Optimizing the doping efficiency and thermoelectric properties of isoindigo-based conjugated polymers using side chain engineering

Abstract

The chemical design of side chains and the physical control of molecular aggregation on conjugated polymer semiconductors have been demonstrated to be effective strategies for achieving efficient doping and charge transport in preparing doped conjugated polymer thermoelectric devices. Herein, our study introduces a unique approach for regulating the doping efficiencies and the thermoelectric properties of polymers, which involves manipulating the hydrophilicity and asymmetric side chain engineering. From isoindigo–bithiophene (IID–T2) donor–acceptor conjugated polymers, three polymers named P(Si–Si), P(Si–O), and P(Si–F) with the symmetric carbosilane side chains, asymmetric carbosilane/oligoether, and carbosilane/semifluorinated side chains, respectively, attached on the IID rings are investigated for their FeCl₃-doped thermoelectric films. It is found that the morphological structures and molecular packing can be controlled by altering the hydrophilicity and asymmetricity of side chain substituents. The doped P(Si–O) from the asymmetric side chain with oligoether moieties exhibits the maximum power factor of 23.4 μW m⁻¹ K⁻², which is attributed to the high doping efficiency of the polymers due to the intercalation of dopant molecules in the polymer side chains (without chain orientation disruption) while maintaining the charge transport percolation morphologies. The poor affinity between the semifluorinated side chains in P(Si–F) and dopants results in aggregation morphologies with low thermoelectric performance. These findings suggest that the combined asymmetric and hydrophilic side chains in conjugated polymers can effectively facilitate their miscibility with dopants for improving thermoelectric properties.