Multilayer OLED

Using both low-molecular evaporated films and polymer films, multilayer OLEDs of high efficiency have been constructed. The structures of some molecules often used for evaporated films are shown in Fig. 11.4, and the monomers for polymer films in Fig. 11.5. The purpose of the fabrication of multilayer OLEDs is the independent optimisation of the individual processes which were described in Sect. 11.1.2, with the goal of achieving high-efficiency OLEDs injection and transport of the electrons and holes, balance between the currents of electrons and holes, recombination, use of triplet states, and reduction of reflections in the transmission of the luminescence to the outside of the OLED. In the following, we will treat a few typical examples of OLEDs prepared with low-molecular evaporated films. We emphasize, however, that also multilayer OLEDs made with polymers can yield comparable results and information. 

The simplest multilayer OLED consists of two organic layers (Fig. 11.6), one EML which is at the same time an ETL (e.g. Alq3), and an HTL (e.g. NPB). )Alq3 is a singlet emitter. The layer thicknesses can be optimised so that the electron and hole currents are equal and that the recombination and thus the emission occur in the Alq3, and there in a thin layer near the interface to the HTL This allows the 

5 PEDOT = poly-3,4-ethylene-dioxi-thiophene PSS = polystyrol-sulfonate PEDOT PSS (= BAYTRON) is used as anode material. 

With a dielectric cover layer of ZnSe over the semitransparent Ca cathode, by a suitable choice of the layer thickness, both a high efficiency and good colour purity could be obtained. Here, the constructive interference of the directly emitted radi- 

The most important electrooptical properties of this and similar highly efficient multilayer OLEDs made of molecular evaporated layers of polymer layers will be discussed in the following subsection on the basis of examples. 

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FIGURE 3.1 Schematic of a multilayer OLED device structure. 

The configuration and construction of monolayer and multilayer OLEDs have undergone substantial changes and modifications since these first reports of organic electroluminescence from low-molar-mass materials. Several types of OLEDs using small organic and organometallic molecules are described schematically below. 

A further potential problem associated with the practical fabrication of multilayer OLEDs with low-molar-mass calamitic and columnar liquid crystals with a charge-transport layer, especially as the HTL, is the fluid nature of the liquid crystalline state. This could permit contamination of the columnar liquid crystal during the deposition of subsequent layers by the process of vapour deposition or spin-coating. The columnar materials could well be soluble in the solvent used to deposit the next layer. This would lead to chemical contamination of the individual layers due to mixing of one component into another, the... 

The use of insoluble, highly cross-linked anisotropic networks created by the polymerisation of photoreactive monomers, eliminates the problem of crystallisation, at least for organic materials, since polymer networks are macromole-cular structures incapable of crystallising, see Chapter 6. Furthermore, the fabrication of multilayer devices would be facilitated by the use of a cross-linked stable HTL next to the anode on the solid substrate surface, onto which subsequent layers can be deposited by vapour deposition. Multilayer OLEDs are intrinsically more stable than monolayer devices due to a better balance of charge-carriers and concentration of the charged species away from the electrodes. The synthesis and cross-linking of a suitable aromatic triarylamine derivative with a polymerisable oxetane group at each end of the molecule for use as a HTL has been reported recently, ... 

Conjugated organic polymers such as those shown in the Tables have been used in multilayer OLEDs as the HTL or combined HTL and emission layers or as the ETL or combined ETL and emission layer. The combined polymers (75-77) shown in Table 6.15 have been used as combined ETL, HTL and emission layers in various OLED configurations. Blends of these polymers have also been used to maximise OLED efficiency, although phase separation is always a problem with mixtures (blends) of main-chain polymers. 

The work on Alq3 and other small 7r-conjugated molecules that followed shortly thereafter13 14 demonstrated that multilayer OLEDs could be fabricated simply by thermal evaporation of these molecules. In 1990 Friend and coworkers described the first PLED,15 in which the luminescent poly(p-phenylene vinylene) (PPV)... 

In the following sections, we give an overview of the various attempts to fabricate crosslinked layers for use in multilayer OLEDs categorized by the reactive group used in the precursor materials. We start with the [2+2] cycloaddition of cinna-mates and the radical polymerization of acrylates and styrene derivatives. The emphasis of the chapter is on our own work, which is focused on the cationic ringopening polymerization (CROP) of oxetane-functionalized materials. Finally, we summarize the less-frequently employed synthetic routes. 

In this final subsection on OLEDs, we wish to show some selected, characteristic experimental results using the example of the multilayer OLEDs of the type shown in Fig. 11.9, and to explain them [10]. 

Fig. n.lO The current densityj and the luminosity Lq of the multilayer OLED of the type shown in Fig. 11.9, both plotted logarithmically as functions of the applied voltage on a linear scale. Three experimental curves for three Ni anodes of differing... 

The level of quality that has been achieved in OLEDs can be seen also from their radiation characteristics Fig. 11.13 shows the angular distribution of the emitted intensity in a polar diagram for the multilayer OLED (Figs. 11.9 and 11.10). Its... 

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