Electronic Processes Associated with Electroluminescence in Conjugated Polymers

Processible conjugated polymers can be used to fabricate a range of thin-film diodes which can be designed to show good characteristics both as electroluminescent diodes and also as photoconductive diodes. We consider the present understanding of the operation of light-emitting diodes which use conjugated polymers for both charge transport and emission. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and thus bring about electron-hole capture to generate excitons at this interface. We present results on the photophysical properties of the cyano-substituted poly(phenylene vinylene)s which provide electron transport layers in these heterostructure devices; we find evidence for strong interchain interactions (excimer formation) for the neutral excited states in these polymers, with strong red-shifts of the luminescence, though preserving high photoluminescence efficiencies.