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Electrochemical measurements

Natalia Zawacka

Conjugated polymers present a continuous and uninterrupted network of alternated double and single bonds, which leads to the formation of π-states delocalized along the polymer conjugation length. These π-states form the frontier electronic bands, with a corresponding band gap, $E_{opt}$. When light reaches the photoactive layer, the conjugated polymer absorbs photons of energy equal to or higher than the band gap and the electrons are excited from the HOMO level to the LUMO level (transition from $S_0$ to $S_1$).

Electrochemical measurements are used as a method for evaluation of the position of HOMO and LUMO energy levels and the band gap of the polymers. Electrochemical techniques are also useful for investigating reversibility, stability and rearrangements of the polymer films deposited on the electrode. Voltammetry measurements give information regarding redox properties, the oxidation and reduction potentials. The oxidation corresponds to electron extraction from the HOMO level and can be correlated to the ionization potential, whereas the reduction potential is associated with electron affinity and indicates the LUMO level. In the main electrochemical technique, cyclic voltammetry (CV), the current is measured while the voltage is continuously varied (linear sweep). The input parameters are therefore the initial and final voltage and the scan rate, while the output parameters are the voltages values at which the peaks occur and current intensities. For some polymers, either anodic or cathodic signals are not well resolved, in such cases differential pulse voltammetry (DPV) and squarewave voltammetry (SWV) techniques, where the current is measured before and after each potential step change (staircase voltammetry) is used.

Figure 2. Exemples of voltammograms: squarewave (SWV) of oxidation (top left) and reduction (top right) process and CV of a low-band gap polymer. Extraction of onset potentials is indicated.

The onset potentials of oxidation and reduction are determined from intersection of the tangents between the baseline and the signal current. The use of a reference compound is required and the comparison of the potential values for the two materials, with subsequent conversion to vacuum level, provides the HOMO and the LUMO energy levels and the electrochemical band gap, $E_g$ of the analyzed material. Usually the electrochemical band gaps of the polymer films are higher than the values extracted from the optical absorption spectra, due to electrode-film interface charge barrier. The usual practice is to calculate the energy of the LUMO level by subtracting the electrochemically obtained HOMO value from the optical band gap; the HOMO corresponds to ionization potential, whereas the LUMO refers to an excited state and the optical measurement should be more meaningful.

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