Layers blocking

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 11-15. Calculated electric field as a function of bias in the center of the blocking layer and in Ihc center of the luminescent layer of the 0.5 cV barrier structure of Fig. 11-13.

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.

Acid-Soluble Mineral Fibers. Mineral fibers that are highly soluble in acid can be used to control the permeability [1242,1243] of formations. The fibers are 5 to 15 p in diameter and are formed into pellets of to inch diameter. A fluid-blocking layer formed of semidispersed pellets can bridge the face pores of the formation. After well rework, the plugging layer is treated with an acid solution to quickly dissolve the mineral fibers to the desired extent to control porosity. 

F. Montgomery, S. Montgomery, and P. Stephens. Method of controlling porosity of well fluid blocking layers and corresponding acid soluble mineral fiber well facing product. Patent US 5222558, 1993. 

The purpose of the n-type doped a-Si H layer is to prevent injection of charge from the substrate into the photoconductor. Thus it serves as a blocking layer. Injection of surface charge into the photoconductor is prevented by the surface... 

The spinel blocks in (3-alumina are related by mirror planes that mn through the conduction planes that is, the orientation of one block relative to another is derived by a rotation of 180°. A second form of this compound, called (3"-alumina, has similar spinel blocks. However, these are related to each other by a rotation of 120°, so that three spinel block layers are found in the unit cell, not two. The ideal composition of this phase is identical to that of (3-alumina, but the unit cell is now rhombohedral. Referred to a hexagonal unit cell, the lattice parameters are a = 0.614 nm, c = 3.385 nm. The thickness of the spinel blocks and the conduction planes is similar in both structures.3... 

PLD has also been utilized to produce bilayer electrolytes. In one study, aNiO-YSZ (anode support, tape cast)/NiO-SDC (AFL, screen-printed)/ScSZ-SDC (electrolyte bilayer, PLD)/SSC (cathode, screen-printed) cell showed excellent performance (0.5 and 0.9 W/cm2 at 550 and 600°C, respectively), with an OCV of 1.04 V at 600°C, indicating that the PLD technique was successful in depositing a sufficiently dense ScSZ electronic blocking layer to suppress electronic conduction normally observed across single-layer SDC electrolytes, and which typically result in lower OCV values (0.87 V, 600°C) [46, 127],... 

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]. 

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. 

FIGURE 3.23 External quantum efficiencies of PtOEP/CBP and PtOEP/Alq3 devices as a function of current with and without a BCP blocking layer (left). Emission spectra of CBP-based electroluminescent devices with and without a BCP exciton blocking layer (right). (From O Brien, D.F., Baldo, M.A., Thompson, M.E., and Forrest, S.R., Appl. Phys. Lett., 74, 442, 1999. With permission.)... 

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). 

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. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer, Appl. Phys. Lett., 79 156-158 (2001). 

Homogeneous melt, Todt < Tc > Tg. In diblock copolymers exhibiting homogeneous melts, microphase separation is driven by crystallization if Tg of the amorphous block is lower than Tc of the crystallizable block. This generally results in a lamellar morphology where crystalline lamellae are sandwiched by the amorphous block layers and spherulite formation can be observed depending on the composition [6-10]. 

E. Palomares, J.N. Clifford, S.A. Haque, T. Lutz, J.R. Durrant, Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers,... 

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