Synthesis and aggregation behavior of a glycolated naphthalene diimide bithiophene copolymer for application in low-level n-doped organic thermoelectrics

The synthesis of a naphthalene diimide bithiophene copolymer P(EO-NDIT2) with branched, base-stable, and purely ether-based side chains is presented. Stille polycondensation leads to high molecular weights that are limited by methyl transfer and eventually T2 homocouplings. While extensive solution aggregation hampers molecular weight determination by conventional methods, NMR spectroscopy allows identification of both T2- (H and methyl) and NDI-related (methyl) end groups, enabling the determination of absolute number average molecular weights larger than Mn,NMR ∼100 kg/mol. Solvent- and temperature-dependent aggregation in solution is investigated by NMR and UV-vis spectroscopy. These results are used for solution doping of P(EO-NDIT2) with N-benzimidazole-based n-dopants. Spin coating from heated chlorobenzene solutions and using 4-(2,3-dihydro-1,3-dimethyl-1H-benzoimidazol-2-yl)-N,N-diisopropylaniline (N-DiPrBI) as the dopant leads to homogeneous films with highest conductivities up to 10-2 S/cm. Generally, N-DiPrBI concentrations as low as ∼5 wt % are sufficient to increase conductivity by orders of magnitude. Strikingly, maximum power factors up to 0.11 μW/mK2, although limited by conductivity, are achieved for the highest molar mass sample at a low dopant concentration of 2 wt % N-DiPrBI only.