Photopotentials transients

Photopotential transients have also been studied [181]. The light pulse will generate a non-stationary concentration of electrons and holes analysis reveals that these separate rapidly in the depletion layer (type semiconductor) and an exponential concentration of electrons at the inner edge of the depletion layer, as discussed above. This new charge distribution will alter the potential distribution and numerical integration for an n-type wide bandgap material shows that, if... 

Experiments with laser light pulses provide further insight into the action of the light. The separation of the charge careers by the Ught pulse generates a photopotential. A photopotential transient follows the decay of the photopotential.The equivalent circuit in Figure 9.12 used for the interpretation of IMPS also explains the processes on the semiconductor surface after a laser pulse. 

For the measurement of the photopotential and photopotential decay the usual potentio-stat/galvanostat could not be used and special equipment was developed. An example of a photopotential transient measured on n-GaAs is shown in Figure 9.13. 

Figure 9.13 Photopotential transient of p-GaAs. Reduced presentation E IE and presentation of the reciprocal values, 0.05 mol-cm HjSO Excimer laser, X = 308 nm and external...

Fig. 106. Photopotential decay transients following a ns illumination pulse on n-CdSe in contact with selenide solution. The lower curves show the mechanically polished electrodes with a damaged surface layer and the upper curves the same after etching.

As in all potentiostatic techniques, the double layer charging is a parallel process to the faradaic reaction that can substantially attenuate the photocurrent signal at short-time scale (see Section 5.3)" . This element introduces another important difference between fully spectroscopic and electrochemical techniques. Commercially available optical instrumentation can currently deliver time resolution of 50 fs or less for conventional techniques such as transient absorption. On the other hand, the resistance between the two reference electrodes commonly employed in electrochemical measurements at the liquid/liquid interfaces and the interfacial double layer capacitance provide time constants of the order of hundreds of microseconds. Consequently, direct information on the rate of heterogeneous electron injection from/to the excited state is not accessible from photocurrent measurements. These techniques do allow sensitive measurements of the ratio between electron injection and decay of the excited state under pho-tostationary conditions. Other approaches such as photopotential measurements, i.e. relative changes in the Fermi levels in both phases, can provide kinetic information in the nanosecond regime. 

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