High‐Performance n‐Type Organic Thermoelectrics Enabled by Synergistically Achieving High Electron Mobility and Doping Efficiency

n‐Doped polymers with high electrical conductivity (σ) are still very scarce in organic thermoelectrics (OTEs), which limits the development of efficient organic thermoelectric generators. A series of fused bithiophene imide dimer‐based polymers, PO8, PO12, and PO16, incorporating distinct oligo(ethylene glycol) side‐chain to optimize σ is reported here. Three polymers show a monotonic electron mobility decrease as side‐chain size increasing due to the gradually lowered film crystallinity and change of backbone orientation. Interestingly, polymer PO12 with a moderate side‐chain size delivers a champion σ up to 92.0 S cm(−1) and a power factor (PF) as high as 94.3 µW m(−1) K(−2) in the series when applied in OTE devices. The PF value is among the highest ones for the solution‐processing n‐doped polymers. In‐depth morphology studies unravel that the moderate crystallinity and the formation of 3D conduction channel derived from bimodal orientation synergistically contribute to high doping efficiency and large charge carrier mobility, thus resulting in high performance for the PO12‐based OTEs. The results demonstrate the great power of simple tuning of side chain in developing n‐type polymers with substantial σ for improving organic thermoelectric performance.