Effect of dye concentrations in blended-layer white organic light-emitting devices based on phosphorescent dyes

The electronic and optoelectronic behavior of white organic\ud light-emitting devices (OLEDs) based on blue (FIrpic) and red\ud [Ir(piq)(2)(acac)] phosphorescent dyes doped into the same layer of a\ud polyvinylcarbazole (PVK) host are reported. The conductivity of all the\ud OLEDs studied appeared to be dominated by space-charge injection\ud effects, exhibiting a current I versus voltage V dependence of the form\ud I alpha V-n, with n approximate to 7 at applied voltages at which\ud electroluminescence was observed. Systematic studies of the current\ud versus voltage and light-emitting behavior of the OLEDs have identified\ud different excitation processes for the two dyes. It is suggested that\ud electroluminescence from the FIrpic molecules originates by direct\ud transfer of the exciton energy from the PVK to the dye molecules, while\ud the process of light emission from the Ir(piq)(2)(acac) molecules\ud involves carrier trapping. The efficiency of the devices can be tuned,\ud to some extent, by varying the thickness of the organic film. Luminous\ud efficiencies and luminous power efficiencies of 8 cd A(-1) and 3 lm W-1\ud were measured for these blended-layer OLEDs, with Commission\ud Internationale de l'Eclairage coordinates of 0.35, 0.35. \u