White polymer light-emitting diodes co-doped with three phosphorescent iridium(III) complexes aimed at improvement of color rendering properties

To obtain white electroluminescence (EL) with a high color rendering index (CRI), we fabricated polymer light-emitting diodes (PLEDs) co-doped with high-efficiency blue-, yellow- and red-phosphorescent bis-cyclometalated iridium(III) complexes (Ir-1, Ir-2, and Ir-3, respectively) in a single emitting layer based on a PVCz-OXD-7 blended film (PVCz, poly(9-vinylcarbazole); OXD-7, 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene). When the constituent ratio of the emitting layer (PVCz:OXD-7:Ir-1:Ir-2:Ir-3) was adjusted to 10:3.0:1.2:0.060:0.036 in weight ratio, an approximately white PLED (WPLED) was obtained showing a Commission Internationale de l׳Eclairage (CIE) chromaticity coordinate of (0.38, 0.44) and a CRI of 83 at 1000 cd m−2. To elucidate the energy transfer among the phosphorescent emitters, we also investigated the photophysical properties of the PVCz-based thin film whose constituent ratio is the same as the emitting layer of the WPLED, and it was found that a cascade of the energy transfer from the blue to the red via the yellow emitter considerably contributes to the emission process along with the direct energy transfer from the blue to the red and yellow emitters.