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Anion-Dependent Molecular Doping and Charge Transport in Ferric Salt-Doped P3HT for Thermoelectric Application

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journal contribution
posted on 24.02.2021, 17:33 by Lili Wu, Hui Li, Haoyu Chai, Qing Xu, Yanling Chen, Lidong Chen
The effect of different iron (III) dopants on the doping process and charge transport properties based on a poly­(3-hexylthiophene) (P3HT) film was investigated. It is found that the doping level is dependent on not only the driving force for charge transfer but also the miscibility between a polymer and a dopant, while the mobile carrier transport is significantly controlled by the microstructure upon doping. A high electrical conductivity (128 S cm–1) is obtained for a FeCl3-doped P3HT film among three different doped P3HT combinations, although a low doping level is observed in this film. In contrast, a highest doping level but a low electrical conductivity (65 S cm–1) is achieved for Fe­(OTf)3-doped P3HT. Another ferric salt with a larger size anion and strong oxidation ability, Fe­(Tos)3, endows both much low doping level and low electrical conductivity (9 S cm–1). Grazing-incidence wide-angle X-ray scattering (GIWAXS) shows that a much stronger π–π stacking of P3HT and larger crystalline domains may exist in Fe­(OTf)3-doped P3HT compared with those of FeCl3-doped P3HT. However, Hall-effect measurements show that the high electrical conductivity of FeCl3-doped P3HT is mainly attributed to higher carrier mobility. Temperature-dependent conductivity experiments demonstrate that smaller activation energy for carrier transport is needed for a FeCl3-doped P3HT film. These results indicate that smooth and continuous transport paths are formed in a FeCl3-doped film, contributing to high carrier mobility while discrete domains in Fe­(OTf)3-doped film hamper the carrier transport. A prototype device with a five-leg FeCl3-doped P3HT film connected with a silver paste was fabricated. The measured maximum output power is about 4.64 nW at the temperature difference of 23.3 K. Our results suggest that the interaction between dopant anions and polymer chains is crucial for high electrical conductivity by improving morphologies to achieve ionized carriers’ transfer into much mobile carriers.