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 using a side-chain random polymer based on a high electron affinity (EA) naphthalimide moiety (PNI). The chromophore is attached to a polymethacrylate backbone through a spacer, and emits in the green with high efficiency (30% photoluminescence quantum yield). In single-layer light-emitting diodes (LEDs), we find that the electroluminescence quantum efficiency is not limited by Al cathodes as for poly(⁠$p$-phenylene vinylene), PPV, and we attribute this to the increased 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. Compared to some oxadiazole based electron injection/transport 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 this layer. We discuss the properties of the different device configurations from the viewpoint of the electronic structure of the materials and, in particular, the influence of the thickness of the individual layers on both quantum (photon/electron) and luminous (Lumen/$W$⁠) efficiency and driving conditions. Unexpectedly, we find that not only does the dye doping of PNI redshift 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.