Photoexcitation, electron-hole pair

Since photoexcited electron-hole pairs are formed only within a limited depth from the semiconductor surface to which the irradiating photons can penetrate, the photon-induced split of the Fermi level into the quasi-Fermi levels of electrons and holes occurs only in a surface layer of limited depth as shown in Fig. 10-2. 

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] ... 

In photoexcited n-type semiconductor electrodes, photoexcited electron-hole pairs recombine in the electrodes in addition to the transfer of holes or electrons across the electrode interface. The recombination of photoexcited holes with electrons in the space charge layer requires a cathodic electron flow from the electrode interior towards the electrode interface. The current associated with the recombination of cathodic holes, im, in n-type electrodes, at which the interfadal reaction is in equilibrium, has already been given by Eqn. 8-70. Assuming that Eqn. 8-70 applies not only to equilibrium but also to non-equilibrium transfer reactions involving interfadal holes, we obtain Eqn. 10-43 ... 

Semiconductor electrodes whose band gap is relatively narrow receive photon energy and produce photoexcited electron-hole pairs in the space charge layer. The photoexcited electron-hole pair formation significantly increases the concentration of minority charge carriers (holes in the n-type), but influences little the concentration of majority carriers (electrons in the n-type). The photoexcited electrons and holes set their energy levels not at the electrode Fermi level, ef, but at what we call the quasi-Fermi levels, n p and p p, respectively. The quasi-Fermi level for majority carriers is close to the electrode Fermi level, F, but the quasi-Fermi level for minority carriers is far away from the electrode Fermi level. 

An electric field E is applied across the photoconductor to measure the photoconductivity. The field drives electrons toward x — l and holes toward X=0 see Fig. 4.8. The electrical contacts are assumed to be metallic and ohmic, so that photoexcited electron-hole pairs recombine there instantaneously thus... 

Fig. 11.29 A newly proposed photocatalysis model based on nonadiabatic chemical dynamical processes and ground state reactions. In this model, photoexcited electron-hole pairs are nonadiabatically recombined to convert the excited electronic state taiergy to the ground state energy, which drives the chemical reactions (m the ground state surface (Reprinted with permission from Ref. [192]. Copyright 2015 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences)...

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