Photoexcited electrode reaction current Photocurrent

Photoexcited electrons or holes migrate in a space charge layer towards the electrode interface, where they participate in transfer reactions of cathodic electrons or anodic holes to provide a reaction current as shown in Pig. 10-9. Such a reaction current of photoexcited electrons or holes is called the photoexcited reaction current or simply the photocurrent. 

As shown in Fig. 10-9, the photoexcited reaction current occurs only when an appreciable electric field exists in the space chai ge layer. No photocurrent occurs at the flat band potential because no electric field that is required to separate the photoexcited electron-hole pairs is present. The photocurrent occurs at any potentials different from the flat band potential hence, the flat band potential may be regarded as the potential for the onset of the photocurrent. It follows, then, that photoexcited electrode reactions may occur at potentials at which the same electrode reactions are thermodynamically impossible in the dark. 

The rate of the formation of photoexcited electron-hole pairs, Gix), is given as a function of the intensity of photon beam h, the absorption coefiicient of photons a, and the depth of photon-penetration x as shown in Eqn. 10-12 [Butler, 1977]  

Integration of Eqn. 10-12 yields the total photocurrent iph inEqn. 10-13 [Memming, 1983]  

It follows from Eqn. 10-13 that, if a 6sc is much larger than 1 (a 5sc 1, both a and being great), all the photoexcited minority charge carriers will be consumed in the interfacial reaction (ipb = e Iq ). In such a case, the photocurrent is constant at potentials away from the flat band potential as shown in Fig. 10-11 this figure plots the anodic ciirrent of photoexcited dissolution for a gallium arsenide electrode as a function of electrode potential. 

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Fig. 10-9. Photoexcited reaction current (photocurrent) at semicon ductor electrodes (a) photoexcited reaction of cathodic electron transfer (OX + e - RED) at p-type semiconductor electrode, (b) photoexcited reaction of anodic hole transfer (RED - OX + e) at n-type semiconductor electrode, iph = photocurrent.

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