Naphthalimide polymers for organic light-emitting diodes

Abstract

We used a side-chain polymer based on a high-electron affinity (EA) naphthalimide moiety (PNI), to fabricate single and double-layer light-emitting diodes (LEDs) with improved efficiency in the green spectral region. The chromophore is attached to a polymethacrylate backbone through a spacer and is characterised by a 30% photoluminescence quantum yield. In single-layer light-emitting diodes we find that the electroluminescence efficiency is not limited by A1 cathodes as for poly( p-phenylene vinylene), PPV, as expected from consideration of the EA. We also report maximum 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. Tuning of emission in the red is possible by dye-doping (at high concentration) the PNI and causing the emission to happen in this layer. Unexpectedly, not only does the dye-doping of PNI red-shifts the emission spectrum, but also affects significantly the charge transport properties and in particular reduces the driving field necessary for electroluminescence in both single- and double-layer LEDs and we propose this effect as one of the factors for the lifetime increase upon doping recently reported in the literature.