B ← N Unit Enables n-Doping of Conjugated Polymers for Thermoelectric Application

Only very few conjugated polymers can be n-doped for thermoelectric applications. In this work, for the first time, we report that incorporation of Boron-Nitrogen coordination bond (B ← N unit) to a donor-acceptor (D-A) type conjugated polymer enable n-doping for thermoelectric application. The incorporation of B ← N unit into the polymer backbone leads to not only a downshift of LUMO/HOMO energy levels by 0.27 eV/0.33 eV, but also diminished intramolecular D-A character of the polymer backbone. As a result, while the control polymer cannot be n-doped, the polymer containing B ← N unit (PI-BN) can be n-doped by 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-dimethylaniline (N-DMBI). Finally, PI-BN exhibits an electrical conductivity (σ) of 0.97 × 10^-3 S cm^-1, Seebeck coefficient (S) of -453.8 μV K^-1, and power factor (PF) of 0.02 μW m^-1 K^-2 when doped with 5 wt % N-DMBI. A great advantage of PI-BN is its excellent miscibility with the n-dopant because of its amorphous nature and large pendent substituents. This work indicates that organoboron polymers can be n-doped and can be used for thermoelectrics.