High‐Performance n‐Type Thermoelectric Polymers via Ion‐Exchange Tuning of Polaron–Cation Interactions

Abstract The thermoelectric (TE) properties of the n‐type conjugated polymer (CP) polybenzodifuran‐dione (PBFDO) are systematically modulated via counter‐ion exchange, in which protons initially present as the counter‐ion are replaced with various cations of increasing ionic radius. Ion exchange varied polaron‐cation coupling, macroscopic conducting network, and macroscopic structural order, enabling purpose‐oriented TE properties control, depending on the cation. In potassium‐exchanged PBFDO film, a weakened polaron‐cation coupling and improved structural order facilitated a transition from hopping to band‐like transport, doubling electrical conductivity to realize an electrode‐suitable material. Meanwhile, ammonium exchange not only increased the electrical conductivity but also maintained a high Seebeck coefficient through hydrogen bonding, yielding a record‐high power factor of 225 µW m ⁻¹ K ⁻² for n‐type CPs, demonstrating a strategic tuning of TE properties in PBFDO. The ammonium‐exchanged PBFDO film thereby achieved a high TE figure‐of‐merit, ZT of 0.13, which is the highest reported for n‐type CPs exhibiting metallic conductivity.