High‐Performance n‐Type Organic Thermoelectrics Enabled by Control Over Microstructure, Orientation and Contact Plane in Naphthalenediimide‐Bithiazole‐Copolymer Films

Conjugated polymers are promising thermoelectric materials offering solution processability, flexibility, and versatile synthesis. However, n‐type polymers lag behind their p‐type counterparts. Herein, we demonstrate that the combination of precise doping with control over crystalline contact plane, orientation, and microstructure helps design efficient n‐type films of a naphthalenediimide‐bithiazole‐based copolymer (PNDIOE7‐Tz2). Favorable fibrillar morphology and enhanced planarity lead to tight packing in films doped with N‐DMBI, resulting in a high electrical conductivity σ ≈ 10 S cm −1 . GIWAXS indicates a predominantly face‐on orientation in pristine films with a short π–π stacking distance of 3.47 Å. Incremental concentration doping enhances the σ to 45 S cm −1 with a power factor (PF≈17.5 µW m −1 K −2 ). Rubbing alignment causes a detrimental shift from face‐on to edge‐on, reducing σ below 5 S cm −1 and PF below 4 µW m −1 K −2 . Subsequent solvent vapor annealing converts aligned crystallites to mixed face‐on/edge‐on, promoting a strong enhancement of TE performance with a high PF up to 41µW m −1 K −2 , among the highest reported for n‐type polymers. This work underscores that controlling the contact plane effectively decouples σ and S for high‐performance n‐type polymer thermoelectrics.