Electron-hole pair generation

Direct splitting of water can be accomplished by illuminating two interconnected photoelectrodes, a photoanode, and a photocathode as shown in Figure 7.6. Here, Eg(n) and Eg(p) are, respectively, the bandgaps of the n- and p-type semiconductors and AEp(n) and AEF(p) are, respectively, the differences between the Fermi energies and the conduction band-minimum of the n-type semiconductor bulk and valence band-maximum of the p-type semiconductor bulk. lifb(p) and Utb(n) are, respectively, the flat-band potentials of the p- and n-type semiconductors with the electrolyte. In this case, the sum of the potentials of the electron-hole pairs generated in the two photoelectrodes can be approximated by the following expression ... 

How are electron-hole pairs generated Exemplify for the case of titania. 

Because there is no band gap, and electron-hole pair generation is not involved in the detector response, there is no issue of bleaching (carrier depletion) under high levels of irradiance, so the detector can achieve good linearity performance. 

The difference between a photoconductive detector and a photodiode detector lies in the presence of a thin p-doped layer at the surface of the detector element, above the bulk n-type semiconductor. Holes accumulate in the p-layer, and electrons in the n-type bulk, so between the two there is a region with a reduced number density of carriers, known as the depletion layer. The important effect of this is that electron-hole pairs, generated by photon absorption within this depletion layer, are subjected to an internal electric field (without the application of an external bias voltage) and are automatically swept to the p and n regions, and... 

Ne is the number of photons utilized for electron-hole pair generation and Nx is the total number of absorbed photons, (j) =1 in the ideal case where all the photons of energy E > Eg are utilized for carrier generation. 

Electron hole pairs generated by photon absorption enable oxygen to desorb from the surface (bottom of Fig. 3). The oxygen desorption annihilates some of the holes, thereby decreasing the surface, so that electrons are now able to move from one ZnO grain to another. Thus, photoconductivity of the layer is produced. In the dark period which follows, the photoconductivity of the layer is preserved for some time due to the large number of shallow electron traps. 

Electron-hole pairs generated in the body are separated by the pn-junctions 12 by extraction of electrons from the p-layer 14 into the depleted layers 13 where they drift to the n-regions 3 and by extraction of holes from the depleted layers 13 into the p conductive paths of the layers 14 where they drift to the common connection 4, 24, 34. A close spacing of the junctions 12 gives a high efficiency of minority-carrier extraction from the layers 13 and 14. 

A p-type HgCdTe layer 3 is attached to a silicon read-out substrate 1. N-type regions 6 are formed on the side-walls of holes which go through the HgCdTe layer. A protective film 5 is formed in the vicinity of the regions 6. An anodized film 8 is formed on the layer 3 and an inverted layer (n-type) will be formed underneath the film. Electron-hole pairs generated between the detector elements will be absorbed by the inverted layer thereby suppressing cross-talk between the detector elements. 

The decay of the nanoparticle plasmons can be either radiative, ie by emission of a photon, or non-radiative (Figure 7.5). Within the Drude-Sommerfeld model the plasmon is a superposition of many independent electron oscillations. The non-radiative decay is thus due to a dephasing of the oscillation of individual electrons. In terms of the Drude-Sommerfeld model this is described by scattering events with phonons, lattice ions, other conduction or core electrons, the metal surface, impurities, etc. As a result of the Pauli exclusion principle, the electrons can be excited into empty states only in the CB, which in turn results in electron-hole pair generation. These excitations can be divided into inter- and intraband excitations by the origin of the electron either in the d-band or the CB (Figure 7.5) [15]. 

Gold nanoparticles are virtually not luminescent, but silver nanoparticles show plasmon emissions with reasonable quantum yields. Furthermore, the non-radiative decay, resulting in electron-hole pair generation, may be used for photosensitization of wide bandgap semiconductors (see Figure 7.5) [16,17]. Similar effects may also be observed as direct photoinduced electron transfer between metal surfaces and surface-bound molecules [18]. 

Step 2 This virtual electron-hole pair generates or annihilates a phonon and a second virtual electron-hole pair is formed. 

On the other hand, molecular crystals are characterized by the existence of strongly bound (Frenkel type) excitons, and it has been shown that the lower-energy part of the absorption spectrum (say, the first 2 eV) is completely dominated by these excitons [168], even to the extent that the absorption corresponding to electron-hole pair generation is completely hidden in the exciton spectrum [128] and is revealed only by such methods as modulated electrorefletance [169]. The only states in the exciton bands that are accessible by photon absorption are those at the center of the Brillouin zone, so the absorption is not a continuous band as for semiconductors, but a sharp line. The existence of this sharp line therefore does not mean that the exciton band is narrow (i.e., that its dispersion relation in the Brillouin zone is flat). On the contrary, since that dispersion is caused by dipolar interactions, exciton bandwidths can be several eV [168,170] the total bandwidth is four times the coupling term. This will be particularly... 

The local superficial rate of electron-hole pair generation can be computed considering a wavelength averaged primary quantum yield for the generation of charge carriers on the catalytic surface... 

Figure 4.4. Schematic of recombination of electron-hole pairs generating either a photon of energy or heat.

Figure 3. A localized picture of electron-hole pair generation (see also Figure 2a) in silicon.

Figure 1.4 Multiple internal reflections within ultra-thin semiconducting films (penetration depth of light 8 df). The extension of the space charge layer scl depends on the applied potential U and on the electronic layer properties (defect density Nd, dielectric constant e). It is assumed that only electron/hole pairs generated within the scl contribute to the photocurrent [74].

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