Electroluminescent device

Trends in solution are sometimes reversed in the solid state and there are clearly many factors and compromises which must be borne in mind when formulating strategies for synthesis of new oligomers optimised for both ease of fabrication (film deposition) and efficient operation in devices. However, as a general rule of thumb, it may be advantageous to employ relatively disordered films within EL devices, while better ordered films are more suitable for field-effect transistors and photovoltaic applications. 

In organic electroluminescent devices, the semiconductor layer is sandwiched between two electrodes, as shown in Fig. 10a. One electrode, such as gold (Au) or indium tin oxide (ITO), is chosen to have a high work function, for injection of positive charges (holes). The other electrode, often aluminium (Al), calcium (Ca) or 

Calhode with Iqw wodcfuncNon (e.g, AI,Ca,Mg ConjugatOfJ oligorrar or polymer AnoOe with high workluriction e.g ITO, Au) 

Additional semiconductor layers (charge-transport layers) may be included between the emissive layer and the electrode to facilitate transport of charges of one polarity, while impeding charges of the opposite polarity, thereby encouraging radiative recombination within the emissive layer, as shown in Fig. 11, for a polymer heterojunction device [30]. 

The efficiency of EL diodes made with organic semiconductors is critical to their usefulness in display (or possibly lighting) applications. The overall quantum efficiency for generation of light within the diode (internal quantum efficiency), that-of a diode that operates according to the scheme shown in Fig. 10b has been summarised as the product of three terms  

The pulse response of emission from a PAT, e.g., PODT, consists of two independent parts a fast and a slow transition part. The fast response corresponds to carrier transit between electrodes, and the anomalous slow response, which becomes significant at higher current, is explained by heating at the junction due to the injection current [717], The use of poly(thienylene vinylene) thin film as a buffer layer between ITO and poly(2,5-dialkyloxy-p-phenylene vinylene) results in increasing the breakdown voltage and increasing luminescence [718]. Further organic electroluminescence devices are described in literature [719,720]. 

The polymeric film is very thin so that electric field strengths in the range of grater than 10 Vm occur. The quantum efficiency, expressed as the number of photons related to the number of electrous iujected is in the range of 0.1-5%. Other performance indicators are the luminous efficiency and the power efficiency. 

The indium tin oxide (ITO) anode layer is kept as thin as 15 mu to allow passage of the light emitted in the polymer. The cathode is fabricated from a material that has a small energy barrier with respect to electron emission, e.g., aluminum, calcium, or barium. 

To improve the operational stability, low work function metals such as Mg and Li are alloyed with more stable metals with higher work function, such as A1 or Ag, and used as cathode material. Most simply, the polymeric layer consists of a single layer. Multilayer structures of different polymers are more common. Multilayer organic devices, such as are conventionally constructed in a sequential manner  

A transparent electrode, usually ITO is vacuum sputtered on a glass substrate. 

A hole transport layer, such as poly(3,4-ethylenedioxythiophene) 

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Another recently discovered form of epitaxy is graphoepitaxy (Geis et al. 1979). Here a non-crystalline substrate (often the heat-resistant polymer polyi-mide, with or without a very thin metallic coating) is scored with grooves or pyramidal depressions the crystalline film deposited on such a substrate can have a sharp texture induced by the geometrical patterns. More recently, this has been tried out as an inexpensive way (because there is no need for a monocrystalline substrate) of preparing oriented ZnS films for electroluminescent devices (Kanata et al. 1988). 

The use of 1 l//-pyrido[2,l-Z)]quinazolin-l 1-ones in an organic electroluminescent device was patented (99JAP(K)99/74080). 2//-Pyrimido[2,l-n]isoquinolin-7-ols were patented as multi-functional fuel and lube additives (97USP5646098). 

Polyfarylene vinylene)s form an important class of conducting polymers. Two representative examples of this class of materials will be discussed in some detail here. There are poly(l,4-phenylene vinylcne) (PPV) 1, poly(l,4-thienylene viny-lenc) (PTV) 2 and their derivatives. The polymers are conceptually similar PTV may be considered as a heterocyclic analog of PPV, but has a considerably lowci band gap and exhibits higher conductivities in both its doped and undoped stales. The semiconducting properties of PPV have been shown to be useful in the manufacture of electroluminescent devices, whereas the potential utility of PTV has yet to be fully exploited. This account will provide a review of synthetic approaches to arylene vinylene derivatives and will give details an how the structure of the materials relate to their performance in real devices. 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

Electrical industries, polyimides in, 269 Electroluminescent devices, 271 Electronic applications, 26 polyimide, 269... 

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

Interfaces between two different media provide a place for conversion of energy and materials. Heterogeneous catalysts and photocatalysts act in vapor or liquid environments. Selective conversion and transport of materials occurs at membranes of biological tissues in water. Electron transport across solid/solid interfaces determines the efficiency of dye-sensitized solar cells or organic electroluminescence devices. There is hence an increasing need to apply molecular science to buried interfaces. 

Electroluminescent devices were made to demonstrate the possible application of these a-Si H materials. Er-doped p-n diodes in c-Si show electroluminescence, both in forward and reverse bias [670-672]. Under forward bias the electrolu-... 

PbS has attracted much attention due to its special direct band gap energy (0.4 eV) and a relatively large exciton Bohr radius (18 nm) and their nanoclusters have potential applications in electroluminescent devices such as light-emitting diodes. PbS nanocrystals with rod like structures with diameters of 20-60 nm and lengths of 1-2 pm have been obtained using the sonochemical method and by using PEG-6000 [66]. Addition of PEG and the time of sonication have been found to play a key role in the formation of these rods. 

Kim SH, Han SK, Park SH, Park LS (1998) A new dithiosquarylium dye for use as an electron transport material in an organic electroluminescent device having poly(p-phenylene vinylene) as an emitter. Dyes Pigm 38 49-56... 

Novel Organo-electroluminescent Devices/Inorganic Devices 709... 

Emergency lighting systems based on electroluminescent devices... 

Van Slyke, S. A. Blue Emitting Internal Junction Organic Electroluminescent Device. U.S. Patent 5,151,629 Sept 29, 1992. 

Research on semiconductor nanoparticle technology by chemists, materials scientists, and physicists has already led to the fabrication of a number of devices. Initially, Alivisatos and co-workers developed an electroluminescence device from a dispersion of CdSe nanoparticles capped with a conducting polymer349 and then improved on this by replacing the polymer with a layer of CdS, producing a device with efficiency and lifetime increased by factors of 8 and 10, respectively. 0 Chemical synthetic methods for the assembly of nanocrystal composites, consisting of II-VI quantum dot polymer composite materials,351 represent one important step towards the fabrication of new functional devices that incorporate quantum dots. 

Bharathan, J. Yang, Y. 1998. Polymer electroluminescent devices processed by inkjet printing I. Polymer fight-emitting logo. Appl. Phys. Lett. 72 2660-2662. 

Aspects of Covalent Fullerene Chemistry Regioselective Multiple Functionalization, Optically Active Carbon Allotropes, and Electroluminescent Devices (LEDs)... 

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