The Effect of Macroscopic Fields and Electrochemical Potential on Ion Transport in Organic Electrochemical Devices

Organic electrochemical devices include electrochemical transistors, electrochromic devices, biosensors, light emitting electrochemical cells, and energy storage devices. The fundamental mechanism of ion transport through an organic conducting or semiconducting film remains a topic of widespread interest. My group has championed diffusion as the primary mechanism of ion transport on the basis of charge balance considerations. However, it is common to view nanostructured polymer semiconductors and conductors as analogous to activated carbon, which operates in a manner similar to an RC circuit. In this model, the resistance can be ohmic and time independent or deviate strongly from simple behavior both with respect to voltage and time. However, fundamental to the concept of resistance as a description of the motion is charged species is that they drift in response to an applied field. We present scan rate dependent cyclic voltammetry and spectroelectrochemical dynamics on PEDOT:PSS and ProDot thin films. The dynamics are insensitive to electrode placement at the top of the film or the bottom of the film, indicating that the presence of an electric field across the film thickness has no influence on ion transport. It is tempting, given this result, to infer that the applied potential has no influence on the ion transport rate. We demonstrate that this inference is also incorrect, the ion transport rate can be strongly influenced by the applied potential. Interestingly, we frequently observe that turn-on dynamics are much faster than turn-off dynamics, in contradiction to recent reports.¹ Approaching the ion transport problem from the perspective of polymer mixing thermodynamics using a voltage dependent enthalpy of mixing resolves the apparent contradictions. Increasing the applied potential increases the thermodynamic driving force for ion transport, accelerating the transport rate exponentially. Recent advancements on determining the functional form of the thermodynamic diffusion constant will be discussed that provide insight into why a strong voltage dependence on the ion diffusion rate in PEDOT:PSS is not always observed.

1. Guo, J.; Chen, S. E.; Giridharagopal, R.; Bischak, C. G.; Onorato, J. W.; Yan, K.; Shen, Z.; Li, C.-Z.; Luscombe, C. K.; Ginger, D. S., Understanding asymmetric switching times in accumulation mode organic electrochemical transistors. Nat. Mater. 2024, 23 (5), 656-663.