Quantifying anisotropic mobility enhancement in uniaxially aligned polythiophene films via ion-exchange doping

Abstract

We quantitatively investigated the anisotropic enhancement of hole mobility induced by structural modifications upon ion-exchange doping in highly uniaxially aligned poly(3,3′′′-didodecyl-quaterthiophene) (PQT-12) films. Large-area, uniform alignment was achieved via the floating film transfer method (FTM), giving a high dichroic ratio of 15. Anion-exchange p-doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) yielded a carrier density of 8.06 × 10²⁰ cm⁻³ while preserving uniaxial order and improving crystallinity. Compared to undoped films, the hole mobility increased by 850-fold along the backbone direction and 1700-fold along the π–π stacking (perpendicular) direction, demonstrating that doping optimizes structural order for charge transport in both directions, with a more pronounced relative enhancement perpendicular to the chains. Using the stronger oxidant magic blue with LiTFSI further increased the doping level, achieving a higher conductivity of 251 S cm⁻¹. These results reveal how ion-exchange doping and structural anisotropy cooperatively govern charge transport in aligned conjugated polymers.