What Drives the Kinetics and Doping Level in the Electrochemical Reactions of PEDOT:PSS?

Significance 

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 PSSmatrix.

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.

What Drives the Kinetics and Doping Level in the Electrochemical Reactions of PEDOT:PSS? - Advances in Engineering
Figure 1: Enthalpy drives dedoping in PEDOT:PSS electrochemical reactions, while entropy drives the redoping at an appropriate voltage.

About the author

Natalie Banerji is a Full Professor of Chemistry at the University of Bern. She is the leader of the FemtoMat group. Her research interests include the study of organic and hybrid materials using ultrafast spectroscopy, in view of bioelectronic and photovoltaic applications. She studied Chemistry at the University of Geneva and obtained her PhD in Physical Chemistry in 2009 (with Prof. Eric Vauthey). She studied organic photovoltaics during a post-doctoral stay (2009–2011) with Nobel Laureate Prof. Alan J. Heeger at the University of California in Santa Barbara, and consequently held independent positions at EPFL (Ambizione, 2011-2014) and at the University of Fribourg (Associate Professor, 2014-2017).

About the author

Gonzague Rebetez is a PhD candidate in the FemtoMat research group. He developed Vis-Nir and THz spectroelectrochemical instruments to study, in operando, the fundamental processes and working principles of organic mixed ionic-electronic conductors. He obtained his Master’s degree in chemistry from the University of Fribourg in Switzerland and did his Master project in the group of Prof. Rafal Klajn at the Weizmann Institute of Science on the synthesis and characterization of light-sensible molecules.

Reference

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.

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