Injection mechanisms

Besides injection mechanisms, in order to describe the l/V characteristics of LEDs, the charge transport mechanisms in the bulk have to be taken into account. 

A polymer layer al a contact can enhance current How by serving as a transport layer. The transport layer could have an increased carrier mobility or a reduced Schottky barrier. For example, consider an electron-only device made from the two-polymer-layer structure in the top panel of Figure 11-13 but using an electron contact on the left with a 0.5 eV injection barrier and a hole contact on the right with a 1.2 cV injection barrier. For this case the electron current is contact limited and thermionic emission is the dominant injection mechanism for a bias less than about 20 V. The electron density near the electron injecting contact is therefore given by... 

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

Fig. 27 Rotary die machine. (A) Ribbon casting drum (B) ribbon (C) filling leads (D) injection mechanism (E) rotary die (F) capsule wash (G) infrared dryer. (Courtesy of R. P. Scherer North America, Clearwater, FL.)... 

The semiconductor structure is crucial for both electron injection and charge transport after the exciton separation. Meng et al. [35] published a theoretical study focused on the electron injection mechanism in dyad anthocyanine-Ti02 nanowires. 

Dye sensitization plays an important role in photography. The sensitization mechanism for ZnO-materials as used in electro-photography is obviously in complete correspondence with these electrochemical experiments as shown for single crystals under high vacuum conditions by Heiland 56> and for imbedded ZnO-particles by Hauffe 57). Even for silver halides where electron injection as sensitization mechanism has been questioned by the energy transfer mechanism 58> electrochemical experiments have shown that the electron injection mechanism is at least energetically possible in contact with electrolytes 59>. Silver halides behave as mixed conductors with predominance of ionic conductivity at room temperature. These results will therefore not be discussed here in any detail since such electrodes are quite inconvenient for the study of excited dye molecules. 

For chemical reactions there is a convenient and natural way to specify the noise properties of the injection mechanism, which has already been used in some examples. One supposes that the molecules X are produced from a compound B, which is present in large amount and slowly decays into X. The production is then practically constant and the reverse reaction negligible, just as the production of helium by uranium may be regarded as constant. In other words, one describes the open system as a limiting case of a closed system that is not in equilibrium. The role of B is reduced to that of a reservoir. 

With hydrostatic injection mechanisms, injection reproducibility can be better than 1-2% RSD. The volume of sample loaded is a function of the capillary dimensions, the viscosity of the buffer, the applied pressure, and the time, and it can be calculated using... 

Figure 74 Comparison of field-assisted thermionic injection mechanisms in wide- (a) and narrow-band (b) materials. An x0 close to the geometrical contact region is distinguished where Eq. (198) is not applicable. The inapplicability can be associated with field or coordinate dependence of /1, D and s or the coexistence of some other processes such as bimolecular or tunneling recombination which are not included in Eq. (198). After Ref. 361. Copyright 1989 Jpn. JAP, with permission.

The above considerations show that the identification of the injection mechanism based upon the shape of a j(F) curve only is highly uncertain and conclusions must be drawn with great caution. 

The main problem with this approach is that the intermediate must be brought to reaction conditions almost instantaneously. A transient of any length of time, say, to reach the elevated temperature and pressure of the reaction under study, is apt to falsify the results seriously. Equipment of the type described in Figure 3.3 in Section 3.1.1 or some other injection mechanism is needed. 

FIGURE 2.1. Two possible carrier injection mechanisms at the organic/metal electrode interface (a) Schottky-type carrier injection via impurity or structural disordered levels with thermal assistance and (b) Fowler-Nordheim tunneling carrier injection with the assistance of a local high electric field (106—107 V/cm). 

Accuracy) Stability Ruggedness Robustness precision Peak area precision Applied voltage Capillary temperature Injection mechanism pressure/vacuum applied Time of applied pressure/ vacuum Height of vial and transition time treatment, capillary temperature, buffer ionic strength, organic modifiers... 

Injection Injection mechanism Electrokinetic applied voltage and time of application Hydrodynamic units of pressure or vacuum applied or height of displacement and time of application... 

Figure 2 Scheme of a bottom-contact OFET illustration of the current research topics including the stmcture and morphology of organic semiconductor films as well as charge carrier transport and injection mechanism, which are the subject of this book. 

FIGURE 2.4.4 Comparison of different charge injection mechanisms at a biased metal-semiconductor contact (a) thermionic emission, (b) field emission (tnnneling), (c) defect assisted injection. 

Figure 6.1 Schematic diagram of the shock tube with test section, droplet injection mechanism, and recording devices 1 — droplet generator 2 — high-speed pressure transducers 3 — low-speed pressure transducer 4 — vacuum port 5 — thermocouples 6 — diaphragm chamber bleed 7 — low-speed pressure transducer S — thermocouple and 9 — helium input and low-speed pressure transducer.

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