LED devices

Further developments in this area have included the neparation of several additional N,N -diaryl indolo[3,2-h]carbazoles with substituents such as m-tolyl, ffi-anisoyl, or triarylamine-containing species. Like 221, these compounds, possessing excellent hole-transport properties, also occurred in stable amorphous states and displayed high glass-transition temperatures. LED devices involving these systems were also constructed and showed promising characteristics [OOSMO11-112)42]]. 

Figure 11-1. Chemical structure of poly(pura-phenylene vinylene) (PPV) and schematic polymer LED device structure.

Poly(l,4-phenylene vinylcne) and its Derivatives 2 The Basic Polymer LED Device Architecture 4 Substituted Poly(phcnylene vinylcne)s 6 Poly(anthrylenevinylcne)s 10 Step-Growth Routes to PPV Derivatives 10 PPV Copolymers 11... 

For typical polymer LED device parameters, currenl is space charge limited if the energy barrier to injection is less than about 0.3-0.4 eV and contact limited if it is laiger than that. Injection currents have a component due to thermionic emission and a component due to tunneling. Both thermionic emission and tunneling... 

LC state. See Liquid crystalline (LC) state LED devices. See Light-emitting diode (LED) devices... 

In this work, we outline the methodology for realistic analysis and first-principles prediction of the optical properties useful for obtaining phosphors, ameliorating the emission of LED devices by shifting the blue and UV predominance realizing... 

Although the exact mechanism of the fluorenone formation is not known, it is believed that the monoalkylated fluorene moieties, present as impurities in poly(dialkylfluorenes), are the sites most sensitive to oxidation. The deprotonation of rather acidic C(9)—H protons by residue on Ni(0) catalyst, routinely used in polymerization or by metal (e.g., calcium) cathode in LED devices form a very reactive anion, which can easily react with oxygen to form peroxides (Scheme 2.26) [293], The latter are unstable species and can decompose to give the fluorenone moiety. It should also be noted that the interaction of low work-function metals with films of conjugated polymers in PLED is a more complex phenomenon and the mechanisms of the quenching of PF luminescence by a calcium cathode was studied by Stoessel et al. [300],... 

Other fluorene-oxadiazole copolymers, such as fully conjugated 268a,b [360] or 269a-c, with conjugation interrupted by o-links [361], have been synthesized. For both series of polymers, emission was in the blue region at very similar wavelengths, but no LED device was reported. 

When working with metal electrodes, the energy of the electrons in the metal is lower than the vacuum level by the work function of the metal, which tends to be 3-5 eV. Work functions of some materials relevant to LED devices are collected in Table 10.2 [11]. The work function can vary depending upon the crystal facet from which emission is measured (or if the metal is amorphous), and sample preparation details. The photoelectric (PE) effect is exploited in XPS (ESCA) or UPS to measure the work function. It is very critical to realize that, in these experiments, what is measured is the energy required to remove an electron to a point just outside the surface of the solid, not to infinity. At this range, the dipolar forces at the surface are still active, and one can learn about surface dipoles in the material. 

Work Functions of Some Materials Relevant to LED Devices. The Work Function Can Vary Depending upon the Crystal Facet from Which Emission Is Measured, and Sample Preparation Details... 

Cyano-substituted PPVs have low electrical conductivity due to the electron withdrawing ability of cyano group, however, they have become important materials for the fabrication of LEDs. While LED devices constructed from unsub-stituted-PPV exhibit efficiencies of less than 1%, similar LED devices fabricated with cyano-substituted PPVs have displayed efficiencies of over 4%. To further enhance device performance, proccessible polymers containing cyano functionalities were synthesized utilizing monomers possessing solubilizing alkoxy chains for ease of device construction. 

The materials (metals and conjugated polymers) that are used in LED applications were introduced in the previous chapter. The polymer is a semiconductor with a band gap of 2-3 eV. The most commonly used polymers in LEDs today are derivatives of poly(p-phenylene-vinylene) (PPV), poly(p-phenylene) (PPP), and polythiophene (PT). These polymers are soluble and therefore relatively easy to process. The most common LED device layout is a three layer component consisting of a metallic contact, typically indium tin oxide (ITO), on a glass substrate, a polymer film r- 1000 A thick), and an evaporated metal contact4. Electric contact to an external voltage supply is made with the two metallic layers on either side of the polymer. 

The current density vs electric field curves and the emitted light intensity vs current density for the three LED devices fabricated from P-1 and P-4 are com-... 

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