Here, an effective design strategy of polymer thermoelectric materials based on structural control in doped polymer semiconductors is presented. The strategy is illustrated for two archetypical polythiophenes, e.g., poly(2,5‐bis(3‐dodecyl‐2‐thienyl)thieno[3,2‐b]thiophene) (C12‐PBTTT) and regioregular poly(3‐hexylthiophene) (P3HT). FeCl3 doping of aligned films results in charge conductivities up to 2 × 105 S cm−1 and metallic‐like thermopowers similar to iodine‐doped polyacetylene. The films are almost optically transparent and show strongly polarized near‐infrared polaronic bands (dichroic ratio >10). The comparative study of structure–property correlations in P3HT and C12‐PBTTT identifies three conditions to obtain conductivities beyond 105 S cm−1: i) achieve high in‐plane orientation of conjugated polymers with high persistence length; ii) ensure uniform chain oxidation of the polymer backbones by regular intercalation of dopant molecules in the polymer structure without disrupting alignment of π‐stacked layers; and iii) maintain a percolating nanomorphology along the chain direction. The highly anisotropic conducting polymer films are ideal model systems to investigate the correlations between thermopower S and charge conductivity σ. A scaling law S ∝ σ−1/4 prevails along the chain direction, but a different S ∝ −ln(σ) relation is observed perpendicular to the chains, suggesting different charge transport mechanisms. The simultaneous increase of charge conductivity and thermopower along the chain direction results in a substantial improvement of thermoelectric power factors up to 2 mW m−1 K−2 in C12‐PBTTT.

Thermoelectricity has gained considerable interest in the last decade due to the advent of organic thermoelectric materials. Crystallinity and doping level crucially determine the thermoelectric figure of merit of semi-conducting polymers. Hence, detailed insight into these factors is prerequisite for developing efficient devices. Here we show that the semicrystalline structure of aligned P3HT films shows superior thermoelectric efficiencies as compared to the smectic-like phase because of both a higher in-plane orientation and a higher doping level. Conductivities up to 160 S/cm and power factors of 56 W m-1 K-2 along the rubbing direction are obtained versus a few W m-1 K-2 for non-oriented films. Different intercalation mechanisms of F 4 TCNQ in the layers of alkyl side chains are evidenced by electron diffraction in doped oriented films of the smectic-like and the semi-crystalline phases. We provide compelling evidence that doping of the smectic-like phase promotes ordering of P3HT backbones along the chain direction within individual -stacks whereas for the semi-crystalline phase, dopant intercalation reorganizes the arrangement of successive stacks and perturbs the packing of alkyl side chains. Insight in the orientation of F 4 TCNQanions in the layers of alkyl side chains of P3HT crystals was further retrieved from a detailed polarized UV-vis-NIR spectroscopic analysis. Our results demonstrate that both orientation of the polymer chains and crystallinity enhance the thermoelectric properties as well as the doping level. We anticipate that detailed control of polymer morphology in films further improves the thermoelectric figure of merit of semiconducting polymers.

We report a record thermoelectric power factor of up to 160 μW m –1 K –2 for the conjugated polymer poly(3-hexylthiophene) (P3HT). This result is achieved through the combination of high-temperature rubbing of thin films together with the use of a large molybdenum dithiolene p-dopant with a high electron affinity. Comparison of the UV–vis–NIR spectra of the chemically doped samples to electrochemically oxidized material reveals an oxidation level of 10%, i.e., one polaron for every 10 repeat units. The high power factor arises due to an increase in the charge-carrier mobility and hence electrical conductivity along the rubbing direction. We conclude that P3HT, with its facile synthesis and outstanding processability, should not be ruled out as a potential thermoelectric material.