Space electroluminescence 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. 

Two main methods have been used to measure the charge carrier mobility in electroluminescent polymers time of flight (TOF) carrier transit time measurements and analysis of the current-voltage (1-V) characteristics of single carrier devices in the space charge-limited current (SCLC) regime. A summary of the results for the hole mobility of PPV and PPV-related polymers is given in Table 11-1 [24, 27-32]. For... 

FIGURE 4.5. Electroluminescence spectra from three devices which have different optical thicknesses. The presence of a second mode in one of the spectra is a result of a closer than ideal mode spacing, If the refractive index difference between the materials that constitute the QWS is enhanced, the total optical thickness will be reduced and the mode spacing increased. 

The EL spectrum has multiple peaks when more than one mode of the cavity overlaps the free-space emission spectrum. It is possible to realize a white LED with a single electroluminescent material such as Alq by employing a two-mode microcavity device structure in which one of the modes is centered near 480 nm and the other near 650 nm. Such an electroluminescence spectrum, for which the CIE coordinates are (0.34, 0.386),13 is shown in Fig. 4.6. The approximate spectrum calculated with Eq. (4) is also shown in Fig. 4.6. With very minor changes in the device design, it is easy to achieve (0,33, 0.33). For comparison, the CIE coordinates of a noncavity Alq LED are (0.39, 0.56). 

Several different materials can be used as transparent electrodes, most of them as anode material ITO [113,194,195,200], pofyaniline and polyaniline blends [206, 207, 209, 210], TO [201, 202,204], and F-doped TO [112,205]. The use of transparent material as a cathode has also been reported [202]. In the case of PPV and several of its derivatives, the effective mobility of the electrons is lower than that of the holes, implying a reduction in the extent of the recombination zone in the electroluminescent polymer layer, as observed in PPP LEDs [164]. Further, the values of polymer electroaffinity and ionization potential make the injection and transport of holes etisier than that of electrons in single-polymer-layer devices. The injection dynamics also depends on the injected carrier that remains in the polymer (space charge), modifying the electric field distribution in the device [217]. For these reasons, different materials are tested as cathode and anode and, in several cases, intermediate layers are also introduced in order to improve the injection of a specific charge carrier type or to block its transport through the device [212,213,218-220]. 

Ferroelectric Polymers (1995), Nonlinear Optics of Organic Molecules and Polymers (1997) and Organic Electroluminescent Materials and Devices (1997), he has authored over 100 scientific publications in leading refereed journals and books, and has 18 patents either issued or applied for on electronic and photonic materials. He is the founder and Editor-in-Chief of the Journal of Porphyrins and Phthalocyanines and serves on the editorial board of Applied Organometallic Chemistry, Journal of Macromolecular Science-Physics and Photonics Science News. A member of the American Chemical Society and American Association for the Advancement of Science, he has also been awarded a number of prestigious fellowships in India and abroad National Merit Scholarship, Indian Space Research Organization (ISRO)... 

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