Hole blocking

Carbon brick and block ate used to line the cupola well (73) or cmcible. When properly installed and cooled carbon linings last for many months or even years of intermittent operation. Their resistance to molten iron and both acid and basic slags provides not only insurance against breakouts but also operational flexibility to produce different iron grades without the necessity of changing refractories. Carbon is also widely used for the tap hole blocks, breast blocks, slagging troughs, and dams. 

In bilayer LEDs the field distribution within the device can be modified and the transport of the carriers can be controlled so that, in principle, higher efficiencies can be achieved. On considering the influence of the field modification, one has to bear in mind that the overall field drop over the whole device is given by the effective voltage divided by the device thickness. If therefore a hole-blocking layer (electron transporting layer) is introduced to a hole-dominated device, then the electron injection and hence the efficiency of the device can be improved due to the electric field enhancement at the interface to the electron-injection contact, but only at expense of the field drop at the interface to the hole injection contact This disadvantage can be partly overcome, if three layer- instead of two layer devices are used, so that ohmic contacts are formed at the interfaces [112]. 

Figure 11-18. Calculated current (solid line) and iccuinbiiuuion current (dashed line) density as a function of voltage bias for a single-layer structure, a two-layer structure with a hole-blocking layer and a two-layer structure in which the hole-blocking layer also serves as an electron transport layer.

While sodium wire was being pressed into ether, the die-hole blocked. Increasing the pressure on the ram to free it caused ignition of the ejected sodium and explosion of the flask of ether. Pressing the sodium into less flammable xylene or toluene and subsequent replacement of solvent with ether was recommended. 

Due to the electronegative atom, polypyridines are good electron acceptors from UPS and UV-vis absorption spectra, Yamamoto and coworkers [669] estimated /iA 3.5 eV, /P = 6.3eV. Chen and coworkers [670] reported L a 2.9 cV, /p = 5.7 eV based on electrochemical measurements. For the double-layer ITO/PPV/562/A1 device, in which 562 acts as electron transport and hole-blocking layer, Chen and coworkers [670] reported a EL efficiency of 0.12cd/A that is 17 times higher than for an single-layer PPV-based PLED. The improvement in d by a factor of 60 (from 0.004 to 0.25%) for this device configuration was demonstrated by Monkman and coworkers [665]. 

J. Lu, Y. Tao, M. D iorio, Y. Li, J. Ding, and M. Day, Pure deep blue light-emitting diodes from alternating fluorene/carbazole copolymers by using suitable hole-blocking materials, Macromolecules, 37 2442-2449, 2004. 

M.-Y. Hwang, M.-Y. Hua, and S.-A. Chen, Poly(pyridine-2,5-diyl) as electron-transport/hole blocking layer in poly(phenylene vinylene) light-emitting diode, Polymer, 40 3233-3235, 1999. 

P. Posch, R. Fink, M. Thelakkat, and H.-W. Schmidt, A comparison of hole blocking/electron transport polymers in organic LEDS, Acta Polym., 47 487-494, 1998. 

Low molecular weight and polymeric heterocyclics as electron transport/hole-blocking materials in organic light-emitting diodes... 

T. Lee, O.O. Park, L. Do, T. Zyung, T. Ahn, and H. Shim, Polymer light emitting devices using ionomers as an electron injecting and hole blocking layer, J. Appl. Phys., 90 2128-2134 (2001). 

I. 3,5-triazine ethers as hole-blocking materials in electroluminescent devices, Chem. Mater., 10 3620-3625 (1998). 

Y. Sato, S. Ichinosawa, T. Ogata, M. Fugono, and Y. Murata, Blue-emitting organic EL devices with a hole blocking layer, Synth. Met., 111-112 25-29 (2000). 

M. Kinoshita, H. Kita, and Y. Shirota, A novel family of boron-containing hole-blocking amorphous molecular materials for blue- and blue-violet-emitting organic electroluminescent devices, Adv. Func. Mater., 12 780-786 (2002). 

Figure 3.27. Energy level scheme of the device in Figure 3.26, consisting of the electrode work functions and the molecular HOMOs and LUMOs. The relative energy level of HOMOs and LUMOs can he determined hy cyclic voltammetry and optical spectroscopy. Note the hole blocking character of the electron-transport layer. This feature is important since holes that proceed via the HOMO levels have much higher mobilities than electrons proceeding via the LUMO levels.

Compared to Alqa, the oxadiazoles have a lower tendency toward reduction and thus a higher barrier for electron injection. Spiro-PBD (140), for example, can accept four electrons, the first electron transfer (merged wave for two electrons) taking place at -2.46 eV vs. Fc/Fc" " (Fig. 3.29) [87]. The oxadiazoles 17a, 18, and 29 exhibit reversible reduction waves at -2.39 and -2.18 eV and an irreversible reduction, respectively [238]. Since the HOMO-LUMO gap is >1 eV larger than for Alqs, the hole blocking properties are better for the oxadiazoles. 

ETL materials that are used most often are emissive metal complexes, especially aluminium but also beryllium and lanthanides such as europium and terbinm, of ligands such as 8-hydroxyquinoUne, benzoquinolines and phenanthroUne, whilst other effective compounds inclnde extended conjugated compounds, e.g. distyrylarylene derivatives. Some ETL materials are chosen because they are non-emissive to act as combined ET and hole blocking layers. A selection of these ETL materials is illustrated in Figure 3.35. 

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