White organic light-emitting diodes (WOLEDs) attract much attention in recent years due to their potential use in back light unit of flat panel displays, full color display and solid state lighting applications. The applications require WOLEDs possess high efficiency, appropriate color temperature, high color rendering index, and high color stability. [1] Various approaches have been reported to improve the performance, which include doping of several fluorophors or phosphors in a single emitting layer (EML), [2][3][4][5][6][7][8] synthesis of polymers incorporating different color emitting moieties, [9][10][11] use of excimer or exciplex formed by one or two dopants, [12][13][14] stacked several organic light emitting diodes (OLEDs), [15][16][17] use of microcavity effect from single emission layer, [18] down conversion of blue light, [1,19] and multi-EML structure doped with different color emitting dopants. [20][21][22][23][24][25] Among them, multi-EML structure has advantages over other architectures in terms of efficiency and color controllability because the recombination current, singlet and triplet energy transfer and performance of each layer can be controlled by layer thickness, doping concentration and charge blocking layers. One drawback of the WOLEDs with multi-emissive layers is the color shift with increasing voltage. [21][22][23][24][25] The color shift is believed to be originated from the shift of recombination zone with increasing voltage and easier formation of high energy excitons at higher voltage. [5] In this paper, we report the fabrication of efficient, color-stable, multi-EML WOLEDs using three phosphorescent dopants; iridium(III)bis [(4,6-difluorophenyl . The WOLEDs showed color temperature about 4300 K and CRI over 87, which are appropriate to indoor lighting application. We interpret the high color stability of the WOLEDs based on the HOMO and LUMO level alignment of the middle green dopant against the host, which controls the recombination current in each layer to be maintained in the same proportion. Figure 1 shows the structures of the devices, and the energy levels of the materials. The molecular structures of the dopants used in this study are shown in Figure 1b. Three kind of devices were fabricated with different green dopants; Ir(chpy) 3 for device 1, Ir(mchpy) 3 for device 2 and fac tris(2-phenylpyridine) iridium (Ir(ppy) 3 ) for device 3, respectively. The device 3 was fabricated for comparison purposes. Figure 2 displays the electroluminescent (EL) spectra of the WOLEDs at several different luminances of 10, 100, 1000, and 5000 cd m À2 . The EL spectra exhibited the peak wavelengths at 472, 536, and 620 nm in the device 1 and 2. They correspond to the peak wavelengths of the EL spectra of the single color OLEDs using FIrpic, Ir(chpy) 3 (or Ir(mchpy) 3 ) and Ir(piq) 3 dopants, respectively. The EL spectra of these WOLEDs covered all wavelengths from 450 nm to 750 nm and were stable as the luminance varied. At 1000 cd m À2 , CRI and color temperature were calculated to reach...

Ahnak protein plays an important scaffolding function connecting Erk and Rac activation in PDGF-dependent migration of ASMC.

Iridium complexes containing cyclometalated 2‐cyclohexenylpyridine derivatives with rigid and bulky cyclohexene units are synthesized, and found to be highly efficient materials in EL devices (see figure). Devices based on these iridium complexes emit yellowish‐ green light with the very high external quantum efficiency of 18.7%.

No abstract

No abstract