NAPHTHALIMIDE POLYMERS FOR ORGANIC LIGHT-EMITTING-DIODES

We used a side-chain polymer based on a high-electron affinity TEA) na phhalimide moiety (PNI), to fabricate single and double-layer light-em itting diodes (LEDs) with improved efficiency in the green spectral re gion. The chromophore is attached to a polymethacrylate backbone throu gh a spacer and is characterised by a 30% photoluminescence quantum yi eld. In single-layer light-emitting diodes we find that the electrolum inescence efficiency is not limited by Al cathodes a!, for poly(p-phen ylene vinylene), PPV, as expected from consideration of the EA. We als o report maximum internal quantum efficiencies of about 1.7% for Ca an d 0.9% for Al in double-layer devices where PPV serves as both hole-in jector and emitter. Tuning of emission in the red is possible by dye-d oping (at high concentration) the PNI and causing the emission to happ en in this layer. Unexpectedly, not only does the dye-doping of PNI re d-shifts the emission spectrum, but also affects significantly the cha rge transport properties and in particular reduces the driving field n ecessary for electroluminescence in both single-and double-layer LEDs and we propose this effect as one of the factors for the lifetime incr ease upon doping recently reported in the literature [J. Shi, C.W. Tan g, Appl. Phys. Lett. 70 (1997) 1665.]. (C) 1998 Elsevier Science B.V.