Kinetic Studies by Photoluminescence Measurement

Theoretically, the luminescence intensity is proportional to the rate of electron-hole recombination, i.e. I oc n z)p z). In the simplest case it is assumed that the majority carrier density is not substantially changed upon illumination. TTien the luminescence intensity is given by 

7 Charge Transfer Processes at the Semiconductor-Liquid Interface 

It increases with the square root of the potential across the space charge region, sc-Accordingly, the minimum luminescence should further decrease with increasing anodic potentials. This effect can be quantified by performing the integration in Eq. (7.115) under the condition that s DIL within the range of djc — t — (instead of 0 z °o). Instead of Eq. (7.119) we obtain then 

More recently time-resolved techniques have been applied for studying photocarrier dynamics at the semiconductor-liquid interface. One of the main motivations is that such studies can lead to an estimation of the rate at which photo-induced charge carriers can be transferred from the semiconductor to a redox acceptor in the solution. This method is of great interest because rate constants for the transfer of photocarriers cannot be obtained from current-potential curves as in the case of majority carrier transfer (Section 7.3.5). The main aim is a detailed understanding of the carrier dynamics in the presence of surface states. The different recombination and transfer processes can be quantitatively analyzed by time-resolved photoluminescence emitted from the semiconductor following excitation by picosecond laser pulse. Two examples are shown in Fig. 7.60 [82, 83]. 

62) is only valid for small changes in majority carrier densities, i.e. Ap po. Since in most cases fairly intense laser pulses were used, a more complex equation must be used which finally leads to quadratic concentration terms in Eq. (7.115). The exact procedure for the evaluation cannot be given here and the reader is referred to 

The figure shows how the luminescence decay observed with a p-GaAs electrode varies upon the addition of an electron acceptor such as cobaltocene (CoCpp and ferrocene (FeCpJ). The GaAs surface was passivated in order to keep the surface recombination low. The evaluation of such a decay curve is not easy. In principle, the continuity equation (Eq. (7.62)) must be solved again, this time under conditions of short pulse illumination the resulting minority carrier 

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