Measurements of Surface Recombination and Minority Carrier Injection

3 Measurements of Surface Recombination and Minority Carrier Injection 

The surface recombination is defined by Eq. (2.47) in Chapter 2. It can be measured by the thin slice method as described below. This method involves mainly a p-n junction at the rear of the electrode in addition to the ohmic contact mentioned in Section 4.1. Surface recombination is measured after excitation of electron-hole pairs near the electrode surface. They can diffuse towards the electrode surface but also towards the p-n junction at the rear. The electron-hole pairs are separated at the p-n junction which leads to a corresponding short-circuit current across this junction. As already discussed in Section 2.5, this current is proportional to the light intensity and therefore to the density of electron-hole pairs created by light excitation. If there is some additional surface recombination in a certain potential range, then fewer electron-hole pairs reach the rear p-n junction. A quantitative relation between the short-circuit current at the rear p-n junction j and light intensity and surface recombination can be derived as follows. 

Taking a p-type electrode as an example, the electrode is illuminated through the electrolyte as illustrated in Fig. 4.6. The diffusion of the excess minority carrier density An is given by the continuity equation [10] 

The generation rate depends on the light intensity /, and on the absorption coefficient a. The short-circuit current is defined as 

The surface recombination velocity can be introduced by a further boundary condition. At the semiconductor-liquid interface the recombination rate is given by 

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