Injection efficiency

This confinement yields a higher carrier density of elections and holes in the active layer and fast ladiative lecombination. Thus LEDs used in switching apphcations tend to possess thin DH active layers. The increased carrier density also may result in more efficient recombination because many nonradiative processes tend to saturate. The increased carrier confinement and injection efficiency faciUtated by heterojunctions yields increasing internal quantum efficiencies for SH and DH active layers. Similar to a SH, the DH also faciUtates the employment of a window layer to minimise absorption. In a stmcture grown on an absorbing substrate, the lower transparent window layer may be made thick (>100 /tm), and the absorbing substrate subsequendy removed to yield a transparent substrate device. 

Based on extensive screening of hundreds of ruthenium complexes, it was discovered that the sensitizer s excited state oxidation potential should be negative of at least —0.9 V vs. SCE, in order to inject electrons efficiently into the Ti02 conduction band. The ground state oxidation potential should be about 0.5 V vs. SCE, in order to be regenerated rapidly via electron donation from the electrolyte (iodide/triiodide redox system) or a hole conductor. A significant decrease in electron injection efficiencies will occur if the excited and ground state redox potentials are lower than these values. 

In search of new sensitizers that absorb strongly in the visible region of the spectrum, Arakawa and co-workers have developed several sensitizers based on 1,10-phenanthroline ligands. Among these new compounds, (29) and (30) are noteworthy they show an intense and broad MLCT absorption band at 525 nm in ethanol. The energy levels of the LUMO and HOMO for (29) were estimated to be —1.02 and 0.89 eV vs. SCE, respectively, which are slightly more positive than those of the sensitizer (22). These sensitizers, when anchored onto a Ti02 surface, yield more than 85% photon to electron injection efficiencies.8,52,53... 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

E.W. Forsythe, M.A. Abkowitz, and Y. Gao, Tuning the carrier injection efficiency for organic light-emitting diodes, J. Phys. Chem. B, 104 3948-3952 (2000). 

To facilitate good charge transport in an OLED, the organic materials must satisfy three key requirements they must have a high mobility for either electrons or holes, a good injection efficiency from the contact electrode, and suitable band offsets with other organic layers within the device. These processes are discussed in detail by, for example, Kalinowski [73] and Greenham and Friend [74],... 

Injection into unsaturated deposits must also consider that when the injected water leaves the injection tubing, it is at atmospheric pressure, and the driving head is lost which lowers injection efficiency. In addition, the capillary and surface tension forces in the void spaces provide resistance to water movement, thus limiting injection efficiency. Injection into only the saturated zone (Figure 8.8b) maintains a positive head pressure on the water until it exits the well screen. This limits the chemical reaction that might occur in the well bore. Since the void spaces are saturated, there are no capillary or surface tension forces to overcome. 

Electrophoretic injection can be used as a means for zone sharpening or sample concentration if the amount of ions, particularly salt or buffer ions, is lower in the sample than the running buffer. Because sample ions enter the capillary based on mobility, low-mobility ions will be loaded to a lesser extent than high-mobility ions. For this reason, the presence of nonsample ions will reduce injection efficiency, so electrophoretic injection is very sensitive to the presence of salts or buffers in the sample matrix. The disadvantages of electrophoretic injection argue against its use in routine analysis except in cases where displacement injection is not possible, e.g., in capillary gel electrophoresis (CGE) or when sample concentration by stacking is necessary. 

Displacement injection is usually the preferred method because analyte ions are present in the sample zone in proportion to their concentration in the bulk sample. In addition, injection efficiency is less sensitive to variations in sample ionic strength. However, it should be noted that the presence of high salt can affect detector response and variations in the sample viscosity due to temperature, or the presence of viscosity-modifying components can affect displacement injection efficiency. 

Returning to Fig. 10, because the majority of the alkyl groups are flexible and because the interfacial electrostatic binding geometries are unknown, the rate of falloff of kbET or Hab with semiconductor/molecule separation distance cannot be evaluated quantitatively. A curious finding that remains unexplained is that the falloff with osmium complexes is considerably weaker than with ruthenium species. Finally, although the injection reaction was not the focus of the study, spacers clearly do decrease its rate, as shown, for example, by an increase in emission quantum yield (decrease in injection efficiency) with the largest spacers. 

It is fascinating to note that this class of dyes are injecting efficiently into the conduction band of TiO2, despite the fact that the pyridyl orbitals do not participate in the tt-tt excitation, which is responsible for the 650-nm absorption band. This phenomenon shows that the electronic coupling of the excited state of the dye to the Ti (3d) conduction band manifold is strong enough through this... 

Sanii L, Schuster GB (2000) Long-distance charge transport in DNA sequence-dependent radical cation injection efficiency. J Am Chem Soc 122 11545-11546 Saran M, Bertram H, Bors W, Czapski G (1993) On the cytotoxcity of irradiated media. To what extent are stable products of radical chain reactions in physiological saline responsible for cell death Int J Radiat Biol 64 311-318... 

As a result, nearly perfect interfaces between the ferromagnetic material and the semiconductor are not a prerequisite for efficient spin injection. It is for example possible to insert a non-magnetic seed layer between the ferromagnetic base layer and the semiconductor collector. Since hot electrons retain their spin moment while traversing the thin non-magnetic layer this will not drastically reduce the spin polarization of the injected current. Finally, since electron injection is ballistic in SVT and MTT devices the spin injection efficiency is not fundamentally limited by a substantial conductivity mismatch between metals and semiconductors [161, 162], The latter is the case in diffusive ferromagnetic metal/semiconductor contacts [163],... 

Pending further work on these new magnetic semiconductors, metallic ferromagnets are in principle, the most convenient spin polarized sources for spin device work. The obvious configuration of direct Ohmic contact between metal and semiconductor proved to have fundamental shortcomings. The conductivity mismatch between the two materials implies very indifferent spin injection efficiency [174, 175], However it transpires that this difficulty is surmountable [176] by placing a tunnel barrier between the... 

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