High electron affinity polymers for LEDs

Several high electron affinity cyano derivatives based on poly(phenylenevinylene) (PPV) have been synthesised via a Knoevenagel condensation reaction. The HOMO-LUMO gap and hence the electro-optical properties of the polymers can be altered by judicious choice of the monomer units or the solubilising groups. The construction of efficient polymer LEDs on both glass and silicon substrates using air stable aluminium negative contacts can therefore be realised. Dialkyl substituents on the aromatic ring were found to lower the conjugation through the chain, resulting in large blue shifts in the UV/VIS absorption maximum. The fluoresence emission of the alkyl substituted polymers appears to be lower than that of the equivalent dialkoxy-substituted polymers. The inclusion of thiophene units in the main chain lowers the HOMO-LUMO gap and shifts the luminescence emission into the infra-red.