Measurements of photogeneration

Time-resolved measurements of photogenerated (very intense illumination, up to 0.56 GW/cm ) electron/hole recombination on CD (selenosulphate/NTA bath) CdSe of different crystal sizes has shown that the trapping of electrons, probably in surface states, occurs in ca. 0.5 ps, and a combination of (intensity-dependent) Auger recombination and shallow-trapped recombination occurs in a time frame of ca. 50 ps. A much slower (not measured) decay due to deeply trapped charges also occurred [102]. A different time-resolved photoluminescence study on similar films attributed emission to recombination from localized states [103]. In particular, the large difference in luminescence efficiency and lifetime between samples annealed in air and in vacuum evidenced the surface nature of these states. 

Nardello, V. and Aubry, J.-M. (2000) Measurement of photogenerated singlet oxygen in aqueous media. Methods in Enzymology, 319, 50—58. 

The increased lifetime of photogenerated minority carriers can be measured experimentally. This is shown for a single-crystal ZnO-electrode (Fig. 22). Both the stationary PMC peak and the potential-dependent lifetime in the depletion region, measured with transient microwave conductivity techniques are plotted.25 It is seen that the stationary PMC peak coincides with a peak in the lifetime of minority carriers. This... 

This means that the minority carriers are measured, however formally, with an effectively changed mobility, which also includes the mobility of photogenerated majority carriers. 

In this chapter we have attempted to summarize and evaluate scientific information available in the relatively young field of microwave photoelectrochemistry. This discipline combines photoelectrochemical techniques with potential-dependent microwave conductivity measurements and succeeds in better characterizing the behavior ofphotoinduced charge carrier reactions in photoelectrochemical mechanisms. By combining photoelectrochemical measurements with microwave conductivity measurements, it is possible to obtain direct access to the measurement of interfacial rate constants. This is new for photoelectrochemistry and promises better insight into the mechanisms of photogenerated charge carriers in semiconductor electrodes. 

The resist films were irradiated with the same incident doses used for the determination of acid content. A postbake was performed at 100°C for one minute. The carbonyl absorption was measured before and after irradiation and postbake. The difference was used to determine the number of t-BOC groups removed for a given weight of resist film. By dividing the number of t-BOC groups by the amount of photogenerated acid, the catalytic chain length was determined (Table II). 

The cathodic dark currents measured under negative bias in a DSSC (Fig. 4) are related to the recombination reactions (4) and (5) in the illuminated cells. However, the relation between the dark current and reaction (4), especially, is difficult to quantify. In conventional solar cells, dark current measurements provide quantitative information about the photorecombination processes [34] because (1) the number of photogenerated charge carriers is only a small perturbation on the dark carrier density and (2) the current flows along the same pathway in the light and in the dark. Neither of these conditions hold for a DSSC. 

For positive lit electrodes one can register the drift of holes, and for negative ones- the drift of the electrons. The photosensitizer (for example Se) may be used for carrier photoinjection in the polymer materials if the polymer has poor photosensitivity itself. The analysis of the electrical pulse shape permits direct measurement of the effective drift mobility and photogeneration efficiency. The transit time is defined when the carriers reach the opposite electrode and the photocurrent becomes zero. The condition RC < tlr and tr > t,r should be obeyed for correct transit time measurement. Here R - the load resistance, Tr -dielectric relaxation time. Usually ttras 0, 1-100 ms, RC < 0.1 ms and rr > 1 s. Effective drift mobility may be calculated from Eq. (4). The quantum yield (photogenerated charge carriers per absorbed photon) may be obtained from the photocurrent pulse shape analysis. 

On the fundamental side, the research on photocatalysis has focused on several topics, including a) the primary processes involved in the production and trapping of photogenerated electrons and holes, using pulsed femtosecond or picosecond laser techniques, b) measurements on the kinetics of the photodecomposition processes on longer time scales, and c) measurements on the kinetics on small size scales. For the first topic, the reader is referred to several recent publications.69-7 This work is of great practical importance, because it helps to point out the critical factors involved in the photocatalytic materials themselves. 

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