Dynamic Charging Mechanism of Organic Electrochemical Devices Revealed with In Situ Infrared Spectroscopy

The steps responsible for device charging in organic electrochemical transistors were investigated using in situ infrared spectroscopy. Metal-electrolyte-dielectric-organic semiconductor capacitor structures were fabricated on infrared waveguides and measured using the total internal reflection sampling method. Upon the application of a voltage, charges were induced in the device, creating polaron absorption features in the mid-infrared region. The dynamics of the device charging were investigated by varying the channel length, dielectric layer thickness, and organic semiconductor layer thickness. Device charging was independent of the channel length but depended strongly on the semiconductor and dielectric layer thicknesses, indicating that the movement of ions is the primary determining factor for device charging kinetics. A quantitative model is developed combining an resistor-capacitor (RC) circuit-model for the dielectric layer and a mixed ion-carrier diffusion model for the organic semiconductor layer. The model is evaluated against an RC circuit model for electrochemical charging dynamics, which is used in the popular Bernards model. Thickness-dependent dynamics cannot be adequately explained using the RC circuit model. The model presented here is significantly more appropriate for fundamental studies of ion dynamics in pi conjugated materials.