Photoelectrochemical Impedance Measurements

The general transfer hmction appears as the product of three transfer functions, i.e., 

This frequency-response analysis offers unique insights into complex photo- 

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EIS measurements can also be carried out under conditions where illumination of the semiconductor generates a photocurrent. The technique is then referred to as photoelectrochemical impedance spectroscopy, PEIS. Interpretation of the results in terms of passive RC circuit elements is no longer appropriate since the system contains a current source. A more satisfactory approach is to relate the impedance response directly to the physical processes responsible for the photocurrent (Ponomarev and Peter, 1995 Peter, 1999 Peter and Vanmaekelbergh, 1999). 

In addition to performance evaluations, many photoelectrochemical experiments are aimed to identify performance-Umiting steps or to determine certain materials properties. Examples of the latter are donor or acceptor densities and the flatband potential of a material, which can be determined by electrochemical impedance measurements. The challenge with these measurements is that they always yield data, but that it can be difficult - and sometimes even impossible - to translate the measured data to the desired materials parameters. Carefully performed control experiments and a good basic understanding of the measurement equipment -in particular, the potentiostat and the frequency response analyzer (FRA) - are essential for obtaining meaningful results. 

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). 

The three- or two-port photoelectrochemical ceUs (e.g.. Fig. 3.7b) should be made of material that is transparent to the illumination spectrum of interest. If UV irradiation is required, quartz windows must be used in the cell, as opposed to borosilicate glass which generaUy absorbs wavelengths below 360 nm. The distances between all electrodes should be minimized to limit the effect of electrolyte resistance on the electrochemical test. For electrical measurements, a low impedance ammeter can be used to measure the short-circuit current density. 

In the case of a semiconductor-based photoelectrochemical system, the measurement of the electrochemical admittance serves two purposes. As is explained in Sect. 2.1.3.1, it allows on the one hand the in situ determination of the energetics of the (bulk) semiconductor surface. On the other hand, it makes the dynamics of various (photo)electrochemical processes experimentally accessible. Clearly, EIS is also possible using an illuminated semiconductor, an experimental method sometimes referred to as PEIS. Finally, it should be noted that although the electrochemical admittance is determined experimentally (the applied electrode potential is used as the perturbation), the electrochemical impedance is generally plotted as the result of an EIS measurement. 

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