Light-emitting devices based on a poly(p-phenylene vinylene) derivative with ion-coordinating side groups

We report the phase-separation and the optoelectronic properties of an alte rnating poly(p-phenylene vinylene)-based copolymer, to which poly(ethylene oxide), PEO, and or lithium triflate are added. The alternating units of th is copolymer (DB-BTEM-PPV) are 2,3-dibutoxy-1,4-phenylene vinylene, and 2,5 -bis(triethoxymethoxy)-1,4-phenylene vinylene, a moiety containing side gro ups allowing ion solvation and transport. Upon addition of the ion-transpor ting polymer PEO to DB-BTEM-PPV blended with lithium triflate, we have foun d a sizeable efficiency increase (from 0.9 to 1.5 cd/A), with a concomitant increase of the response time. We propose that this is due to solvation an d complexation of lithium triflate by the PEO, which simultaneously reduces the quenching of photoluminescence (and electroluminescence) efficiency by the ionic charge, and the effectiveness of formation of highly doped, low- barrier, polymer/electrode interfaces. We discuss charge transport and inje ction in the copolymer and in the blend with reference to the diodes charac teristics, and to the phase separation of PEO, which we investigated with a tomic force microscopy. (C) 1999 American Institute of Physics. [S0021-8979 (99)03323-X].