DYE CONCENTRATION STUDY IN PVK BASED LIGHT-EMITTING-DIODES

Light emitting diodes made of a single spin-coated layer of poly(9-vin ylcarbazole) doped with coumarin-515 dye have been prepared. The influ ence of dye concentration on emission and electrical characteristics i s evidenced. Two different regimes are identified. At low concentratio ns, hole injection barrier raises, holes are trapped and mobility decr eases. External quantum efficiency increases with concentration. At co ncentrations larger than 10%wt, coumarin aggregates, photoluminescence yield drops and electroluminescence yield increases up to 0.1 %-photo ns per electron. Coumarin in a single-layer diode improves electron-ho le injection and recombination balance more than an additional hole-bl ocking layer.