Trapping of electrons and holes

Therefore, we are able to discuss the existence of defect states (recombination centers and traps) in the forbidden gap of solid organic dyes characterized by different trapping probabilities (ranging from 10-12 cm2 to 10-20 cm2) for electrons and holes. Hence, asymmetric trapping of electrons and holes leading to n- and -photoconductivity is very probable. 

One of principal causes of increase of P-conductivity in MF can be magnetosensitive non-equilibrium processes connected with charges carriers transport or change of intensity of capture (or release) by traps of electrons and holes. High times of increase and decrease of P-conductivity confirm the given assumption, indicating the contribution of defect structure to P-conductivity of C6o single crystal in MF. 

Table 8.1 sununarizes the results for trapping of electrons and holes into the different charge states of the defects, based on measurements of Pd cAq and the assumption that a is 5 A for electrons and 10 A for holes (Street 1984). The capture cross-sections are in the range... 

In order to improve the emission efficiency of these devices, quantum well structures are introduced for efficient trapping of electron and hole pairs. A quantum well is a multilayered structure where a semiconductor material with a low bandgap (well) is sandwiched between two layers of semiconductor materials with higher bandgap (barriers). A typical InGaN/GaN multiple quantum well structure in a blue InGaN based LED is shown in Figure 9. 

On the absorption of light and the trapping of electrons and positive holes in crystalline dielectrics. Physik. Z. Sowjetunion 9, 158 (1936). 

A representative example for the information extracted from a TRMC experiment is the work of Prins et al. [141] on the electron and hole dynamics on isolated chains of solution-processable poly(thienylenevinylene) (PTV) derivatives in dilute solution. The mobility of both electrons and holes as well as the kinetics of their bimolecular recombination have been monitored by a 34-GHz microwave field. It was found that at room temperature both electrons and holes have high intrachain mobilities of fi = 0.23 0.04 cm A s and = 0.38 0.02 cm / V s V The electrons become trapped at defects or impurities within 4 ps while no trapping was observed for holes. The essential results are (1) that the trap-free mobilities of electrons and holes are comparable and (2) that the intra-chain hole mobility in PTV is about three orders of magnitude larger than the macroscopic hole mobility measured in PTV devices [142]. This proves that the mobilities inferred from ToF and FET experiments are limited by inter-chain hopping, in addition to possible trapping events. It also confirms the notion that there is no reason why electron and hole mobilities should be principally different. The fact... 

For all TSR processes (with the corresponding redistribution of electrons and holes over the states in the gap) to be discussed, it seems useful to give a criterion permitting the classification of states under consideration as traps and recombination centers, respectively. 

A series of papers has recently appeared in the literature concerning the dynamics of photo-induced formation of electron and hole centres at the surface of MgO powders.12 14 Monochromatic excitation of the sample with 282 nm photons leads to the creation of well-separated electron and hole centres at the surface which were monitored via EPR in term of a trapped hole (O ion) and a trapped electron according to the following equation ... 

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

The majority of inorganic systems reported to exhibit photochromism are solids, examples being alkali and alkaline earth halides and oxides, titanates, mercuric chloride and silver halides.184 185 The coloration is generally believed to result from the trapping of electrons or holes by crystal lattice defects. Alternatively, if the sample crystal is doped with an impurity capable of existing in variable oxidation states (i.e. iron or molybdenum), an electron transfer mechanism is possible. 

Fig. 3.7. Trap-controlled carrier recombination 1 - excitation of solid with creation of electron ( ) and hole (0) pair 2, 3 - their localization (trapping) by defects 4 - thermal ionization of electron from a trap 5 - its recombination with the recombination centre.

The bipolar single-trap model assumes that both electrons and holes share identical trap centers. Since sequential trappings of the electrons and holes by the identical centers mean the neutralization of the electric charge, the effective space-charge field will depend on the relative power (i.e., the mobilities) of electron and hole transports. The expressions for the writing and erasing diffraction efficiency are [100] ... 

Electron-Hole Pairs. Localization of electrons and holes occurs via the processes of non-radiative charge trapping. These processes are represented phenomenologically in Figure 1. An example of a simple trapping process is the coulombic attraction of an electron and an... 

In general, it is accepted that recombination of electrons and holes, trapping of electrons by oxygen deficiency sites and a low mobility of the holes, cause a low conductivity and accordingly a low photoresponse for hematite. Electron mobility in the range 0.01 [60] to 0.1 cm2/V-s [17] has been reported. In the latter case, it was found that the electron mobility was independent of donor concentration. More recently, an electron mobility of about 0.1 cm2/V-s has been measured with doped single crystals and the mobility was also here independent of donor concentration [5]. A diffusion length of holes has been determined to be only of 2-4 nm [6], which is about 100 times lower than many other (III-V) oxides. 

Improved Carrier Extraction by Intercalating Membranes. With light trapping, the condition for good extraction of electrons and holes requiring Le,h l, is also relaxed, since a smaller thickness of the solar cell is possible. For low mobility organic materials, this condition is still a problem. It ensures that electrons and holes generated in the absorber reach the membrane within their recombination lifetime. They can then pass into the external circuit. The distance of the membranes, however, is not limited by the thickness l of the absorber, as can be seen in Fig. 4.11, and can be made arbitrarily short. 

Figure 7.10 Schematic illustration of electron and hole trapping at dopant sites and subsequent donor-acceptor-mediated photon emission. (Adapted from Ozawa and Itoh [33])...

Alberici et al. [226] used two forms of reactor to analyse the breakdown of TCE—fixed bed and fluidised bed. In the presence of 20% O2 the main breakdown products identified using GC-MS were phosgene, carbon tetrachloride, dichloroacetyl chloride, dichloroacetic acid and pentachloroethane. In the absence of oxygen the products detected were pentachloroethane, 1-pentachloropropene and 1,1,3,4-tetrachloro-l,3-butadiene. Kim et al. [230] concluded that the degradation rate of TCE decreased with increasing water vapour. They concluded that there was an optimum water concentration of 0.383 mol m-3 (vol%). It was also reported that molecular oxygen was an essential component because it trapped photogenerated electrons on the semiconductor surfaces and decreased the recombination of electrons and holes [230]. 

The individual specific rate and adsorption equilibrium constants are defined in Table 1. In Equations (4) and (5) [Sitesjrefer to the available concentration of sites for adsorption on the Ti02 film, [O2] to the liquid phase oxygen concentration, [M] to the concentration of water, atomic or free radical species, reactor walls or other surfaces trapping atomic chlorine, and Rg to the superficial rate of electrons and holes generation. 

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