Thermally Controlled Polymorphism and Doping Gradients in Poly(didodecyl-quaterthiophene) for Functionally Graded Organic Thermoelectrics

Conjugated polymers are essential for flexible organic thermoelectrics (OTEs). Herein, we demonstrate that polymorphism in 3,3‴-poly(didodecyl-quaterthiophene) (PQT) significantly dictates doping efficiency and charge transport upon 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) vapor doping. Using X-ray scattering, ellipsometry, and molecular dynamics simulations, we characterize how thermal annealing induces distinct side-chain and backbone orientations. 130 °C annealed samples reach a conductivity (σ) of 8.6 ± 0.1 S/cm, while 100 °C samples only exhibit a conductivity of 0.3 ± 0.2 S/cm due to inhibited doping from the polymorph with stronger interdigitation of the side chains. Leveraging these findings, we fabricated functionally graded (FG) PQT films using a custom thermal annealing station to induce spatial microstructural gradients. Modeling the cooling performance via linear constitutive relations and experimental transport profiles reveals that FG films outperform equivalent uniform counterparts in cooling performance (ΔTc = TH − Tc). Overall, this work establishes a robust strategy to enhance OTE performance by leveraging thermally induced microstructural gradients in semiconducting polymers.