTE materials.