Raoul Cervini

High electron affinity polymers for LEDs

Summary form only given. A new series of alternating copolymers based on poly(p-phenylenevinylene) (PPV) was synthesized. The electron affinity of the polymers is substantially increased through the use of electron-withdrawing substituents. Optimum conditions for the condensation polymerization were explored, and polymers with a M/sub n/ in excess of 30,000 were produced. The synthesis is extremely flexible and allows a large variety of conjugated polymers with different emission properties to be synthesized. Cyclic voltammetry was used to estimate the position of the HOMO and LUMO relative to poly(2,5-dialkoxy-1,4-phenylenevinylene) analogues. Fabrication of efficient bilayer polymer LEDs using PPVin conjunction with the new polymers and stable metal negative contacts such as aluminum and gold is reported.

4-AcNI—a new polymer for light-emitting diodes

Abstract We report investigations on a 4-acetamido-substituted naphthalimide polymer (4-AcNI) as a component for light-emitting diodes (LEDs). Devices prepared with 4-AcNI as the active material are relatively inefficient (4 × 10−4%). Higher internal quantum efficiency values of 0.4% are achieved if 4-AcNI is used as an electron-transporting layer (ETL) in devices which use poly(p-phenylene vinylene) (PPV) as a hole transport layer. For Al cathodes, this material proves to be superior to similar devices which use 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as the ETL. The results are used to shed some light on the function of ETLs in organic LEDs.

New CF3-substituted PPV-type oligomers and polymers for use as hole blocking layers in LEDs

Abstract New CF 3 -substituted PPV derivatives have been synthesised by Wittig-Horner polycondensation. To obtain a better understanding of the relationship between absorption and luminescence properties and structure a single crystal X-ray analysis of a model oligomer has been performed. The effect of electron withdrawing trifluoromethyl groups at vinylidene linkages in PPVs on absorption, luminescence, hole blocking and electron injecting properties has been investigated.

Synthesis of new conjugated thiophene polymers

Results are presented which explore the synthesis, characterization and photoluminescence of new types of conjugated copolymers which incorporate fused thiophene units. Such materials possess intense purple-red colours and some absorb in the near-infrared. The redox processes showed that the polymers were both p- and n-dopable.

Light-emitting polymer LEDs

Simple light emitting diodes can be constructed using fluorescent organic materials. Conjugated polymers can be used both for charge transport and for light emission. It is considered necessary for maximum device efficiency to balance the rates of electron and hole injection. We report the synthesis of a poly(cyanoterephthalylidenene) that was designed to exhibit an increased electron affinity. Electrochemical measurements showed a significant shift in the oxidation and reduction potentials due to the cyano functionality. The use of this polymer in a range of electroluminescent devices is described. Internal quantum efficiencies of up to 4% can be achieved in a bilayer device using stable electrode materials. The route used to synthesize this polymer is amenable to considerable variation in the subunits employed. This allows tuning of both the band-gap and the electron affinity of the resulting polymer.

Precursor route chemistry and optoelectronic properties of poly(pyridine vinylene)

Several soluble precursor routes, involving sulfonium-, methoxy-, and chloro-substituted polymers, have been investigated for the synthesis of poly(pyridine vinylene)(PPyV). The thermal conversions of these precursors to fully conjugated polymer PPyV have been characterised by TGA, FT-IR and UV-VIS spectroscopy in conjunction with microanalysis. Of all the precursors, the chloro-substituted polymer is the most convenient for forming good quality thin films. PPyV fluoresces orange red and has a HOMO-LUMO band gap of 2.2 eV. PPyV has a lower luminescent quantum efficiency compared with its analogue, poly(phenylene vinylene) (PPV), but a higher electron affinity according to CV measurements (with reduction potential of -1.3 V). PPyV was used as an orange red emissive layer in a LED using aluminium as cathode.

Model compounds for novel high electron affinity polymers

Abstract The development of improved polymers for light emitting diodes (LED)s requires fluorescent materials with high electron affinities to act both as emitters and as charge transport materials. In this paper we report the synthesis of novel distyrylpyrazines and distyrylpyrazine- N-oxides as model compounds for novel high electron affinity polymers. The UV-VIS and PL emission spectra demonstate that the emission of these materials can be tuned from blue to red by changing the substituents. Cyclic voltammetry reveals that these compounds possess high electron affinities