Electroluminescence in polymers

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

Electroluminescence in Polymer-Nanocomposites and Light Emitting Diodes... 

The enormous progress in the field of electroluminescent conjugated polymers has led to performances of oiganic light-emitting devices (LEDs) that are comparable and in some aspects superior to their inorganic counterparts 11). Quantum efficiencies in excess of 5% have been demonstrated [2] and show that a high fraction of the injected carriers in a polymeric electroluminescence (EL) device form electronic excitations which recombine radiatively. 

Since multiple electrical and optical functionality must be combined in the fabrication of an OLED, many workers have turned to the techniques of molecular self-assembly in order to optimize the microstructure of the materials used. In turn, such approaches necessitate the incorporation of additional chemical functionality into the molecules. For example, the successive dipping of a substrate into solutions of polyanion and polycation leads to the deposition of poly-ionic bilayers [59, 60]. Since the precursor form of PPV is cationic, this is a very appealing way to tailor its properties. Anionic polymers that have been studied include sulfonatcd polystyrene [59] and sulfonatcd polyanilinc 159, 60]. Thermal conversion of the precursor PPV then results in an electroluminescent blended polymer film. 

G. Leising, S. Tasch. W. Graupncr, Fundamentals of Electroluminescence in Paraphcnylene-Typc Conjugaled Polymers and Oligomers, in Handbook of Conducting Polymers (Eds. T. Skotheim, R. Elsenbaumer, J. Reynolds), Marcel Dekker, New York 1997. 

Light-emitting diodes (LEDs) based on the electroluminescent conjugated polymers (Fig. 47) have attracted significant attention, both in academic research and industrial development, and are now on the edge of commercialization (Burroughs et al. 1990, Barth and Bassler 1997). Polymer LEDs require properties such as shown in Table 11. Recently, ef-... 

A Kraft, AC Grimsdale, and AB Holmes, Electroluminescent conjugated polymers — seeing polymers in a new light, Angew. Chem. Int. Ed., 37 402-428, 1998. 

Y Cao, ID Parker, G Yu, C Zhang, and AJ Heeger, Improved quantum efficiency for electroluminescence in semiconducting polymers, Nature, 397 414—417, 1999. 

Q. Pei and Y. Yang, 1,3,4-Oxadiazole-containing polymers as electron-injection and blue electroluminescent materials in polymer light-emitting diodes, Chem. Mater., 7 1568-1575, 1995. 

FIGURE 5.7 Schematic Representation of typical, (partially) electroluminescent LC polymer architectures. (a) Rodlike structure, (b) Hairy-rod structure, (c) Combined main-chain-side-chain system, (d) Semiflexible segmented structure, (e) Semiflexible segmented structure with disklike mesogen. (After Weder, C. and Smith, P., Main-chain liquid-crystalline polymers for optical and electronic devices, in Encyclopedia of Materials Science and Technology, Buschow, K.H., Cahn, R.W., Flemings, M.C., Ilschner, B., Kramer, E.J., and Mahajan, S., Eds., Elsevier Science, New York, 2001.)... 

P.K.H. Ho, M. Granstrom, R.H. Friend, and N.C. Greenham, Ultrathin self-assembled layers at the ITO interface to control charge injection and electroluminescence efficiency in polymer light-emitting diodes, Adv. Mater., 10 769-774, 1998. 

BH Cumpston and KF Jensen, Electromigration of aluminum cathodes in polymer-based electroluminescent devices, Appl. Phys. Lett., 69 3941-3943, 1996. 

Friend RH, Gymer RW, Holmes AB, Burroughes JH, Marks RN, Taliani C, Bradley DDC, Dos SDA, Bredas JL, Logdlund M, Salaneck WR (1999) Electroluminescence in conjugated polymers. Nature (London) 397 121-128... 

Kraft A, Grimsdale AC, Holmes AB Angew Chem Int Ed (1998) 37 403-428 Electroluminescent conjugated polymers - seeing polymers in a new light... 

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