Coumarin concentration effect on PVK-based blue light-emitting diodes

We study the influence of the doping ratio on current-voltage characteristics and electroluminescence quantum efficiency of single-layer poly(9-vinylcarbazole) (PVK)-polymer blue light-emitting diodes. Driving voltage increases monotonically with coumarin doping ratio, up to 10−1% photons/electron for a 30 wt % doping ratio. Time-of-flight experiments give evidence of a hole-trapping effect of the coumarin. It improves the balance for injection of holes and electrons. Holes which drift through the film are expected to have a higher probability of recombination with electrons on the coumarin luminescent centers.