CIE coordinates

FIGURE 2.18 The efficiency-luminance plot for ITO/PEDOT/228/Ca device. The numbers along the curve are the CIE coordinates at the corresponding brightness levels. The insert shows the EL emission spectrum of a diode driven at 3.6 V. (From Miteva, T., Meisel, A., Knoll, W., Nothofer, H.G., Scherf, U., Muller, D.C., Meerholz, K., Yasuda, A., and Neher, D., Adv. Mater., 13, 565, 2001. With permission.)... 

Recently, Chen s group reported a deep blue OLED based on an asymmetric mono(styryl) amine derivative DB1 (192) as shown in Scheme 3.59. PL spectra of this deep blue dopant in toluene solution showed a peak emission of 438 nm, which is about 20 nm hypsochromic shift compared with DSA-amine symmetric dopant, due to the shorter chromophoric conjugated length of the mono(styryl) amine. OLED device based on this blue dopant achieved a very high efficiency of 5.4 cd/A, with CIE coordinates of (0.14, 0.13) [234]. 

Perylene (199) and its derivative (TBP, 200) have been widely used as blue dopant materials owing to their excellent color purity and stability. Efficient blue emitters with excellent CIE coordinates are found in biaryl compound 2,2 -bistriphenylenyl (BTP, 201) as shown in Scheme 3.62 [145]. A device of structure ITO/TPD/BTP/TPBI/Mg Ag emits bright blue emission with CIE (0.14, 0.11). A maximum brightness of 21,200 cd/m2 at 13.5 V with a maximum EQE of 4.2% (4.0 cd/A) and a power efficiency of 2.5 lm/W have been achieved. 

Spiro-FPAl/TPBI/Bphen Cs/Al. A very low operating voltage of 3.4 V at luminance of 1000 cd/m2 was obtained, which is the lowest value reported for either small-molecule or polymer blue electroluminescent devices. Pure blue color with CIE coordinates (0.14, 0.14) have been measured with very high current (4.5 cd/A) and quantum efficiencies (3.0% at 100 cd/m2 at 3.15 V) [245]. In another paper, Spiro-FPA2 (126) was used as a host material with an OLED device structure of ITO/CuPc/NPD/spiro-FPA2 l%TBP/Alq3/LiF that produces a high luminescent efficiency of 4.9 cd/A [246]. 

Stable white emission with CIE coordinates of (0.3519, 0.3785) was obtained in such a rare-earth-based OLED device. The authors mentioned that the QE of the device was not good, possibly due to the inefficient energy transfer process between the ligand and the rare-earth metal. A suitable choice of the ligand may improve this type of device performance. 

Another interesting white emitting diode based on a boron hydroxyphenylpyridine complex (247) was reported by Wang et al. [279]. The PL emission of such a material in fluid solution as well as in the solid state is blue (450 nm). However, the EL spectrum of ITO/NPD/(mdppy)BF/LiF/Al gives a broad emission band with a stable CIE coordinate... 

CIE coordinates of the TOLED (—0.06, —0.01) more than that of the bottom-emitting device (—0.03, +0.01) when one considers a viewing angle of 60°. However, this is of minor concern for green emission but care must be taken when applying this design to blue and red pixels. 

Figure 16.3 Structural formulae of the ligands L and emission colors of their L2Ir(acac) complexes in the Commission Internationale de l E clairage (CIE) coordinates. (See the color version of this figure in Color Plates section.)...

Due to the large band gap and high triplet energy level of the poly(3, 6-dibenzosilole) 5, the copolymer is an excellent host for the fabrication of blue polymer phosphorescent light-emitting diodes. A high external quantum efficiency (t/el) of 4.8% and a luminance efficiency of 7.2 cd/A at 644 cd/m2 have been achieved for blue phosphorescence devices (emission peak (AEL) at 462 nm, CIE coordinates x = 0.15,y = 0.26). The performances of the devices are much better than those reported for blue phosphorescent devices with poly(A--viny 1 cabarzo 1 e) (PVK) as the host.32... 

Copolymers 10 derived from 3,6-dibenzosilole and 2,7-fluorene are blue electroluminescent SCPs.27 When the copolymers are used as the emissive layer in EL devices, highly efficient pure blue emissions with CIE coordinates of (x = 0.16, y = 0.07), a 7EL of 3.34%, and a luminance efficiency of 2.02 cd/A at 326 cd/m2 are achieved from the copolymer with 90% fluorene content. The blue color matches the NTSC blue standard (x = 0.14, y = 0.08) quite well. The EL spectral stability of the devices is quite good, even under operation at elevated temperatures. Copolymer 9 derived from 3,6- and 2,7-dibenzosiloles also exhibits high performance with a jyEE of 1.95%, a luminous efficiency of 1.69 cd/A, and a maximal brightness of 6000 cd/m2, with the CIE coordinates of (x = 0.162, y = 0.084).26... 

Multilayer devices with lanthanide chelate complexes. In these complexes, efficient energy transfer from the singlet or triplet exciton on the ligand of the complex to the lanthanide atom at its center results in efficient, atomic-like line emission spectra from the latter. By adjusting the identity and concentration of the different lanthanide complex dopants, a line spectrum with white CIE coordinates was achieved.77... 

FIGURE 4.6. Electroluminescence spectrum of a two-mode LED which has CIE coordinates close to that of pure white. The CIE coordinates are (0.34, 0.386) and the Alq thickness is 30 mn. Also shown is the calculated spectrum using Eq. (4.4). 

The EL spectrum has multiple peaks when more than one mode of the cavity overlaps the free-space emission spectrum. It is possible to realize a white LED with a single electroluminescent material such as Alq by employing a two-mode microcavity device structure in which one of the modes is centered near 480 nm and the other near 650 nm. Such an electroluminescence spectrum, for which the CIE coordinates are (0.34, 0.386),13 is shown in Fig. 4.6. The approximate spectrum calculated with Eq. (4) is also shown in Fig. 4.6. With very minor changes in the device design, it is easy to achieve (0,33, 0.33). For comparison, the CIE coordinates of a noncavity Alq LED are (0.39, 0.56). 

FIGURE 8.17. (a) From left to right electroluminescence of the PHP organic LED photo luminescence of the green dye (—), and the red dye excited by the PHP EL emission the spectra are normalized for better visibility, (b) CIE graph containing the CIE coordinates of the produced RGB colors (squares) compared to RGB colors used in cathode-ray tubes (circles) (the dotted line marks all colors, which can be produced by the RGB colors). 

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