Infrared Photospectroelectrochemistry of Germanium/Pedot/Electrolyte Interfaces
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ABSTRACT Difference infrared spectra between illumination and dark of a polyethylenedioxythiophene (PEDOT) covered germanium electrode in contact with the electrolyte solution and under applied potential are shown. The spectra show a different behavior at different redox (doping) states of the polymer. The photoinduced formation of positive charge carriers was found by illumination of the neutral (undoped) form, wheras no significant spectral changes occur in the oxidized (p-doped) and reduced (n-doped) form of PEDOT.
INTRODUCTION Due to the inexpensiveness and easy fabrication, the use of organic materials is very attractive for photovoltaic energy conversion. In photoelectrochemical cells, thiophene based conjugated polymers show interesting properties in liquid [1] as well as solid [2] electrolyte systems. For a better understanding of the behavior of the photoactive semiconducting polymer, information on a molecular level of the processes in the substance in contact with the electrolyte solution during illumination is necessary. In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy has proved to be a valuable tool for providing these information [3,4]. In this paper, we present results on in situ ATR-FTIR spectroscopic studies of polyethylenedioxythiophene (PEDOT), a low band gap conjugated polymer, in a photospectroelectrochemical cell. Difference spectra between illumination and dark conditions of the polymer on a germanium reflection element in contact with the electrolyte solution under applied potential are shown.
EXPERIMENTAL PEDOT was electropolymerized from monomer solution on a germanium reflection element. The electrolyte was 0.1 M tetrabutyammoniumperchlorate in acetonitrile. As reference electrode Ag/AgCl wire, as counter electrode Pt foil was used. The illumination was performed with an Ar+ laser at 488 nm, 30 mW/cm-2. The FTIR spectrometer was a Bruker IFS66S with an MCT detector, spectral resolution 4 wavenumbers. The measurement procedure for a photoinduced spectrum was a coaddition of 30 repetitions of 10 scans (light on) and 10 scans light off. The spectra are shown as -∆T/T, (∆T = T(light on) - T(light off)). Spectroelectrochemistry was done with a potential scan rate of 5 mV/s and a simultaneous measurement of IR spectra by coadding of 32 interferograms
for one spectrum, covering about 85 mV in the cyclic voltammogram. Difference spectra with the spectrum of the neutral form as reference are shown. The photospectroelectrochemical cell is shown in Figure 1.
Figure 1: Photospectroelectrochemical cell.
RESULTS AND DISCUSSION Figure 2 shows difference spectra recorded during oxidation (p-doping, Figure 2a) and reduction (n-doping, Figure 2b) of PEDOT [5]. Different spectral signatures for both types of doping can be seen. The spectra are the basis for the identification of the photoinduced charge carriers during the photospectroelectrochemical experiments.
Figure 2: a) Difference spectra during oxidation (p-doping). Upper spectrum: full scale, lower spectr
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