Interfacial electron-hole pairs

This exciton then diffuses to a donor/acceptor interface where it is dissociated, producing an interfacial electron-hole pair. Dissociation of this interfadal electron-hole pair can also be problematic (see Section 14.5.1) with their sogeminate recombination thought to be a dominant loss mechanism in all-polymer cells. Under favorable conditions, the electron and hole are separated from the interface and are collected at the relevant electrode (electrons at the top, low-work-function electrode and holes at the bottom, high-work-function electrode) avoiding the bimolecular recombination of charges. 

This case is shown in Fig. 10.6c and d where through absorption of light a photohole in the vb and a photoelectron in the cb are formed. The probability that interfacial electron transfer takes place, i.e. that a thermodynamically suitable electron donor is oxidized by the photohole of the vb depends (i) on the rate constant of the interfacial electron transfer, kET, (ii) on the concentration of the adsorbed electron donor, [Rads]. and (iii) on the rate constants of recombination of the electron-hole pair via radiative and radiationless transitions,Ykj. At steady-state of the electronically excited state, the quantum yield, Ox, ofinterfacial electron-transfer can be expressed in terms of rate constants ... 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

From this discussion, it can be seen that while control over semiconductor potential does not provide control over the interfacial energetics, it does allow one to control the efficiency with which electron-hole pairs are allowed to recombine or alternatively undergo interfacial charge transfer. This process is usually discussed in terms of a quantum yield for electron flow, d>e, as defined in... 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

We see therefore that photoactive semiconductor particles provide ideal environments for control of interfacial electron transfer. Photoinduced electron-hole pairs formed on irradiated semiconductor suspensions, as in photoelectrochemical cells, allow for reactivity control not available in homogeneous solution. This altered activity derives from controlled adsorption on a chemically manipula-ble surface, controlled potential afforded by the valence band edge positions, controlled kinetics by virtue of band bending effects, and controlled current flow by judicious choice of incident light intensity. 

In order to account for such a mechanism, photochemical excitation of a semiconductor surface might induce the promotion of an electron from the valence band to the conduction band. Since relaxation of the high-energy electron is inhibited by the absence of intra-states, if the lifetime of this photo generated electron-hole pair is sufficiently long to allow the interfacial electron transfer from an accumulation site to an electron acceptor, as well as the interfacial electron transfer from an adsorbed organic donor to the valence-band hole, the irradiated semiconductor can simultaneously catalyze both oxidation and reduction reactions in a fashion similar to multifunctional enzymes reactions [232]. 

Although photoelectrochemistry has been known as a field for over thirty years, its full impact on organic synthesis has yet to be revealed. This article has dealt with a variety of examples that show how chemical conversions can be induced by photo-electrochemical activation of light-sensitive semiconductor surfaces. Photoexcitation causes the promotion of an electron from the valence band to the conduction band, thus producing a surface-confined electron-hole pair. The charges represented by this pair are then trapped by interfacial electron transfer. The oxidized and reduced... 

In order to achieve higher photovoltages, the rates of photogenerated carrier separation, transport and interfacial transfer across the semiconductor interface must all be fast compared with the rate of carrier cooling (Boudreaux et al, 1980 Nozik, 1980 Williams and Nozik, 1984). The achievement of higher photocurrent requires that the rate of II (rn) or electron-hole pair multiplication is greater than the rates of... 

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