Control of Chain Alignment and Crystallization Helps Enhance Charge Conductivities and Thermoelectric Power Factors in Sequentially Doped P3HT:F₄TCNQ Films

Thermoelectricity has gained considerable interest in the past decade due to the advent of organic thermoelectric materials. Crystallinity and doping level crucially determine the thermoelectric figure of merit of semiconducting 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 μW m^–1 K^–2 along the rubbing direction are obtained versus a few μW m^–1 K^–2 for nonoriented films. Different intercalation mechanisms of F₄TCNQ in the layers of alkyl side chains are evidenced by electron diffraction in doped oriented films of the smectic-like and the semicrystalline phases. We provide compelling evidence that doping of the smectic-like phase promotes ordering of P3HT backbones along the chain direction within individual π-stacks, whereas for the semicrystalline phase dopant intercalation reorganizes the arrangement of successive π-stacks and perturbs the packing of alkyl side chains. Insight into the orientation of F₄TCNQ^– anions 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.