Intrinsically conducting polymers (ICPs) have received significant attention due to their exceptional properties. These include excellent chemical, thermal, and oxidative stability, tunable electrical properties, catalytic abilities, optical and mechanical features, and more. ICPs are used in myriad applications: in sensors, antistatic coatings, light-emitting diodes, transistors, flexible devices, and as the active material in electrochromic devices e.g., «smart» windows which regulate the amount of light that passes through.

Poly(3,4-ethylenedioxythiophene), otherwise known as PEDOT, is one of the most promising ICPs on the market. This is due to its high conductivity, electrochemical stability, catalytic properties, high insolubility in almost all common solvents, and interesting electrochromic properties (i.e., transparent in the doped state and colored in the neutral state). In this Application Note, PEDOT film is evaluated by spectroelectrochemical techniques.

This Raman characterization study was carried out using a SPELEC RAMAN (785 nm laser) instrument (Figure 1a), a Raman probe corresponding to the laser wavelength, and a Raman spectroelectrochemical cell for screen-printed electrodes (SPEs).

UV-Vis spectroelectrochemical measurements were performed using a SPELEC instrument (Figure 1b), a reflection probe for this spectral range, and a reflection cell for SPEs.

Gold SPEs (220AT) modified with a PEDOT film were used in this study. This setup allows users to obtain clear and detailed yet concise information about the behavior of PEDOT located on the electrode surface.

The SPELEC and SPELEC RAMAN instruments were controlled with DropView SPELEC software. DropView SPELEC is a dedicated software that provides spectroelectrochemical information and includes tools to perform adequate treatment and analysis of the collected data. All hardware and software used for this study is compiled in Table 1.

Raman spectroelectrochemistry was employed for the fingerprint characterization of the different oxidation states, neutral and doped, of PEDOT deposited on the Au SPE. The spectrum of the neutral state was obtained at -0.40 V (Figure 2, blue line) and p-doped PEDOT at +0.50 V (Figure 2, red line) in a 0.1 mol/L lithium perchlorate (LiClO₄) aqueous solution.

Assignments of the vibrational modes for each Raman band are listed in Table 2. The characteristic vibrational modes depend on the polymer oxidation state, particularly those located in the Raman shift region (1100–1600 cm^-1). Several Raman bands of PEDOT are up-shifted in the doped state. Note that although the C_α-C_α’ inter-ring stretching vibrational mode is not detected in neutral PEDOT, it is observed at 1293 cm^-1 in the doped state.

Valuable qualitative information provided by UV-Vis spectroelectrochemistry allows the complete characterization of the PEDOT film previously deposited on the gold working electrode. Spectroelectrochemical experiments were performed in a 0.1 mol/L LiClO₄ aqueous solution, scanning the potential from 0.00 V to +0.70 V and back to -0.40 V at 0.05 V/s for two cycles. UV-Vis spectra were recorded in reflection configuration (300 ms integration time), resulting in almost 300 spectra collected during the electrochemical experiment. Synchronization of the electrochemical and spectroscopic responses is completely assured by the SPELEC instrument.

Cyclic voltammetry (Figure 3a) does not show any remarkable electrochemical peaks associated with the change of the oxidation state of PEDOT. However, a UV-Vis band centered at 525 nm is clearly observed in the simultaneously recorded spectra (Figure 3b).

Evolution of the absorption band at 525 nm with changing potential is shown in Figure 4. Initially, absorbance decreases from 0.00 V to +0.70 V. In the backward scan, absorbance increases up to -0.40 V and decreases until 0.00 V, where it reaches a similar value as was at the beginning of the experiment. In the second scan, the spectroscopic signal shows the same spectroelectrochemical behavior. Absorbance at 525 nm at -0.40 V achieves the same value in both cycles, demonstrating the stability of this film for at least two cycles.

Evolution of this absorbance band with potential agrees with the electrochromic properties of PEDOT, being colorless in the doped state at positive potentials, while it is colored in the neutral state at negative potentials.

Figure 5 displays the relevant derivative voltabsorptogram (dAbs/dt vs. potential) at 525 nm. The derivative curve is only related to the faradaic component of the concomitant current flow. As can be observed in Figure 5, this derivative curve proves the polymer doping and de-doping processes through its reversible behavior.

Spectroelectrochemistry is a multi-response technique that provides outstanding results in the characterization of electrochromic materials, e.g., PEDOT polymer.

Raman spectroelectrochemistry gives fingerprint results that allow discrimination between neutral and doped states of the sample since the position of Raman bands depends on the oxidation state. In addition, UV-Vis spectroelectrochemistry shows the presence of an absorption band in the visible region that enables the spectral monitoring of the electrochemical characterization of PEDOT. Absorbance decreases at positive potentials (doped state) while increasing at negative potentials (neutral state).

Analysis of the stability of the PEDOT coating with potential as well as achieving a complete understanding of its optical properties are crucial in the development of new applications.