The effect of different iron (III) dopants on the doping process and charge transport properties based on a poly(3hexylthiophene) (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 FeCl 3 -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 FeCl 3 -doped P3HT. However, Hall-effect measurements show that the high electrical conductivity of FeCl 3 -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 FeCl 3 -doped P3HT film. These results indicate that smooth and continuous transport paths are formed in a FeCl 3doped 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 FeCl 3 -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.

Chemical doping of donor–acceptor (D–A) polymers is essential for their usage in highly efficient optoelectronic devices. The crucial challenge remains to synergistically improve the carrier concentration and mobility of these polymers via a single solution doping method. Here, a D–A polymer, Pg32T‐OTz is designed and synthesized, containing a weak‐acceptor–strong‐donor backbone with nonpolar (alkoxy) and polar (ethylene glycol) side chains. Pure integer charge transfer (ICT) is shown to occur in 2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane (F4TCNQ)‐doped D–A polymer at a low doping level. It is shown that the ordered edge‐on orientations of ICT structures overcome the Coulomb interactions and endow a long‐range hole delocalization, while few charge transfer complex states form in amorphous regions and bridge the crystalline ICT structures at high doping level, creating a network that sustains efficient charge transport. As a result, simultaneously high carrier concentration and high mobility are realized for F4TCNQ‐doped Pg32T‐OTz and thus a high electrical conductivity up to 550 S cm−1 is approached, which is the highest value among any doped polymers via a single‐solution doping process. This work demonstrates that the modulation of the acceptor strength combined with side‐chain engineering is an effective molecular design strategy to promote both the doping efficiency and carrier transport property of D–A polymers.