Luminescent layer

The utility and importance of multi-layer device structures was demonstrated in the first report of oiganic molecular LEDs [7]. Since then, their use has been widespread in both organic molecular and polymer LEDs [45, 46], The details of the operating principles of many multi-layer structures continue to be investigated [47—49], The relative importance of charge carrier blocking versus improved carrier transport of the additional, non-luminescent layers is often unclear. The dramatic improvements in diode performance and, in many cases, device lifetime make a detailed understanding of multi-layer device physics essential. 

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.

Recent work with multi-layer polymer LEDs has achieved impressive results and highlights the importance of multi-layer structures [46]. Single-layer, two-layer and three-layer devices were fabricated using a soluble PPV-based polymer as the luminescent layer. The external quantum efficiencies of the single-layer, two-layer, and three-layer devices were 0.08%, 0.55%, and 1%, respectively, with luminous efficiencies of about 0.5 hn/W, 3 lm/W, and 6 lm/W. These results clearly demonstrate improvement in the recombination current because of the increase in quantum efficiency. The corresponding increase in luminous efficiency demonstrates that the improvement in recombination efficiency was achieved without a significant increase in the operating bias. 

Y Kawamura, S Yanagida, and SR Forrest, Energy transfer in polymer electrophosphorescent light emitting devices with single and multiple doped luminescent layers, J. Appl. Phys., 92 87-93,... 

The position of the maximum in the photoluminescence spectra is independent of the oxidant type and appears at A, 625 mn. The method of chemical etching is most adapted to mass manufacturing and it is currently used for the preparation of thin homogeneous luminescent layers of sNPS for sensor devices. 

Fig. 2 Different orange-red luminescent layers for O2 sensing, a Indicator layer containing the tris(4,7-diphenyl-l,10-phenanthroline)ruthenium(II) dye adsorbed on CPG particles embedded in a poly(dimethylsiloxane) film [8]. b Perfluorinated (Nation) ionomer membrane doped with the same cationic dye. Although the former appears as a homogeneous material, even under the optical microscope, the luminescent indicator is actually sitting down in various domains as evidenced by its emission kinetics profile [9]...

The advances in nanotechnology and synthesis methods have enabled nanomaterials to be produced in various shapes and structures. Coating of a luminescent layer activated by lanthanide ions on nanoparticles such as SiC>2 or AI2O3 is one of such approaches to develop new nanophosphors. In section 6, we review recent work on interesting spectroscopic features and luminescence dynamics of lanthanide ions in other novel low-dimensional nanostructures including core-shell, one-dimensional (ID) nanowires and nanotubes, two-dimensional (2D) nanofilms, hollow nanospheres, 2D nanosheet and nanodisk which have also attracted extensive attention. 

Figure 11-16. Measured electric field as a function of bias in the blocking layer (P3DA) and in the luminescent layer (MEH-PPV) of an electron-only structure.

Table 3.15 Chemical Structures of Compounds Used in Carrier Transport Layers and Luminescent Layers of Organic Electroluminescent Devices...

Other requirements are placed on electroluminescent devices in addition to supporting efficient hole and electron transport (1) the exciton energy in the transport layer has to be higher than the exciton energy in the luminescent layer (2) the formation of molecular complexes between the fluorescent layers should be precluded (3) the luminescent layer has to possess a high fluorescence quantum yield and (4) the film should be processed readily. 

For a compound to act as an ideal hole-blocking material, it must have a HOMO level deeper than that of the dopant and host material, and also have a triplet energy high enough to efficiently prevent triplet excitons from migrating out of the luminescent layer (Figure 28). The two most common hole-blocking materials are... 

Luminescent layers can be fabricated from a pattern formed by an ink-jet method . A composition with a viscosity of 0.002-0.004 Pas and the surface tension in the range of 0,025-0,040 Nm is used and placed by means of an ink-jet printer. 

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