Photocurrent spectroscopy measurement

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Intensity-modulated photocurrent spectroscopy has been used in combination with microwave reflectivity measurements to investigate hydrogen evolution at a p-type silicon45 and an n-type silicon.46 The measurement of amplitude and phase under harmonic generation of excess carriers, performed by Otaredian47 on silicon wafers in an attempt to separate bulk and surface recombination, should also be mentioned here. 

Electrolyte contacts have been used to characterize as-deposited and annealed CdS/CdTe solar cell structures by photocurrent spectroscopy and electrolyte elec-troabsorbance/electroreflectance measurements (EEA/EER) [267-269]. 

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. 19 Block diagrams for photocurrent measurements with chopped light and lock-in detection (a) as well as for intensity-modulated photocurrent spectroscopy (b). (Adapted from Ref. 85.)... 

Fig. 16 Parameters for defining the charge-transfer state energy cx in organic solar cells. Charge-transfer state energy for MDMO-PPV PCBM blend device determined by Fourier transform photocurrent spectroscopy and electroluminescence measurements. Reprinted figure with permission from [188]. Copyright 2010 by the American Physical Society...

The oxide layer of a metal such as copper may be seen as a semiconductor with a band gap, which may be measured by absorption spectroscopy or photocurrent spectroscopy and photopotential measurements. Valuable additional data are obtained by Schottky Mott plots, i.e. the C 2 E evaluation of the potential dependence of the differential capacity C. For thin anodic oxide layers usually electronic equilibrium is assumed with the same position of the Fermi level within the metal and the oxide layer. The energetic position of the Fermi level relative to the valence band (VB) or conduction band (CB) depends on the p- or n-type doping. Anodic CU2O is a p-type semiconductor with cathodic photocurrents, whereas most passive layers have n-character. 

Current-potential measurements, in the dark and under illumination of the semiconductor working electrode, are extremely useful for first defining the charge-transfer behavior across the interface before more sophisticated experiments are undertaken. The irradiation can be either continuous or intermittent (chopped) the latter mode has the distinct advantage that both the dark and light behavior can be examined in the same scan [55, 58]. Even some dynamic information can thus be extracted under the nominally steady-state conditions typical of a cyclic or linear potential sweep experiment. Another useful steady-state experiment is photocurrent spectroscopy (performed at a fixed DC potential) [55], although this can also be dynamically performed via IMPS (see below). Such measurements not only yield the so-called photoaction spectrum of the semiconductor electrode, but also afford information on surface recombination and surface state activity at the interface as discussed below. 

Photocurrent spectroscopy involves quantitative measurement of the ratio of the electron flux in the external circuit to the incident photon flux (in some cases... 

Photovoltage spectra are measured at open circuit using chopped light of low intensity. It might appear that an advantage of photovoltage spectroscopy over photocurrent spectroscopy is that no photocorrosion occurs. However, this is not necessarily correct, because anodic photocorrosion in the illuminated areas may be balanced by cathodic reduction of solution species such as oxygen or protons. 

The application of photocurrent spectroscopy is not restricted to bulk semiconductors and insulator electrodes. The anodic oxidation of many metal electrodes produces surface films that are insulators or semiconductors, and in spite of the fact that these surface films are often very thin, their characterisation by photocurrent spectroscopy poses few experimental difficulties since photocurrents as small as 10 10 A can be measured by conventional lock-in methods. The instrumentation required for photocurrent spectroscopy is relatively modest and the technique is undemanding in terms of the degree of optical perfection of the electrode surface. Consequently, there seems to be considerable scope for the application of this type of spectroscopy to electrochemical problems such as corrosion, for example, where surface roughening may rule out methods that require an optically flat surface. 

Figure 7(b) contrasts the absorption data obtained by analysis of the photocurrent spectra with the results of the transmission measurements. It is apparent from the figure that the two methods give spectra that almost coincide at short wavelengths. At the same time, the comparison underlines the advantages of photocurrent spectroscopy which were used to derive absorption coefficients as low as 20, two orders of magnitude below the lowest reliable values obtained from the transmission measurements. The identical results obtained by absorption and photocurrent spectroscopy provide a direct proof that the assumptions made in the analysis of the photocurrent spectra were satisfactory. 

Conclusive proof of the identity of the photoactive film on lead follows from the analysis of the photocurrent spectra. Figure 9(b) compares the spectrum of the anodic photocurrent with the absorption spectrum of the tetragonal form of PbO. The oxide film is sufficiently thin for the photocurrent to be a linear function of the absorption coefficient so that direct comparison of the photocurrent and absorption spectra is possible and it is clear from the coincidence of the two spectra that the film consists of tetragonal PbO. The results illustrate the sensitivity of photocurrent spectroscopy as a method for the identification of thin surface phases. In-situ Raman spectroscopy [24] and in-situ X-ray measurements [25] of the same system show no evidence for the formation of PbO unless the electrode is held at a constant potential for a time sufficient for a much thicker layer of oxide to be formed. By contrast, photocurrent spectroscopy is sufficiently sensitive to detect the formation of PbO on the much shorter timescale of a linear sweep measurement and quantitative estimates of the film thickness are feasible. Similar results have been obtained for the reduction of a-Pb02 in alkaline solution, where the existence of the tetragonal form has also been established from the photocurrent spectra [26]. 

Photoeffects at copper electrodes coated with copper phenylacetylide (CuPA) films were discovered during an in-situ laser Raman investigation [35], where substantial cathodic photocurrents were observed. A subsequent detailed study by photocurrent spectroscopy showed that the photocurrent conversion efficiency was of the order of a few percent, even for thicker films which absorbed a substantial fraction of the incident light. Figure 18 contrasts the photocurrent excitation spectrum with the absorption spectrum measured on an OTE coated with the CuPA polymer. The coincidence in the vibrational structure in the two spectra is striking, suggesting that the absorption gives rise to a state with considerable molecular character. 

Enhancing the catalysis at the surface of PEC electrodes results in a lower kinetic overpotential and an increase in photocurrent. The effectiveness of the catalysts after surface treatment can be determined by utilizing three-electrode j-V measurements (see Section Three-Electrode j-V and Photocurrent Onset ) as well as IPCE measurements (see Chapter Incident Photon-to-Current Efficiency and Photocurrent Spectroscopy ). It may also useful to perform Mott-Schottky (see Section Mott-Schottky ) to determine any impacts these catalysts may have on the band structure (e.g., due to Eermi level pinning). 

Again, it should be stressed that the optoelectrical transfer function is defined using small-signal perturbations, superimposed on time-independent bias signals. Measurement of the optoelectrical impedance is often referred to as intensity modulated photocurrent spectroscopy (IMPS). 

The sensitization of such films were smdied in detail by photocurrent spectra, time-resolved photocurrent measurements, and intensity-modulated photocurrent spectroscopy Visible light absorbed in the dyes led to the... 

In the ZAHNER controlled intensity modulated photocurrent spectroscopy (CIMPS) system, the light source and the cells are aligned on an optical track. Switching between the reference and the measurement cell is automatic. A close-up of the measurement cell is shown in Figure 15.41, set up for a study of a multilayer organic solar cell model system, shown schematically on the left side. In the PECC-2 cell shown, the reference electrode is Ag/AgCl and the CE is a Pt coil. The cell construction from PTFE or similar polymers allows the use of aggressive and nonaqueous electrolytes. 

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