Carrier 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. 

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. 

F.R. Zhu, B.L. Low, K.R. Zhang, and S.J. Chua, Lithium-fluoride-modified indium tin oxide anode for enhanced carrier injection in phenyl-substituted polymer electroluminescent devices, Appl. Phys. Lett., 79 1205-1207, 2001. 

Kepler et al. (1995) measured electron and hole mobilities of tris(8-hydroxyquinoline)aluminum (Alq). Alq is of interest for electroluminescent devices. The photocurrent transients for both carriers were highly dispersive. Transit times could be resolved only from double logarithmic transients. The electron mobilities were approximately two orders of magnitude higher than hole mobilities. Figure 46 compares the room temperature electron and hole mobilities. The dashed line represents electron mobilities reported by Hosokawa et al. (1994). At 4 x 105 V/cm, the electron and hole mobilities are 1.4 x 10-6 cm2/Vs and 2.0 x 10-8 cm2 Vs. The activation energy for the electron mobility was reported as 0.56 eV. Later results of Lin et al. (1996) were in excellent agreement with the hole mobilities reported by Kepler et al. 

Figure 1-1. Schematic drawing of a single-layer electroluminescent device. An applied electric field leads to injection of holes (positive charges the majortiy charge carriers in polymers such as PPV) and electrons (usually the minority charge carriers) into the light-emitting polymer film from the two electrode contacts. Formation of an electron-hole pair within the polymer may then result in the emission of a photon. Since holes migrate much more easily through PPV than electrons, electron-hole recombination takes place in the vicinity of the cathode.

Unlike thin film transistors, electroluminescence devices operate in a high electric field, and therefore homogeneity and low defect density of the thin films are more important than the carrier transport characteristics. In this sense, the approach to the use of nematic semiconductors by Kelly and O Neill should be reasonable. 

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

Figure 12.1 Typical organic electroluminescence device configurations showing the arrangement of electrodes, carrier transport and emitting layers. (ETL and HTL refer to electron and hole transporting layers, respectively.)...

Appleyard, S.F.J., S.R. Day, R.D. Pickford, and M.R. Willis. 2000. Organic electroluminescent devices Enhanced carrier injection using SAM derivatized ITO electrodes. / Mater Chem 10 169-173. 

Adachi, C., Tsutsui, T., and Saito, S., Confinement of charge carriers and molecular excitons within 5-nm-thick emitter layer in organ electroluminescent devices with a double hetero structure, Appl. Phys. Lett., 57, 531-533 (1990). 

Egusa, S., Miura, A., Gemma, N., and Azuma, M., Carrier injection characteristics of organic electroluminescent devices, Jpn. J. Appl. Phys., 33, 2741-2745 (1994). 

Organic electroluminescent devices have been the subject of intense research for almost a decade. These organic molecule-based devices use a multilayer cell structure composed of emitted layers and carrier transport layers as shown by Tang et al. [264-5]. The next advance in these devices was the construction of a three-layer cell by Adachi et al. [266] in which the emitter layer was sandwiched between a hole transport layer and an electron transport layer in the belief that these layers would increase electroluminescent efficiency. The device has the type of constmction shown in Figure 12.25. Adachi et al. reported that with a 500 A° emitter thickness, the luminescence was 1000 Cd cm. ... 

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]. 

Soluble 3.4-disubstituted polythiophenes have found application as antistaticcomponents for film materials and are on the market electrochromic and electroluminescent devices are subject to intensive research and surely will be effective in the near future. Biosensor devices with functionalized polythiophene carrier systems for immobilizing enzymes are also applicable for the electro-analytical determination of analytes in micromolar concentrations. 

In this work, we fabricated heterostructure electroluminescent devices by the combination of the PbBr-based layered perovskite as emissive material and organic carrier transporting materials, electron-transporting oxadiazole and hole-transporting copper phthalocyanine, and evaluated their electroluminescent properties. 

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