EFFICIENT LIGHT-EMITTING-DIODES BASED ON POLYMERS WITH HIGH ELECTRON-AFFINITIES

CONJUGATED polymers have been incorporated as active materials into se veral kinds of electronic device, such as diodes, transistors1 and lig ht-emitting diodes2. The first polymer light-emitting diodes were base d on poly(p-phenylene vinylene) (PPV), which is robust and has a readi ly processible precursor polymer. Electroluminescence in this material is achieved by injection of electrons into the conduction band and ho les into the valence band, which capture one another with emission of visible radiation. Efficient injection of electrons has previously req uired the use of metal electrodes with low work functions, primarily c alcium; but this reactive metal presents problems for device stability . Here we report the fabrication of electroluminescent devices using a new family of processible poly(cyanoterephthalylidene)s. As the lowes t unoccupied orbitals of these polymers (from which the conduction ban d is formed) lie at lower energies than those of PPV, electrodes made from stable metals such as aluminium can be used for electron injectio n. For hole injection, we use indium tin oxide coated with a PPV layer ; this helps to localize charge at the interface between the PPV and t he new polymer, increasing the efficiency of recombination. In this wa y, we are able to achieve high internal efficiencies (photons emitted per electrons injected) of up to 4% in these devices.