Impact of Side Chain Chemical Structure on Doping and Thermoelectric Properties of Oriented PBTTT Thin Films

In this contribution, doping of oriented thin films is investigated for three PBTTT polymers bearing different side chains including linear alkyl ─(CH₂)₁₂─H, single ether ─(CH₂)₇─O─(CH₂)₄─H and alkyl-siloxane ─(CH₂)₅─(Si(CH₃)₂O)₂─Si(CH₃)₃ A combination of transmission electron microscopy, polarized UV-vis-NIR spectroscopy and transport measurements helps uncover the essential role of the chemical nature of side chains on the efficacy of the doping and on the resulting thermoelectric performances in oriented PBTTT films. Siloxane side chains help to reach record alignment level of PBTTT with dichroic ratio beyond 50 for an optimized rubbing temperature but they impede effective doping of PBTTT crystals with F₆TCNNQ, resulting in very poor TE properties. By contrast, doping the amorphous phase of all three PBTTTs with magic blue (MB) results in excellent TE performances. Both, chemical nature of side chains and semi-crystalline structure of the polymer determine the efficacy of doping. The use of siloxane side chains further impacts the scaling laws S ∝ σ^-1/s between the Seebeck coefficient S and the charge conductivity σ. An unexpected s = 2 exponent is observed and tentatively attributed to the dimensionality of charge transport in the highly oriented mesophase of PBTTT.