Energy of the valence band edge

Cadmium Sulfide Photoconductor. CdS photoconductive films are prepared by both evaporation of bulk CdS and settHng of fine CdS powder from aqueous or organic suspension foUowed by sintering (60,61). The evaporated CdS is deposited to a thickness from 100 to 600 nm on ceramic substates. The evaporated films are polycrystaUine and are heated to 250°C in oxygen at low pressure to increase photosensitivity. Copper or silver may be diffused into the films to lower the resistivity and reduce contact rectification and noise. The copper acceptor energy level is within 0.1 eV of the valence band edge. Sulfide vacancies produce donor levels and cadmium vacancies produce deep acceptor levels. 

According to our energy conditions of Fig. 7, materials suitable for hole injection must have a relative high energy position of the valence band edge. From Fig. 12 we learn that this is the case for GaP, and CdSe. CdS could be a candidate but it is not available as fi-type material. Besides -GaP only fi-SiC (band gap 3 eV) 7> has been used for the study of hole injection from excited dye molecules. 

In this equation, Ey is the energy level of the valence band edge, Ny the effective density of states of the valence band, p the concentration of photo-generated positive holes, and pp that of Positive holes in thermal equilibrium in the dark which can be eventually ignored in a semiconductor of large band gap such as Zn°. ... 

On the other hand, many organic compounds have a redox potential at a higher energy than the valence band edge of common semiconductor oxides and, therefore, they can act as electron donors and thus yield a radical cation (Fig. 1), which can further react, for example, with H20, 02 , or 02. 

The weak bond model is useful because the distribution of formation energies can be evaluated from the known valence band and defect density of states distributions. Fig. 6.12 illustrates the distribution of formation energies, N iU). The shape is that of the valence band edge given in Fig. 3.16 and the position of the chemical potential of the defects coincides with the energy of the neutral defect gap state. Fig. 6.12 also shows that in equilibrium virtually all the band tail states which are deeper than convert into defects, while a temperatiue-dependent fraction of the states above convert. 

Figure 5.35 Graph illustrating the relevant valence-band (VB) and conduction-band (CB) states in the Brillouin zone for a Ti02 crystal in the X and Z edges, and in the crystal centre F. Note that the lowest energy direct transition from the lowest energy of the valence band to the lowest level of the conduction band at F is forbidden the lowest energy transition is an indirect phonon-assisted transition. Adapted with permission from Emeline et al. (2000c). Copyright (2000) American Chemical Society.

Here is the density of energy states at the upper edge of the valence band and occurs as the energy of the valence band in the exponential term. In the case of a valence band process the cathodic current is constant (Eq. 7.50) whereas the anodic current depends on the hole density at the surface. The latter is given by... 

Tables 10.13a and b demonstrate the effect of the cychc-cluster increase for both HF and DFT-PWGGA methods, respectively. The main calculated properties are the total energy Etot (per primitive unit cell), one-electron band-edge energies of the valence-band top and conduction-band bottom e and Sc, MuUiken effective atomic charges q and full atomic valencies V. As is seen, the result convergence, as the supercell size increases, is quite different for the HF and DFT. We explain the much slower DFT convergence by a more covalent calculated electron-charge distribution, as compared to the HF case. For both methods, the convergence of local properties of the electronic structure is faster than that for the total and one-electron energies.

Fig. 14 Left DOS (states nKmomer eV ) of P3HT fra- layers at different distances from the interface with PCBM. The plots are offset for elaiity. This panel also illustrates that the increased band gap near the interface is mostly due to a reduction of the valence band edge energy. The black curve is the (rescaled) DOS for an idealized isolated chain with no disorder. Center. A schematic of the interface with increased chain disorder near the interface. Right A snapshot from the simulation showing that for the two P3HT/PCBM interfaces per snapshot, the P3HT chains are more disordered near the interface. Reprinted with permission from [109] Copyright 2011 American Chemical Society...

Excitons Excitons occur in solids in which a band gap exists (semiconductors and insulators). These are formed when a sufficient energy is imparted into the solid so as to induce the movement of an electron situated within a level close to the top of the valence band edge to some level close to the bottom of the conduction band edge. When this occurs, a hole is left behind in the valence band. The electron-hole pairs formed can then move together owing to the Coulombic interaction that exists between the two. 

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