Thermoelectric Performance Enhancement of p-Type Pyrrolo[3,2-b:4,5-b′]bis[1,4]benzothiazine Conjugated Ladder Polymer by Pendant Group Engineering

Organic thermoelectrics have attracted widespread attention due to the possibility of harnessing ambient thermal energy to power Internet-of-things and wearables due to their flexibility, low cost, and large-scale production, but research progress still lags behind that of inorganic materials. Rigid ladder-type conjugated polymers consisting of double-stranded fused-ring structure exhibit excellent thermal and electrochemical stabilities and thus are promising materials for organic thermoelectrics. However, effective synthesis, characterization, and solution processing are challenging and thus limit its application in thin-film devices. Here, two p -type pyrrolo[3,2- b:4,5- b ′]bis[1,4]benzothiazine ladder-type conjugated polymers with different pendant groups (methyl: PBBTL-CH 3 and phenyl: PBBTL-Ph) were synthesized, and both can be solution-processed for thin-film thermoelectric applications. PBBTL-CH 3 achieved electrical conductivities as high as 36.29 ± 0.58 S cm –1 and power factors up to 22.7 ± 2.1 μW m –1 K –2 when doped with FeCl 3, which are significantly higher than of PBBTL-Ph with a conductivity of 1.40 ± 0.16 S cm –1 and a power factor of 1.7 ± 0.2 μW m –1 K –2 . Grazing-incident wide-angle X-ray scattering of the polymers showed PBBTL-CH 3 exhibited exclusive edge-on orientation, while PBBTL-Ph exhibited mixed edge-on and face-on orientation. The present study indicates that pendant groups have important influence on the electrical conductivity and thermoelectric performance for the PBBTL series through alteration of HOMO energy levels, polymer packing, and ease of dopant infiltration.