Non‐Monotonic Changes in the Thermal Conductivity of Conjugated Polymer Films with Increasing Levels of Chemical Doping
Reid W. Wilson, Xinping Shi, Diego Garcia Vidales, Abigail Slimp, Zerina Mehmedović, Alexander F. Simafranca, Eric C. Wu, Sarah H. Tolbert, Richard B. Wilson, Benjamin J. SchwartzABSTRACT
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 .