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

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.