Photoexcitation, electron-hole pair generation

Upon illumination, photons having energy higher than the band gap (eg = ec — v) are absorbed in the semiconductor phase and the electron-hole-pairs (e //i+) are generated. This effect can be considered equivalent to the photoexcitation of a molecule (Fig. 5.57) if we formally identify the HOMO with the ec level and LUMO with the v level. The lifetime of excited e //i+ pairs (in the bulk semiconductor) is defined analogously as the lifetime of the excited molecule in terms of a pseudo-first-order relaxation (Eq. 5.10.2). 

Photocatalysis uses semiconductor materials as catalysts. The photoexcitation of semiconductor particles generates electron-hole pairs due to the adsorption of 390 run or UV light of low wavelength (for Ti02). If the exciting energy employed comes from solar radiation, the process is called solar photocatalysis [21],... 

Photoexcitation of TiOz with wavelengths <380 nm generates an electron-hole pair [Eq. (1)], creating the potential for reduction and oxidation processes to occur at the surface of the semiconductor. 

Charge transport in nanocrystalline electrodes is clearly strongly influenced by the inter-penetration of the solid and liquid phases. If electron hole pairs are generated by band to band excitation, it is usually observed that one type of carrier is transferred to the solution, while the other is transported to the substrate contact. In the case of the dye sensitized nanocrystalline systems, an electron is injected into the conduction band from the photoexcited dye and is then transported to the substrate. The dye is regenerated by reaction of its oxidised state with a supersen-sitiser such as 1 as shown in Fig. 8.25. 

The structure and function of this bacterial photosystem reveals important principles for the design of artificial photosystems. First, the sensitizer needs to be posi tioned close to secondary acceptors and donors which themselves are spatially iso lated from each other such that photoexcitation leads to rapid spatial separation of the electron hole pair. Second, compartmentalization of the photosynthetic assembly is likely to be necessary so as to prevent wasteful back reactions. For water splitting, a system in which H2 and O2 are generated in separate compartments would have both safety and efficiency advantages. 

In a bulk semiconductor, photoexcitation generates electron-hole pairs which are weakly bounded by Coulomb interaction (called an exciton). Usually one can observe the absorption band of an exciton only at low temperature since the thermal energy at room temperature is large enough to break up the exciton. When the exciton is confined in an energy potential, the dissociation probability reduces and the overlap of the electron and hole wavefunction increases, which is manifested by a sharper absorption band observable at room temperature. This potential can be due to either a deformation in the lattice caused by an impurity atom or, in the present case, the surface boundary of a nanocluster. The confinement of an exciton by an impurity potential (called bound exciton) is well known in the semiconductor literature [16]. There is considerable similarity in the basic physics between confinement by an impurity potential and confinement by physical dimension. The confinement effects on the absorption cross section of a nanocluster are discussed in Section II. 

The absorption of photons results in the generation of electron-hole pairs. The holes at the n-Si/ electrolyte interface can participate in PS formation. In the case of the pure HF solution (Fig. 2b), the photoexcited holes are hard to drift towards the surface by the very small downward band bending, or possibly by the almost-flat band. Thus, efficient PS formation cannot be expected in pure HF solution. When the Si wafer is dipped in the HF/oxidant solution (Fig. 2d), on the other hand, many photoexcited holes move towards the n-Skelectrolyte interface at the front surface, resulting in the formation of PS with good reproducibility (Xu and Adachi 2006, 2007 Adachi and Kubota 2007 Tomioka et al. 2007). 

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