NAPHTHALIMIDE SIDE-CHAIN POLYMERS FOR ORGANIC LIGHT-EMITTING-DIODES -BAND-OFFSET ENGINEERING AND ROLE OF POLYMER THICKNESS

We report the Fabrication of efficient green light-emitting diodes usi ng a side-chain random polymer based on a high electron affinity (EA) naphthalimide moiety (PNI). The chromophore is attached to a polymetha crylate backbone through a spacer, and emits in the green with high ef ficiency (30% photoluminescence quantum yield). In single-layer light- emitting diodes (LEDs), we find that the electroluminescence quantum e fficiency is not limited by Al cathodes as for poly(p-phenylene vinyle ne), PPV, and we attribute this to the increased EA. We also report ma ximum internal quantum efficiencies of about 1.7% for Ca and 0.9% for Al in double-layer devices where PPV serves as both hole injector and emitter. Compared to some oxadiazole based electron injection! transpo rt layers, PNI gives higher efficiencies at high currents, and longer lifetimes. Tuning of emission in the red is possible by dye doping (at high concentration) the PNI and causing the emission to happen in thi s layer. We discuss the properties of the different device configurati ons from the viewpoint of the electronic structure of the materials an d, in particular, the influence of the thickness of the individual lay ers on both quantum (photon/electron) and luminous (Lumen/W) efficienc y and driving conditions. Unexpectedly, we find that not only does the dye doping of PNI redshift the emission spectrum, but also affects si gnificantly the charge transport properties, and in particular reduces the driving field necessary for electroluminescence in both single an d double-layer LEDs. (C) 1998 American Institute of Physics.