Effect of coumarin on blue light-emitting diodes based on carbazol polymers

Light-emitting diodes made of a single spin-coated layer of poly(9-vinylcarbazole) have been doped with coumarin-515 dye and the effect of the dye concentration on emission and electrical characteristics has been studied. Two different regimes have been identified. For low concentrations, the apparent hole injection barrier rises, holes are trapped, and mobility decreases. External quantum efficiency increases with concentration. At concentrations larger than 10 wt % coumarin aggregates, photoluminescence yield drops and electroluminescence yield increases up to 0.1% photons per electron. As a conclusion, using coumarin dye in a single-layer diode improves electron-hole injection and recombination balance more than using an additional hole-blocking layer. (C) 1998 American Institute of Physics.