Photoelectrolytic cells of two semiconductor electrodes

The photoelectrolytic cell can be composed of both a photoexcited n-type anode and a photoexcited p-type cathode here, the anode and cathode can be the same semiconductor or different semiconductors. The energy diagrams for a photo-electrol3 c cell of an n-type anode and a p-type cathode of different semiconductors are illustrated in Fig. 10-30. 

When the n-type anode and the p-type cathode are in the state of band edge level pinning, the overall potential due to the two space charge layers remains constant even in the state of photoexcitation. 

The photoelectrolytic cell consisting of n-lype and p-lype semiconductor electrodes provides an advantage over the cell consisting of semiconductor and metal electrodes a cell consisting of two semiconductor electrodes with their small band gaps adsorb the energy of solar photons more efficiently than the cell consisting of semiconductor and metal electrodes, in which the semiconductor electrode requires a relatively wide band gap for the decomposition of water. 

The cathodic and the anodic polarization potentials equal each other representing the operation potential if the iR drop in the electrolyte is negligible as shown in Fig. 10-32. 

It follows from Eqn. 10-58 that the affinity of the reaction of the photoelectrolytic cell, for which the polarization curves are shown in Fig. 10-32, is represented by the difference of the quasi-Fermi levels of interfacial minority charge carriers, between the anode and the cathode. Equation 10-58 

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