In-Operando Spectroscopic Characterization of Non-Equilibrium States in a Highly Doped Polymer

Heavily doped organic semiconductors are critical to a variety of applications, including thermoelectrics, bioelectronics, and neuromorphic computing devices. Recent advances have enabled the routine doping of polymer films to extremely high carrier densities on the order of one charge per polymer repeat unit. In this regime charge transport is comparatively poorly understood due to the non-negligible effects of electron-electron and electron-ion interactions, in addition to the ever-present role of static and transient disorder.<br/>Here, we report on dual-gated indacenodithiophene-co-benzothiadiazole (IDTBT) organic electrochemical transistors (OECTs) and corresponding ion-exchange doped films, in which a second field-effect gate enables us to modulate the carrier density independently from the ion density. These devices show unusual behavior; at low temperatures, field effect transfer curves are highly non-linear and in many cases even ambipolar, while at high temperatures a purely linear curve is obtained. Using solid-state NMR and infrared charge modulation spectroscopy, we demonstrate that this non-linearity at low temperatures is a signature of a non-equilibrium state resulting from freeze-out of ionic motion. Under these conditions, we see an enhancement in charge delocalization, electrical conductivity, and Seebeck coefficient, pointing to a new pathway to enhanced thermoelectric performance.