Intercalation and Ordering of F6TCNNQ and F4TCNQ Dopants in Regioregular Poly(3-hexylthiophene) Crystals: Impact on Anisotropic Thermoelectric Properties of Oriented Thin Films

Redox doping is widely used to transform polymer semiconductors such as regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) into conducting polymers. Strong acceptor molecules such as F6TCNNQ and F4TCNQ have been used for sequential doping of aligned P3HT films to produce films with enhanced thermoelectric properties along the chain direction. This study uses polarized UV–vis–NIR spectroscopy and electron diffraction to clarify the way dopant molecules are introduced in the crystals of P3HT. A structural model is obtained for F6TCNNQ-doped P3HT (P-1 symmetry, a = 18.8 Å, b = 8.95 Å, c = 7.75 Å, α = 107.6°, β = 101.5°, and γ = 89.3°). The proposed structure is consistent with the electron diffraction pattern and polarized UV–vis–NIR results. The model predicts intercalation of one dopant per four thiophene monomers, reduction of π-stacking distance, and expansion of the unit cell along alkyl side chains. Both TEM and polarized absorption spectroscopy demonstrate that F6TCNNQ molecules are better ordered than F4TCNQ in the P3HT crystals, leading to improved charge conductivities reaching 500 S/cm with thermoelectric power factors close to 80 μW m–1 K–2.