Soluble electroluminescent poly(phenylene vinylene)s with balanced electron- and hole injections

We report a new route for the design of efficient soluble electroluminescen t PPV-based copolymers bearing electron-deficient oxadiazole (OXD) moieties on side chains. The introduction of OXD through a long alkylene spacer wit h PPV backbone provides a molecular dispersion of OXD in the film; both the side chain OXD and the main chain PPV do retain their own electron-transpo rt and emissive properties, respectively. The use of phenylene vinylene der ivatives with asymmetric and branched substituents and a long spacer provid es solubility for ease of device fabrication as well as amorphous structure to allow a well-mixing of OXD groups with the main chains. By properly adj usting the OXD content through copolymerization, we can tailor the chemical structure of electroluminescent material to give a balance of hole- and el ectron injections for various metal cathodes, such that the quantum efficie ncy is significantly improved and the turn-on voltage is reduced for the de vices with aluminum and calcium. For the device with calcium fabricated in open air, a maximum brightness of 15000 cd/m(2) at 15 V/100 nm and a maximu m luminance efficiency of 2.27 cd/A can be obtained, respectively, about 30 times brighter and 9.4 times more efficient than those with the correspond ing homopolymer, poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] ( MEH-PPV). The use of physical blends to simulate the copolymers provides no significant improvement, since phase-separation structures appear, causing an inefficient utilization of OXD and sometimes voltage-dependent emission spectra. The present route permits a fabrication of single layer PLED with high brightness, high efficiency, and low turn-on voltage.