Positive hole injection

Fig. 83. Surface plasmon absorption band of a 1 x 10 4moldm-3 silver sol (O). Changes in absorption after electron donation (a) and positive hole injection (fc) by free radicals [506]...

H. Kallman and M. Pope, Positive Hole Injection into Organic Crystals, J. Chem. Phys. 32, 300 (1960). 

Since the excess-electron conduction band for hydrocarbon liquids will be in the neighborhood of the vacuum level, neutralization of electrons at a clean metal anode should be facile (Fig. 11). Positive hole Injection would be difficult, however, because the band of positive hole states of the liquid will be at about -6.5 eV and, thus, well below the Fermi level of bare anode metal. Consequently, a barrier of about 2 eV or so to electron transfer and, thus, to hole production at a bare metal anode will exist. 

Turning now to consider breakdown initiation by an anode event, the question is whether positive hole Injection at an insulating micro-region could lead to the development of the low-density rapidly propagating filamentary structures which have been observed. 

Figure 6. (0) Surface plasmon absorption band of a 1.0 x 10" M silver sol (right ordinate scale). Changes in absorption(a) after electron donation and (b) positive hole injection by free radicals" (left ordinate scale).

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

Hole injection into the / -region similarly results in a positive current flow from to, leading to the Shockley diode equation (eq. 12), with a... 

Figure 11-14 shows the calculated hole density (upper panel) and the electric field (lower panel) as a function of position for the three structures. For the devices with a hole barrier there is a large accumulation of holes at the interface. The spike in the hole density at the interface causes a rapid change in the electric field at the interface. The field in the hole barrier layer is significantly larger than in the hole injection layer. For the 0.5 eV hole barrier structure, almost all of the... 

The spectra recorded for the hole injection process by the thianthrene radical cation into the Au(lll) electrode are shown in Fig 4, for three different excitation energies. Once again, square wave modulation was employed with a fixed positive value of +1.0V where the cation radical is produced /4/ to negative values... 

Hole injection into the -region similarly results in a positive current flow fromp to n... 

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. 

The redox potential of this regenerating couple has to be either more negative than the oxidative redox potential of the dye (° d/d+) in the case of electron injection from >, or more positive than the reductive redox potential (°Ed/d ) in the case of hole injection from D. A simple term scheme" of such a charge injection with regeneration of the reaction products is given in Fig. 18. Such a... 

Ohmic contacts. An ohmic contact is defined as one which supplies a particular crystal with an infinite supply of either electrons or positive holes. Under an applied field these charge carriers are drawn into the material setting up a space charge. The subsequent currents are thus termed space-charge limited currents. In general the activation energy required to inject a positive hole from an electrode of work function W into a crystal is Ic — W and that to inject an electron W — Ac. Thus for ohmic contacts the conditions to be satisfied for holes and electrons are respectively (12a) and (12b). Although... 

Fig. 20 Plots of H/kTc against 1 kTQ for electron and hole injection into anthracene crystals grown under different atmospheres open symbols refer to electrons and closed symbols to positive holes. (After Owen et al, 1974)...

In order for injection of an electron from the excited state of the dye species into the conduction band of a semiconductor (as described by Equation (2.39)) to occur, the oxidation potential of the dye excited state (A+ / A ) must be more negative than the conduction band potential of the semiconductor. Conversely, photoinduced hole injection from the excited dye into the semiconductor valence band (Equation (2.40)) requires the excited-state reduction potential of the sensitizer (A /A-) to be more positive than the valence band potential. 

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