Influence of dopant size and doping method on the structure and thermoelectric properties of PBTTT films doped with F6TCNNQ and F4TCNQ
Doped polymer semiconductors are ideal candidates to capture waste heat in ambient conditions using modest temperature gradients close to room temperature. Mastering the doping mechanism of polymer semiconductors is therefore crucial to enhance the performances of these materials. This study focuses on the structure-property correlations in oriented films of poly(2,5-bis(3-dodecyl-2-thienyl)thieno[3,2-b]thiophene) (C12-PBTTT) doped with 1,3,4,5,7,8-Hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) and 2,3,5,6- tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). Slower diffusion of the larger F6TCNNQ molecules in PBTTT crystals results in a better ordering upon intercalation in the layers of alkyl side chains as compared to F4TCNQ. Both, the higher degree of polymer chain oxidation and ordering of F6TCNNQ- dopants in the C12-PBTTT crystals accounts for the improved electrical conductivity and thermoelectric properties. A new doping protocol called “incremental concentration doping” leads to electrical conductivities of up to 2400 S/cm and thermoelectric power factors of 530±200 μWm-1K-2 in aligned C12-PBTTT doped with F6TCNNQ. The progressive intercalation of dopants helps preserve the high level of order initially present in the aligned C12-PBTTT films, hence, to reach higher charge mobilities. The correlations between thermopower S and charge conductivity s measured parallel and perpendicular to the polythiophene chain follow two master curves regardless of the chemical nature of the dopant: S// ∝ (σ//)^-1/4 along the chain direction whereas S⊥∝-ln(σ⊥) perpendicular to the chains.