Experimental aspects of photocurrent spectroscopy

Quantitative measurements are best carried out at much lower light intensities than those responsible for the large effects illustrated in Fig. 5. It is desirable to avoid, as far as possible, the photoelectrochemical oxidation or reduction of the film on the time scale of the measurements and this generally restricts incident power densities to less than 10-4 Wcm 2. Since the photocurrents generated by such low levels of illumination are too small to be measured directly, it is necessary to use a lock-in amplifier in conjunction with a mechanical chopper in the experimental arrangement shown in Fig. 6. The sensitivity of the photocurrent spectrometer is usually determined by the noise current arising from the electrode capacitance and the noise voltage in the system. Under favourable conditions, it is possible to measure photocurrents as small as 10 10 A. For typical illumination inten- 

The spectrometer can also incorporate facilities for single beam transmission and reflectance measurements since it is often necessary to determine the amount of light reflected or absorbed in order to calculate the quantum efficiency of photocurrent generation. Optically transparent electrodes (OTEs) are particularly useful substrates since they allow simultaneous measurements of photocurrents and transmission to be made. Metal films can be prepared by vacuum deposition on the OTEs or, in some cases, the material of interest can be electroplated directly. If Sn02-coated quartz electrodes are used, it is essential to check them periodically to make sure that they do not give rise to background photocurrents since aged electrodes 

The signal-to-noise ratio can be improved if a low-noise battery-operated potentiostat is used and it is usually not necessary to resort to a two-electrode configuration unless the photocurrents are very small or the electrode capacitance unusually large. It is often useful to reduce the sample area to match the illumination spot so as to eliminate the noise contribution from the part of the electrode that is not illuminated. Particular care is necessary to eliminate earth loops and high-frequency pick-up and a screened Faraday cage is essential. Commercial reference electrodes can be replaced by low-resistance electrodes if they cause problems or bridging capacitors can be used to bypass high-resistance liquid junctions. 

Derivation of absorption spectra from photocurrent spectra 

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