Non‐Monotonic Changes in the Thermal Conductivity of Conjugated Polymer Films with Increasing Levels of Chemical Doping

The performance of thermoelectric materials is characterized by the figure of merit ( zT ), which depends on knowing the thermal conductivity ( κ ). Poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) films doped with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyano‐quinodimethane (F 4 TCNQ) are a prototypical conjugated polymer system studied for potential use in thermoelectrics. However, the few reports for this system disagree as to whether κ increases or decreases upon doping. This disagreement is partly due to the use of measurement techniques that rely on knowing the volumetric heat capacity ( C v ) to determine κ , even though the C v of conjugated polymers has not been systematically investigated as a function of doping. Here, we experimentally determine both C v , using differential scanning calorimetry and careful density measurements, and thermal effusivity, using time‐domain thermoreflectance (TDTR), to reveal how the κ of both regioregular and regiorandom P3HT changes as a function of doping with F 4 TCNQ. Our results show that both C v and κ change non‐monotonically with the intercalated F 4 TCNQ:thiophene monomer ratio. At low doping ratios, κ increases because of doping‐induced crystal structure changes, while at higher doping ratios, κ decreases because of the increasing density of F 4 TCNQ¯ counterions in the crystal lattice. We find that F 4 TCNQ‐doped P3HT films can achieve zT values of 5.2 × 10 −3 .