White OLEDs

Methima ole. This compound is a white to pale buff crystalline powder with a faint characteristic odor. It is soluble in water, ethanol, and chloroform (1 g/5 mL) and only slightly soluble in other organic solvents. A detailed chemical, analytical, spectral, and chromatographic description is available (44). It is assayed titrimetrically with NaOH (54). 

The fluorescence color converter technique [32] can, in principle, overcome much of this power loss by replacing the white light emitter with a blue-emitting organic stack, and the absorbing filters with green and red fluorescent dyes. Thus when a green pixel is desired, the OLED underneath is turned on and the blui... 

The performance of AMOLEDs is improved drastically in the past years. In contrast to the data shown in Table 1.2 (which representing development stage in 2002), a set of recent data of a 14.1" WXGA AMLCD made with solution-processed OLED emitters is shown in Table 1.3 [163,175,176], The color gamut is improved to over 60% with respect to NTSC. The luminous and power efficiencies at white point (x 0.28, y 0.31) are >8 cd/A and >5 lm/W. The power efficiency surpasses the performance of AMLCDs, plasma displays, and all other known flat-panel displays in commercial market or under development. A photo of the 14.1" AMOLED display is shown in Figure 1.25b. 

The simulation discussed above was based on full color produced by individual red, green, and blue emitters. Other full-color reproduction approaches have been proposed for OLED displays including color from blue emitter by means of energy down conversion fluorescent filter [177], and color from white emitters by means of transmission color filter sets similar to that used in LCD industry [178,179]. Table 1.5 compares the EL efficiency of equivalent white... 

M Kashiwabara, K Hanawa, R Asaki, I Kobori, R Matsuura, H Yamada, T Yamamoto, A Ozawa, Y Sato, S Terada, J Yamada, T Sasaoka, S Tamura, and T Urabe, Advanced AM-OLED Display Based on White Emitter with Microcavity Structure. SID 04, Digest, 1017-1019, 2004. 

For recent review on white-emitting OLEDs see B.W. D Andrade and S.R. Forrest, White organic light-emitting devices for solid-state lighting, Adv. Mater., 16 1585-1595, 2004. 

The simplest method yet most complex structure for white OLEDs consists of three primary emission colors blue, green, and red. Kido et al. reported using three emitter layers with different carrier transport properties to produce a white emission [273], The multilayer structure of such an OLED is ITO/TPD/p-EtTAZ/Alq3/Alq3 Nile Red/Alq3/Mg Ag, in which a blue emission from the TPD layer, a green emission from the Alq3 layer, and a red... 

Cheon and Shinar demonstrated that by deposition of a thin layer of the blue emitter DPVBI on the DCM-2-doped NPD device (Figure 3.12), an efficient white OLED with a brightness of over 50,000 cd/m2 and a power efficiency of 4.1 lm/W (external efficiency of 3.0%) could be achieved [274]. 

A further strategy to achieve white emission uses rare-earth complexes. For example, a dysprosium complex (245) emits two band emissions a yellow band (580 nm) corresponding to the 4F9/2 —> 6Hi3/2 transition and a blue band (480 nm) corresponding to 4F9/2 — 6H15/2 transition of Dy3+ ion in the complex. Li et al. reported Dy-complex white emission OLEDs of a structure of ITO/PVK Dy complex/Mg Ag device [276], Figure 3.13 shows the PL and EL emission spectra of such a complex and its device, respectively. 

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. 

The broad PL emission spectra of some metal chelates match the requirements for white emission. Hamada et al. investigated a series of Zn complexes and found bis(2-(2-hydroxy-phenyl)benzothiazolate)zinc (Zb(BTZ)2, 246) is the best white emission candidate. An OLED with a structure of ITO/TPD/Zn(BTZ)2/OXD-7/Mg In showed greenish-white emission with CIE (0.246, 0.363) with a broad emission spectrum (FWHM 157 nm) consisting of two emission peaks centered at 486 and 524 nm (Figure 3.14) [277], A maximum luminance of 10,190 cd/m2 at 8 V was achieved. The electronic and molecular structure of Zn(BTZ)2 have been elucidated by Liu et al. [278]. There is evidence that the dimeric structure [Zn(BTZ)2]2 in the solid state is more stable than its monomer Zn(BTZ)2. They also found that the electron transport property of Zn(BTZ)2 is better than that of Alq3. 

T.K. Hatwar, J.R. Vargas, and V.V. Jarikov, Stabilized white-light-emitting OLED devices employing a stabilizing substituted perylene material, U.S. Patent 2,005,089,714, pp. 21 (2005). 

Each of the techniques described above has unique strengths and weaknesses, and the optimum device structure for commercial full-color displays will also be heavily influenced by the ease with which it can be mass-produced. Currently full-color OLED displays have been manufactured commercially by using two of the above described techniques only, i.e., (a) side-by-side pixels deposited by high-precision shadow masking and (b) using white OLEDs and color absorption filters. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

Y Tung, T Ngo, M Hack, J Brown, N Koide, Y Nagara, Y Kato, and H Ito, High Efficiency Phosphorescent White OLED for LCD Backlight and Display Applications, Proceedings of the Society for Information Display, Digest of Technical Papers, Vol. 35, Seattle, 2004, pp. 48-51. 

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