Molecular doping of conjugated polymers (CPs) is a powerful strategy to enhance charge transport and improve the performance of organic electronic devices, particularly organic thermoelectrics. Doped donor–acceptor (D–A) conjugated polymers, which feature a tunable energy offset between the Fermi level and the transport band, can achieve high electrical conductivity (σ) while maintaining a favorable Seebeck coefficient (S). However, while doping has led to remarkable improvements in device efficiency, the thermal stability of chemically doped D–A polymers—a critical factor for long-term device operation—remains poorly understood.

In this study, we systematically investigated the dopant size-dependent thermal stability of a diketopyrrolopyrrole-thiophene (DPP–T) donor–acceptor copolymer, using two molecular p-type dopants with distinct sizes: F₄TCNQ and Mo(tfd-CO₂Me)₃. Temperature-dependent UV–Vis–NIR spectroscopy revealed that DPP–T/F₄TCNQ exhibits significant dedoping under thermal stress, whereas DPP–T/Mo(tfd-CO₂Me)₃ maintains stable optical and electrical properties at elevated temperatures. Although F₄TCNQ doping provides higher initial in-plane conductivity, its conductivity decreases by more than an order of magnitude after annealing at 120 °C for 30 minutes, while the Mo-based doped polymer remains unchanged under the same conditions. Thermogravimetric analysis ruled out dopant sublimation as the primary degradation pathway, suggesting that dopant phase separation and migration drive the observed instability. This mechanism was corroborated by X-ray scattering and nanoscale infrared microscopy and spectroscopy (AFM–IR), which revealed dopant redistribution within the film microstructure.

The insights from this work underscore the importance of dopant molecular design for achieving thermally robust charge transport in doped conjugated polymers. For practical thermoelectric applications, device modules must operate continuously at elevated temperatures where conventional dopants fail to remain stable. Our findings highlight that incorporating sterically bulky dopants can suppress diffusion and phase segregation, providing a rational path toward durable, high-performance organic thermoelectric systems. By establishing the direct link between dopant size, molecular interactions, and long-term stability, this study advances the design principles needed to translate organic thermoelectrics from laboratory demonstrations to real-world energy applications.