Synthesis and Properties of Novel High-Electron-Affinity Polymers for Electroluminescent Devices

Abstract

New conjugated polymers with high electron affinities for use in polymer light-emitting diodes (LEDs) have been synthesized, and their optical and electrochemical properties studied. Incorporation of electronwithdrawing trifluoromethyl substituents was found to markedly enhance the electron affinity of poly(p-phenylene vinylene) (PPV). Polymers incorporating electron-transporting oxadiazole units either in the main -chain or as side-chains have been prepared and evaluated as emissive and charge-transporting materials in LEDs. Copolymers have been made containing both emissive and charge-transporting units as side-chains. The electrochemical oxidation and reduction potentials of all these materials have been measured and compared with those of other conjugated polymers.Since the first report of polymer light-emitting devices (LEDs) using poly(pphenylene vinylene) (PPV) as an emissive layer (1), much progress has been achieved in the understanding of the physics and materials chemistry of LEDs, and also in the search for new electroluminescent polymers. Colors ranging from infrared to violet can be achieved for polymer LEDs by using different emissive polymers. A variety of emissive polymers, such as substituted poly(phenylene vinylene) (2,3),poly(3-alkylthiophene) (4,5) and polyphenylene derivatives (6-8) have been described. These polymers are usually sandwiched between a transparent layer of indium-tin oxide (ITO) and a metal electrode. Singlet excitons are formed under double charge injection and decay radiatively to produce light emission at a wavelength in accordance with the band gap energy of the emissive polymers. Electron Injection and Charge Transport in Polymer Light Emitting DevicesIn addition to the considerable advances in the tuning of emission colors of polymeric LEDs, recent attention has focussed on improving the efficiency of devices. In order

Synthesis, Properties, and Application of New Luminescent Polymers with Both Hole and Electron Injection Abilities for Light-Emitting Devices

“…Both polymers possessed reversible oxidation peaks at relatively low potentials when swept anodically. The onset of the oxidation for 11 was located at 0.4 V and for 12 at 0.5 V, which were lower than that for PPV (0.85 V) and for CN-PPV (1.0 V) . Since the oxidation process for a conjugated polymer in an electrochemical cell is closely related with the removal of electrons (or the injection of the holes) from the HOMO of the material, the lower oxidation potentials for 11 and 12 suggest a better hole injection ability.…”

“…Nevertheless, the CV measurement and related HOMO/LUMO position judgment provide a convenient and useful quantitative comparison for the charge injection ability of conjugated polymers. The CV data for many luminescent polymers and charge transporting polymers have been well-documented in recent years. ,,, Besides, the inonization potential and the electron affinity of a conjugated material can also be judged by using CV redox data. ,

5 Electroluminescent spectrum of 11 (ITO/ 11 /Al) and its comparison with UV and PL spectra.

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7 Electroluminescent spectrum of 12 (ITO/PVK/ 12 /Al) and its comparison with UV and PL spectra.

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“…The onset of the oxidation for 11 was located at 0.4 V and for 12 at 0.5 V, which were lower than that for PPV (0.85 V) and for CN-PPV (1.0 V). 8 Since the oxidation process for a conjugated polymer in an electrochemical cell is closely related with the removal of electrons (or the injection of the holes) from the HOMO of the material, the lower oxidation potentials for 11 and 12 suggest a better hole injection ability. In terms of anodic reversibility, our polymers presented better hole injection ability than the reported oxadiazole/ carbazole 13 or oxadiazole/thiophene 14 bipolar polymers.…”

“…This is due to the difficulty in placing electrons into the higher antibonding molecular obitals. Much effort has been made toward the preparation of high electron-affinity polymers which may provide more efficient electron injection properties …”

“…Much effort has been made toward the preparation of high electron-affinity polymers which may provide more efficient electron injection properties. 8 The introduction of strong electron affinity groups, e.g. cyano groups, onto a PPV backbone has proved to be an effective way to lower the lowest unoccupied molecular orbital (LUMO) of a polymer and therefore enhanced the electron injection ability.…”