Intensity modulated photocurrent spectroscopy IMPS

According to this definition = 1 in the range of saturated photocurrent. The current decrease observed at lower potentials is caused by the recombination of 

Similar to the derivation in Section 4.4, the modulations of these currents are described by 

The relative quantum yield 0, plays a role similar to that of the impedance Z, derived in Section 4.4. It is frequency dependent and (f spectra can be obtained. Examples are given in Section 7.3.5(b). 

If the electrode is illuminated by a light of modulated intensity, the current response of the electrode is a complex quantity consisting of the photocurrent amplitude and the phase shift between light and photocurrent. The method was developed primarily by Peter and 

The light from separating electrons and holes periodically charges the space-charge capacitance C. The recombination of the separated charge can occur internally via the resistance of the space-charge layer R. Impedance represents the internal recombination. The second way for the recombination, which is via the external circuit represented by impedance takes into account the charge transfer resistance and double layer capacitance Cj,. Resistance R represents the electrolyte resistance. 

Evaluation of IMP spectra demands comparison with impedance spectra. Commercial equipment for measuring parallel impedance spectra and IMP spectra is available. 

In this technique, as first developed by Li and Peters [16], the photocurrent instead of the potential is modulated. Hence, it is only applicable for minority carrier processes. The modulation of current is achieved by modulating the exciting light intensity. The current modulation is illustrated by a current-potential curve of an n-type semiconductor electrode (Fig. 4.15). The quantum efficiency is defined as the ratio of the current and intensity modulation ( / = AJ/AT). Since the intensity is not always known it is easier to use a relative quantum yield as defined by 

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The basic experimental arrangements for photocurrent measurements under periodic square and sinusoidal light perturbation are schematically depicted in Fig. 19. In the previous section, we have already discussed experimental results based on chopped light and lock-in detection. This approach is particularly useful for measurement at a single frequency, generally above 5 Hz. At lower frequencies the performance of lock-in amplifier and mechanical choppers diminishes considerably. For rather slow dynamics, DC photocurrent transients employing optical shutters are more advisable. On the other hand, for kinetic studies of the various reaction steps under illumination, intensity modulated photocurrent spectroscopy (IMPS) has proved to be a very powerful approach [132,133,148-156]. For IMPS, the applied potential is kept constant and the light intensity is sinusoid-... 

Fig. 8.7. Experimental arrangement for intensity modulated photocurrent spectroscopy (IMPS). FRA frequency response analyser. LED light emitting diode.

The quasi-Fermi level concept makes it possible to quantitatively relate intensity-modulated photocurrent spectroscopy (IMPS) and impedance data as follows. 

Fig. 7.46 Illustration of intensity-modulated photocurrent spectroscopy (IMPS) in terms of current-potential curves (see text)...

Fig. 8.9 Comparison of experimental and calculated intensity-modulated photocurrent spectroscopy (IMPS) plots for the photodissolution of n-Si (111) in 6.5 M NH4F at low light intensities (photo quadrupling regime) The data used for the theoretical plot are k -2-10 s= 500 s ...

As more recent examples [2, 9,10] reveal, intensity-modulated photocurrent spectroscopy, (IMPS), is also a powerfiil technique for the study of photocurrent multiplication. 

We will distinguish various modes. The techniques most widely used in DSSCs are intensity modulated photocurrent spectroscopy (IMPS) which is, like IPCE, concerned with electron transport under short circuit conditions Intensity modulated photovoltage spectroscopy IMVS, which probes the competition between transport and recombination at open circuit and electrical impedance spectroscopy (EIS), which probes the bias-dependent electrical response, analogous to dark-current transients. Other variants, such as frequency-resolved transmittance, the frequency domain analogs of transient absorption, have been developed [50]. The techniques and... 

Intensity Modulated Photocurrent Spectroscopy IMPS was used on unsensitized nanocrystaUine Ti02 electrodes by de Jongh and Vanmaekelbergh [121]. 

Photocurrent transient methods" " i o-iss intensity modulated photocurrent spectroscopy (IMPS) " ggjj established as good techniques... 

Eermm, D.J., H.D. Duong, Z.F. Ding, P.F. Brevet, and H.H. Girault (1999). Photoinduced electron-transfer at liquid/liquid interfaces—Part It—A study of the electron-transfer and recombination dynamics by intensity-modulated photocurrent spectroscopy (IMPS). Phys. Chem. Chem. Phys. 1, 1461-1467. 

Photocurrent transient methods [44, 150-155] and intensity-modulated photocurrent spectroscopy (IMPS) [142, 151, 156-161] have been established as good techniques for the investigation of semiconductor surfaces [162]. By these methods the working electrode is illuminated with either a light pulse or with sinusoidal modulated light, respectively, and the resulting photocurrent is monitored. Both techniques have been used at phthalocyanine electrodes to analyze in detail the kinetics of the photoelectrochemical reactions. 

The sensitization properties of such films were studied by photocurrent spectra, time-resolved photocurrent measurements and by intensity-modulated photocurrent spectroscopy (IMPS) [207, 209]. Visible light absorbed in the dyes led to the sensitization of ZnO in contact to an organic electrolyte as exemplary shown in Fig. 12. Many experiments were performed in a conventional three-electrode arrangement with the deposited hybrid thin films as working electrode, a Pt counter electrode and a Ag/AgN03 reference electrode. 0.5 M KI in acetonitrile/ ethylenecarbonate (1 4 by volume) was used as the electrolyte. Among TSPcMt... 

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