Effect of Molecular Weight on Thermoelectric Performance of P3HT Analogues with 2-Propoxyethyl Side Chains

By replacing hexyl chains in poly(3-hexylthiophene) (P3HT) with 2-propoxyethyls, four poly(3-(2-propoxyethyl)thiophene) (P3POET) homopolymers with comparable polydispersity indexes (PDI) and regioregularities were prepared herein in addition with step increment of about 7 kDa on number-average molecular weight (M_n) from around 11 to 32 kDa (accordingly denoted as P11k, P18k, P25k, and P32k). When doped in film by FeCl₃ at the optimized conditions, the maximum power factor (PF_max) increases greatly from 4.3 µW·m⁻¹·K⁻² for P11k to 8.8 µW·m⁻¹·K⁻² for P18k, and further to 9.7 µW·m⁻¹·K⁻² for P25k, followed by a slight decrease to 9.2 µW·m⁻¹·K⁻² for P32k. The close Seebeck coefficients (S) at PF_max are observed in these doped polymer films due to their consistent frontier orbital energy levels and Fermi levels. The main contribution to this PF_max evolution thus comes from the corresponding conductivities (σ). The σ variation of the doped films can be rationally correlated with their microstructure evolution.