Measurement of photocurrent

Fig. 8. Experimental set-up for the measurement of photocurrents. M = semiconductor electrode, C = counter electrode, R = reference electrode, Pot = potentiostat, G = voltage generator, Ch = chopper...

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 determination of threshold energies as shown in the inset is based on a modified display of the obtained photocurrent spectrum (see Fig. 5.125). The two linear sections yield two threshold energies, for a detailed treatment, see [722]. Measurements of photocurrents at p-iypc synthetic diamond electrodes that have wavelengths ranging from Xq = 193 to 351 nm yielded results implying that only illumination with the short wavelength (supra-bandgap illumination) was able to excite electrons into the conduction band [723]. 

Finally, IPA or interfacial photoanation reactions have been observed at liquid-liquid interfaces by measurement of photocurrents. The photoinduced exchange of ligands in the bulk is a well-characterized process. Brown has observed... 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

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-... 

Relaxations in photoprocesses, which may be due to surface recombination, minority carrier diffusion, or capacitive discharges, are typically measured as transients of photocurrents or photoprocesses. An analysis of such processes in the time domain encounters some inherent problems. 

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. 

Preliminary measurements with space-resolved PMC techniques have shown that PMC images can be obtained from nanostructured dye sensitization cells. They showed a chaotic distribution of PMC intensities that indicate that local inhomogeneities in the preparation of the nanostructured layer affect photoinduced electron injection. A comparison of photocurrent maps taken at different electrode potentials with corresponding PMC maps promises new insight into the function of this unconventional solar cell type. 

McCann JE, Pezy J (1981) The measurement of the Hatband potentials of n-type and p-type semiconductors by rectified alternating photocurrent voltammetry. J Electrochem Soc 128 1735-1740... 

Irradiation of an ITIES by visible or UV light can give rise to a photocurrent, which is associated with the transfer of an ion or electron in its excited state. Alternatively, the photocurrent can be due to transfer of an ionic product of the photochemical reaction occurring in the solution bulk. Polarization measurements of the photoinduced charge transfer thus extend the range of experimental approaches to... 

The basic idea of most diffusion length measurement techniques is to generate a certain number of minority carriers inside the bulk Si, for example by illumination, and to measure the fraction of these carriers that diffuse to a collecting interface. This fraction can be determined capacitively [Bo6], as well as by measurements of the steady-state photocurrent [Dr2, Lei 1], The parameter obtained by these measurements is the minority carrier diffusion length ID of electrons in... 

A very useful parameter for evaluating the performance of a photoelectrolysis cell is the incident photon to current conversion efficiency (IPCE). This is a measure of the effectiveness in converting photons incident on the cell to photocurrent flowing between the working and counter electrodes. IPCE is also called the external quantum efficiency. 

Fig. 5.39 Variation of photocurrent density (in IM KOH solution) vs. measured potential vs. Ag/AgCl for lOV samples anodized at four anodization bath temperatures, 5, 25, 35 and 50°C. The samples were measured under 320-400nm lOOmW/cm illumination.

The phase-dependent directionality of photocurrents produced by such a detector entails advantageous properties of the photocurrents cross correlations in nonoverlapping time intervals or spatial regions (considered in Section 4.2.2). These directional time-dependent correlations are measured with one detector only. They involve solely terms dependent on LO phases, in contrast to similar correlations measured by conventional photocounters, which inevitably contain terms depending on photon fluxes such as the LO excess noise. Owing to these properties, the mean autocorrelation function of the SL quadrature is shown in the schemes considered here to be measurable without terms related to the LO noise. LO shot noise, which affects the degree of accuracy to which this autocorrelation is measured (i.e., its variance) is easily obtainable from zero time delay correlations because the LO excess noise is suppressed. The combined measurements of cross correlations and zero time delay correlations yield complete information on the SL in these schemes. 

Fig. 15. Dependence of photocurrents (measured in the vidicon arrangement) on light inten-sity log /phot = / ( ) (1) Sb2S3 (2) phthalocyanine ( — 5500 A)...

To compare quantitatively the current-voltage characteristic of an illuminated electrode, given by formula (31), with experimental data, Butler (1977) and Wilson (1977) measured the photocurrent, which arises in a cell with an n-type semiconductor photoanode ( 2, W03) when irradiated with monochromatic light at a frequency satisfying the condition ha>> Eg. In this case a light-stimulated electrochemical reaction of water oxidation with oxygen evolution... 

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