Photogenerated charge carriers

Houle F A 1989 Photochemical etching of silicon the Influence of photogenerated charge carriers Phys. Rev. B 39 10 120-32... 

Microwave measurements are typically performed at frequencies between 8 and 40 Gc/s. The sensitivity with which photogenerated charge carriers can be detected in materials by microwave conductivity measurements depends on the conductivity of the materials, but it can be very high. It has been estimated that 109-1010 electronic charge carriers per cubic centimeter can be detected. Infrared radiation can, of course, also be used to detect and measure free electronic charge carriers. The sensitivity for such measurements, however, is several orders of magnitude less and has been estimated to be around 1015 electronic charge carriers per cubic centimeter.1 Microwave techniques, therefore, promise much more sensitive access to electrochemical mechanisms. 

In the following section the mathematical derivation of the stationary, potential-dependent, photoinduced microwave conductivity signal, which integrates over all photogenerated charge carriers in the semiconductor interface, is explained. This is a necessary requirement for the interpretation of the PMC-potential curves. 

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. 

A characteristic property of these different modes is their spatial resolution. A spatial resolution in the order of the illumination wavelength used can be obtained if lateral diffusion of the photogenerated charge carriers is suppressed. This is not the case in mode 2, or in mode 4 if the electrode is under inversion (with J>JPS). If the electrode is kept under depletion as in modes 3 or 4 (with J[Pg.73]

A series of competing processes arise from photogenerated charge carriers in a semiconductor nanoparticle since a large percentage of the atoms are at the surface and behave differently than those in the bulk. An exciton at the surface is rapidly (picosecond) trapped due to surface defects, with the electron-hole pair subsequently participating in transfer between the semiconductor nanoparticle and the electrolyte adsorbed on its surface ... 

Lubberhuizen WH, Vanmaekelbergh D, Van Faassen E (2000) Recombination of photogenerated charge carriers in nanoporous gallium phosphide. J Porous Mater 7 147-152... 

Photoconducting polymers such as poly (JV-vinylcarbazole) are used in xerography or electrophotography, since the photogenerated charge carriers can travel through the polymer film with relative facility before getting immobilized or trapped (Roberts,... 

The diffusion length of photogenerated charge carriers is one of the important parameters governing the efficiency of a solar cell. In conventional cells, this is an intrinsic property of the semiconductor and its purity [34]. However, in DSSCs, the diffusion length is a function of the rate of reaction (4) and, thus, varies with different redox couples, surface treatments, and so forth. When the oxidation of R [reaction (2)] is chemically irreversible, the diffusion length of electrons is effectively infinite, whereas with kinetically fast, reversible redox couples (see Section VI), it approaches zero with unpassivated interfaces. 

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. 

The photocurrent-voltage curve of a cell made with the I /I2 redox couple (Fig. 8) shows behavior typical of the standard DSSC. The substantial photovoltaic effect is expected from the fact that the dark current (Fig. 4) is negligible positive of about -0.5 V. On the other hand, a cell made with the FcCp2 70 redox couple shows no measurable photoeffect Its current under illumination (Fig. 8) is essentially equal to its dark current (Fig. 4). The photovoltaic effect is negligible because practically all photogenerated charge carriers recombine before they can be collected in the external circuit. In general, fast rates of reactions (4) and (5) tend to eliminate the photovoltaic effect in DSSCs. 

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. 

Although similar grating cancellation and revelation behaviors have been observed in other PR polymeric composites, they are attributed either to the trap s intercommunication or to the residual ionic motion [101-103]. However, the complementary gratings in the present study are formed by the space-charge field of two types of photogenerated charge carriers. 

To improve the resolution of an imager the distance between adjacent photodiodes is reduced. A problem occurs when the distance reaches the same dimensions as the diffusion length of photogenerated charge carriers. In EP-A-0024970 (Thomson-CSF, France, 11.03.81) an insulating opaque film is introduced between the photodiodes allowing a close spacing of the photodiodes. 

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