Organic mixed ionic-electronic conductors have drawn significant research attention lately owing to their ability to conduct electronic and ionic charges. Among them, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have been extensively studied. Besides its commercial availability, PEDOT:PSS has the main advantages of high flexibility, transparency, conductivity and biocompatibility, making it a promising candidate for applications in bioelectronics. PEDOT:PSS film is comprised of PEDOT-rich domains surrounded by a PSS-rich matrix, which absorbs water and solvated ions when immersed in an electrolyte solution.
While ionic transport occurs in the PSS domains, electronic transport is predominant along the conjugated PEDOT backbones. Furthermore, it is possible to modulate the doping state of PEDOT by applying an external voltage between a counter electrode immersed in the electrolyte and the PEDOT:PSS working electrode. Applying a negative voltage results in dedoping of the PEDOT:PSS film, since positively charged ions have the ability to penetrate the film to compensate for the PSS– anions. In contrast, redoping of the film occurs when a positive voltage is applied, and is attributed to the expulsion of the positively charged ions and decompensation of the PSS‑ matrix.
Further development of PEDOT:PSS-based devices requires a thorough understanding of the key processes and mechanisms influencing the switching voltage and changes in the electrical properties due to the electrochemical reactions. The electrical conductivity primarily depends on the nature and concentration of charged species at a given voltage, which can be evaluated using visible-near-infrared spectroelectrochemistry. The majority of reported studies follow the doping level of organic films by measuring the steady-state response at different voltages, but such measurements are not suitable for probing the dynamic of the dedoping and redoping processes in PEDOT:PSS. Therefore, developing more effective strategies for studying the dedoping/redoping kinetics in PEDOT:PSS is highly desirable.
Herein, Dr. Gonzague Rebetez, Dr. Olivier Bardagot, Mr. Joël Affolter, Dr. Julien Réhault, led by Professor Natalie Banerji from the University of Bern in Switzerland, investigated the temperature-dependence of the electrochemical dedoping/redoping dynamics in PEDOT:PSS thin films immersed in sodium chloride electrolyte. These processes were studied using time-resolved spectroelectrochemistry in the near-infrared and visible range at various temperatures. A multivariate curve resolution analysis was used to identify and resolve the spectral signatures of neutral, polaronic and bipolaronic states of PEDOT at different voltages. Furthermore, their dynamics were kinetically modeled to determine what drives the electrochemical processes. The research work is currently published in the journal Advanced Functional Materials.
The authors revealed that both the redoping and dedoping processes involve a sequential conversion between neutral species, polarons and bipolarons and occur within a few milliseconds. The van’t Hoff formalism was used to evaluate the temperature dependence of the doping level, highlighting the critical role of entropy and enthalpy in the establishment of the redox equilibria at a certain voltage bias. While polaronic and neutral species coexisted in equilibrium at negative voltage bias of -0.6 V, both bipolarons and polarons existed at a slightly positive bias voltage of +0.1 V. Using Eyring theory on the temperature-dependent dynamics, the impact of the activation parameters on the reaction rates was evaluated. Overall, the experiment offered a robust and versatile procedure that can be used to compare doping mechanisms between different materials and can be extended to other organic mixed ionic–electronic conductors.
In a nutshell, the study reported the investigation of electrochemical dedoping/redoping reactions in thin PEDOT:PSS films. Results showed that the dedoping and redoping processes were driven by enthalpy and entropy, respectively, while the reaction rates were mostly dependent on the entropic effects related to the changes in the backbone conformation during the reaction processes. The findings also highlighted the possibility of achieving intrinsic device switching speed independent of ionic diffusion. In a statement to Advances in Engineering, Professor Natalie Banerji, the corresponding and lead author, said their study offered valuable insights into the mechanisms determining the rates and extent of electrochemical processes in PEDOT:PSS and will contribute to the design of fast and efficient bioelectronic devices.
Rebetez, G., Bardagot, O., Affolter, J., Réhault, J., & Banerji, N. (2021). What Drives the Kinetics and Doping Level in the Electrochemical Reactions of PEDOT:PSS?. Advanced Functional Materials, 32(5), 2105821.