Conduction band of semiconductor

Fig. 8-23. Energy diagram for a redox electron transfer via the conduction band of semiconductor electrode (a) anodic redox electron transfer, (b) cathodic redox electron transfer.

Figure 8-37 shows a shift in the redox electron level of a reductant from the hydrated state, which can not anodically iiyect electrons into the conduction band because its electron levels are located within the band gap, to the adsorbed state, which then can iiyect electrons into the conduction band of semiconductor electrodes... 

Recently, the electron-transfer kinetics in the DSSC, shown as a schematic diagram in Fig. 10, have been under intensive investigation. Time-resolved laser spectroscopy measurements are used to study one of the most important primary processes—electron injection from dye photosensitizers into the conduction band of semiconductors [30-47]. The electron-transfer rate from the dye photosensitizer into the semiconductor depends on the configuration of the adsorbed dye photosensitizers on the semiconductor surface and the energy gap between the LUMO level of the dye photosensitizers and the conduction-band level of the semiconductor. For example, the rate constant for electron injection, kini, is given by Fermi s golden rule expression ... 

The semiconductor nanocrystallites work as electron acceptors from the photoexcited dye molecules, and the electron transfer as sensitization is influenced by electrostatic and chemical interactions between semiconductor surface and adsorbed dye molecules, e.g., correlation between oxidation potential of excited state of the adsorbed dye and potential of the conduction band level of the semiconductor, energetic and geometric overlapping integral between LUMO of dye molecule and the density of state distribution of the conduction band of semiconductor, and geometrical and molecular orbital change of the dye on the... 

Gregg et at.si) examined photosensitization of perylene pigments (Dye 15-17) on a porous Sn02 thin film instead of Ti02 film as DSC, in view of energy matching with conduction band of semiconductors and LUMO of the sensitizers. When perylene-3,4-dicarboxylic acid-9,10-(5-phenanthroline) carboximide (Dye 15) was used, Jsc of 3.26 mA-cm 2, of 0.45 V, and a photoelectric conversion efficiency of 0.89% were observed under AM 1.5 irradiation. IPCE achieves close to 40% at 460 nm. 

Wc turn finally to materials that are not direct-gap semiconductors. The conduction bands of semiconductors have an analogy in the conjugation of verbs those encountered oftenesl have the exceptional forms. Let us first consider the four homopolar semiconductors diamond. Si, Gc, and Sn. In the text following... 

Data on Conduction Bands of Semiconductors (Room Temperature Data) 4.2.1. Single Valley Semiconductors... 

The dynamics of charge injection from the excited state of squaraines to the conduction band of semiconductors have been investigated by picosecond laser spectroscopy [45,67,101]. The sharp absorption band of bis[4-dimethyl-amino-2-hydroxy)phenyl]squaraine 21 (Strucmre 6) with at 636 nm became broad and red-shifted 670 nm) in the presence of colloidal Ti02 (Fig. 17). The red shift of 35 nm in the absorption band has been attributed to a strong... 

Fig. 2 Correlation between the yield of methanol and the conduction bands of semiconductor catalysts. The dashed line denotes the standard redox potential of the CH3OH/H2CO3 couple vs NPfE. (Reprinted from [61] with permission by the Nature Publishing Group)...

PART B. DATA ON CONDUCTION BANDS OF SEMICONDUCTORS (Room Temperature Data) ... 

I Density of energy states in conduction band of semiconductor. 

At 0 K the conduction band of semiconductors is completely empty (—> band theory). All electrons are in the valence band. With increasing temperature, electrons leave the valence band, overcome the band gap, and transfer into the conduction band. At given temperature an equilibrium between electrons and defect electrons is estabhshed ... 

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