Photo-excitation conduction band

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

This cell involves the absorption of light by dye molecules spread on the surface of the semiconductor, which upon light absorption will inject electrons into the conduction band of the n-type semiconductor from their excited state. The photo-oxidized dye can be used to oxidize water and the complementary redox process can take place at the counter electrode [46,47]. Tandem cells such as these are discussed in Chapter 8. 

The details of the operating principles of the dye-sensitized solar cell are given in Fig. 2. The photo excitation of the metal-to-ligand charge transfer (MLCT) of the adsorbed sensitizer (Eq. 1) leads to injection of electrons into the conduction band of the oxide (Eq. 2). The oxidized dye is subsequently reduced by electron donation from an electrolyte containing the iodide/triiodide redox system (Eq. 3). The injected electron flows through the semiconductor network to arrive at the back contact and then through the external load to the counter... 

In order to obtain high overall light to electric power conversion efficiencies, optimization of the short circuit photo current (z sc) and open circuit potential (Voc) of the solar cell is essential. The conduction band of the TiC>2 is known to have a Nernstian dependence on pH [55,67]. The fully protonated sensitizer 2, upon adsorption transfers most of its protons to the TiC>2 surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic Ru complex and assists electron injection from the excited state of the sensitizer into the titania conduction band, favoring high photocurrents (18-19 mA cm-2). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction band edge induced by the surface protonation. 

To summarize, the chemistry occurring at the surface of a photo-excited semiconductor is based on the radicals formed from 02, H20, and electron-rich organic compounds. Also note that cations in aqueous solution can be directly reduced by conduction band electrons provided that the redox potentials of these cations are adequate (i.e., lying below the conduction band energy) (6). 

A coloured charge transfer complex formed by absorption of ferrocyanide at the surface of Ti02 particles and electrodes, on photo-excitation injects electrons into the conduction band of Ti02 the conduction band electrons can be used to generate a photocurrent (Gratzel, 1987)... 

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]. 

In the case of the injection of an electron from the excited state of a molecular sensitizer into the conduction band of a semiconductor (Eq. (23)), the thermodynamics of the photo-redox reaction requires the oxidation potential of the dye excited state i°(S+/S ) to be more negative than the conduction band flatband potential of the semiconductor, and thus ... 

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