An Overlooked Transport Mechanism in Organic Mixed Conductors

Organic mixed ionic‐electronic conductors (OMIECs) sustain a wide range of bioelectronic, sensing, energy storage, and neuromorphic technologies. While water is known to enhance ionic transport and redox activity in these materials, the role of polarization dynamics in their electrical response remains largely unexplored. Here, eumelanin (EuM) is used as a model platform in which polarization‐assisted charge transport can be systematically resolved by varying hydration. To assess whether this characteristic polarization signature extends beyond EuM, complementary measurements were performed on the widely studied OMIEC PEDOT:PSS. By combining large‐ and small‐signal sinusoidal electrical measurements of EuM films with an analytical transport model, we demonstrate that hydration‐induced polarization dynamics generate displacement currents that can become comparable to conventional conductive currents. The results reveal a progressive transition from low‐hydration regimes characterized by limited charge redistribution and long relaxation times to highly hydrated states exhibiting a large density of polarization‐active sites and shorter relaxation times. These findings show that the electrical response of OMIECs cannot be fully described solely in terms of ionic and electronic conduction mechanisms, establishing a framework for identifying and interpreting polarization‐related contributions to their hysteretic electrical responses and providing a basis for future design strategies in sensing and memory technologies.