Category SOLAR PHOTON CONVERSION

Combination of electrochemistry with vacuum spectroscopy

Ultra-high vacuum surface science techniques use electrons or ions as probes that interact with matter sufficiently strongly that surface-specific information is obtained. Typically, a combination of techniques is used to characterise a surface as completely as possible.

Table 12.1 List of vacuum techniques used to characterise semiconductor surfaces.

Technique

Particle/photons in/out

Information

depth/nm

Type of information

UV photoelectron spectroscopy (UPS)

UV/electrons

0.5-20

electronic structure of valence band interfacial energetics

X-ray photoelectron spectroscopy (XPS)

X-rays/electrons

1-3

elemental composition valence states of elements

Electron loss spectroscopy (ELS, HREELS)

electrons/same electrons

0.5-2

electronic and vibronic excitati...

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Differential mass spectrometry (DEMS)

For fuel-generating photoelectrochemical devices, it is desirable to determine chemical products dynamically in situ during potential variations. This is possible when mass spectroscopy is coupled to electrochemical experiments via specific membranes that are permeable to volatile chemicals but which separate the electrolyte from a vacuum chamber linked to the mass spectrometer (Bittins-Cattaneo et al 1991; Schmidt et al1996). Various arrangements have been developed and applied. Critical factors are the construction of the electrode and the transport mechanism of chemicals to the semipermeable membrane...

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In-situ X-ray analysis and EXAFS

Photoelectrochemical mechanisms may be characterised by potential-dependent structural changes of the photoactive interfaces. These may include stoichiometric changes (e. g. intercalation or excalation of ionic species), phase changes or dynamic molecular changes of photocatalytic species. In all these cases, time-dependent structural information is desirable. This may be accomplished with in-situ X-ray and EXAFS (X-ray absorption fine structure) techniques. For such measurements, the electrochemical cell is integrated into a ‘Plexiglass’ body covered with Capton layers that are penetrated by the X-rays from the diffractometer source or the synchrotron (EXAFS), which are incident at a low grazing angle, as shown in Fig. 12.38.

Schubert et al...

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Imaging techniques

If, instead of measuring integral parameters of an energy-converting interface, a spatially resolved image is produced of a relevant quantity such as photocurrent, photopotential or microwave conductivity, luminescence or electroreflectance, significantly more information is available. Such a spatially-resolved image of a semi­conductor interface typically shows patterns determined by inhomogeneities of the material or surface preparation. If photoelectrochemical reactions are allowed to proceed, it can be observed where they are best catalysed or where photocurrents tend to concentrate...

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Light-modulated microwave reflectivity

The interpretation of IMPS data relies on the competition between charge transfer and recombination to give a semicircle that can be analysed to obtain kct and krec. It follows that IMPS provides no information if recombination is absent. Surface recombination usually becomes negligible at potentials sufficiently far from the flatband potential, where a plateau is seen in the photocurrent-potential plot. IMPS plots measured in the plateau region do not exhibit a semicircle in the upper quadrant; only the lower RC semicircle is seen. Light-modulated microwave reflectance spectroscopy (LMMRS), on the other hand, can provide information about kct even when krec = 0 (Schlichthorl et al., 1995; Peter and Vanmaekelbergh, 1999)...

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Intensity-modulated photovoltage spectroscopy

The basic measurement technique for intensity-modulated photovoltage spectroscopy (IMVS) is the same as for IMPS. In principle, IMVS measurements can be made for any constant current condition, but in practice it is usual to make measurements under conditions where the net current is zero. In the case of a photoelectrochemical solar cell, this corresponds to the open-circuit condition, and a high impedance voltage amplifier is used to ensure that a negligible current is drawn from the illuminated device. The output of the voltage amplifier is fed to the FRA, and the remainder of the set up is the same as for IMPS (cf. Fig. 12.26).

In a conventional solid-state photovoltaic cell or a semiconductor/electrolyte junction, the photovoltage is related to the densities of free electrons and holes...

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Calorimetric measurements

In photocalorimetry, the relative temperature change of a photoactive electrode is measured as a function of the electrode potential under chopped monochromatic illumination (Fujishima et al., 1977; Rappich and Dohrmann, 1989; Dohrmann and Schaaf, 1992; Dohrmann and Reck, 1998). Either pyroelectric or piezoelectric sensors or photoacoustic microphones are coupled to the back surface of the photo­electrode to monitor the temperature change produced by the transmitted heat wave. Alternatively, photothermal laser beam reflection at the front surface of the semiconductorlliquid junction can be utilised to probe the modulated temperature (Royce et al., 1982). Irrespective of the detector scheme, which in modern studies frequently relies on easy-to-handle piezo films (e. g...

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Modulation spectroscopies

12.6.1 Electrolyte electroreflectance spectroscopy

Electrolyte electroreflectance (EER) spectroscopy of semiconductors is based on the use of a transparent electrolyte contact to form a Schottky barrier in which the electric field perturbs the optical constants (i. e. the real and imaginary components of the complex dielectric function) of the solid. Under depletion conditions, the width of the space-charge region, and hence the electric field distribution, is determined by the applied DC potential. If a small square-wave or sinusoidal perturbation of the potential is superimposed on the DC potential, modulation of the optical constants by the AC field manifests itself by a modulation of the reflected light intensity...

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