Recombination electron and hole

The properties of the band gap in semiconductors often control the applicability of these materials in practical applications. To give just one example, Si is of great importance as a material for solar cells. The basic phenomenon that allows Si to be used in this way is that a photon can excite an electron in Si from the valence band into the conduction band. The unoccupied state created in the valence band is known as a hole, so this process has created an electron-hole pair. If the electron and hole can be physically separated, then they can create net electrical current. If, on the other hand, the electron and hole recombine before they are separated, no current will flow. One effect that can increase this recombination rate is the presence of metal impurities within a Si solar cell. This effect is illustrated in Fig. 8.4, which compares the DOS of bulk Si with the DOS of a large supercell of Si containing a single Au atom impurity. In the latter supercell, one Si atom in the pure material was replaced with a Au atom,... 

These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work to modify the electronic property of the catalyst [11,12,14,15],... 

It is important to emphasize that the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light (A < 280 nm) retained the same photocatalytic efficiency as the unimplanted original pure titanium oxides under the same UV light irradiation conditions. When metal ions were chemically dopec into the titanium oxide photocatalyst, the photocatalytic efficiency decreased dramatically under UV irradiation due to the effective recombination of the photo-formec electrons and holes through the impurity energy levels formed by the doped metal ions within the bandgap of the photocatalyst (in the case of Fig. 10.3).14) These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work tc modify the... 

Free radical processes initiated by ionization of DNA are dominated by combination reactions [75], When electrons and holes recombine the result is primarily a return to the parent structure, thus resulting in no damage. On the other hand hole-hole combination reactions result in one site being doubly oxidized with the probability of damage at the site being very high. 

If electrons and holes recombine too close to one of the metal electrodes, the luminescence is quenched by the metal (see Fig. VB-2) [293]. The primary... 

FIGURE 22.24 (a) In this solid-state laser, photons emitted as electrons and holes recombine to stimulate the emission of additional photons, (b) Reflection by a mirror on the right side sends coherent waves back through the laser medium, (c) Further amplification occurs by stimulated emission, (d) Some of the waves pass through a partially reflecting mirror on the left side. 

Titanium dioxide can be improved by doping with metals. Titanium dioxide participates in photochemical processes. Its mechanism involves the formation of positive holes in the valence band and electron promotion to the conductive band irradiated by UV. Both electrons and holes react with the surrounding material. By doping Ti02 crystals with various metals, electron and hole recombination centers are formed. Also, the crystal is coated with a layer of hydrous oxides which decompose hydroxyl radicals. This is applied to various grades of Ti02 which gives them a unique performance in applications where UV durability is required. 

E24.26 An ideal photocatalyst for water splitting would have (a) high efficiency of light absorption (low-intensity light would be sufficient to run the reaction), (b) efficient electron-hole separation process (if this separation process is not efficient then electron and hole recombine fast and do not liberate H2 and O2 respectively), and (c) high surface... 

Fig. 1 Basic set-up of a layered OLED structure. Electrons and holes are injected from the respective electrodes (metal cathode, semiconducting and transparent anode). The charge carriers move from different sides into the recombination/emitter layer, where electrons and holes recombine and excite the doped emitter molecules (asterisks, e.g., or-ganometallic triplet emitters). For more details see Fig. 2. For clarity, light emission is only shown for one direction although the photons are emitted in all directions...

The probability that electrons and holes recombine radiatively is proportional to the electron and hole concentrations, that is, R . n p. The recombination rate per unit time per unit volume can be written as... 

In addition, when the electron and hole recombine, all of their energy, Eg + 3kT, is assumed to be transferred non-radiatively to a third carrier via Auger recombination. But this also becomes excess energy, i.e., more than is needed to maintain the temperature T, and is, therefore, transferred to the lattice atoms. The Auger recombination rate is yne3, where y is the Auger coefficient. 

Using both low-molecular evaporated films and polymer films, multilayer OLEDs of high efficiency have been constructed. The structures of some molecules often used for evaporated films are shown in Fig. 11.4, and the monomers for polymer films in Fig. 11.5. The purpose of the fabrication of multilayer OLEDs is the independent optimisation of the individual processes which were described in Sect. 11.1.2, with the goal of achieving high-efficiency OLEDs injection and transport of the electrons and holes, balance between the currents of electrons and holes, recombination, use of triplet states, and reduction of reflections in the transmission of the luminescence to the outside of the OLED. In the following, we will treat a few typical examples of OLEDs prepared with low-molecular evaporated films. We emphasize, however, that also multilayer OLEDs made with polymers can yield comparable results and information. 

Figure 14.8 The principle of LED (light-emitting diode) operation under a forward bias, electrons and holes recombine in the junction region and emit radiation...

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