Maximum luminance

In push pull polymer 77, both the absorption and emission maxima are red-shifted relative to 1. The LED performance of these materials appeared to be rather low (the EL efficiency of 0.002cd/A and the maximum luminance of 100cd/m2 was achieved at 30 V), and the turn-on voltage for the push-pull polymer 77 (4 V) was lower than that in more electron-deficient polymers 75 and 76. 

Yellow to orange emission was observed in another series of fluorene-phenylene copolymers with CN groups in the vinylene fragment 324-326 (Scheme 2.48) [409]. The PLQY of the copolymers was relatively low (from 3.5% for 326 to 14.7% for 325) and the best results in PLED testing were achieved for copolymer 325. The device ITO/PEDOT/325/A1 showed a turn-on voltage of 5.0 V and a maximum brightness of 7500 cd/m2 at 20 V, with a maximum luminance efficiency of 0.21 lm/W at 6.7 V. 

The absorption and PL spectra of polymers 584a,b are red-shifted, compared to the analog with p-phenylene fragment 583. The effect is much more pronounced in films than in solution (films APL 495 nm for 583 and a broad structureless band at 520-650 nm for 584a,b). Single-layer (ITO/PEDOT/polymer/Ca/Ag) devices fabricated with these polymers showed moderate performance with a maximum luminance and a maximum brightness of 0.17— 0.58 cd/A and 90-150 cd/m2, respectively (Chart 2.137). 

Excellent electron-transporting properties of quinoxaline (also demonstrated for noncon-jugated quinoxaline-containing polymer 588 [684] and quinoxaline-based polyether 589 [685]) resulted in a substantially decreased turn-on voltage of PPV/590 PLED (3.6 V), which is much lower than that of pure PPV in the same conditions (7 V). These diodes showed a maximum luminance of 710 cd/m2 (ca. 40 times brighter than the PPV diode at the same current density and voltage) [686]. 

A new branched carbazole derivative with phenyl ethylene moieties attached, l,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB, 41) (Scheme 3.15), was prepared as a HTM for OLEDs [86], TECEB has a HOMO energy level of —5.2 eV and hole-drift mobility of 1(T 4 cm2/(V s), comparable to NPD. The device performance (maximum luminance of about 10,000 cd/m2 and current efficiency of 3.27 cd/A) in a standard HTL/tris-(8-hydroxyquino-line) aluminum double-layer device is also comparable to NPD, but TECEB has a higher Tg (130°C) and its ease of synthesis is superior to NPD. Distyryl units linked to a TPD derivative, A, A"-bis(4-(2,2-diphenylethenyl)-phenyl)-jY,jV -di(p-tolyl)-bendidine (DPS, 42) (Scheme 3.15), reported by Yamashita and coworkers, showed good hole transport properties and improved thermal stability compared with the parent TPD [87]. 

Jiang et al. were the first to report a relatively stable blue OLED based on anthracene derivative JBEM (120) [240]. With the similar OLED structure as that used above by Kodak of ITO/CuPc/NPD/JBEM perylene/Alq/Mg Ag and using JBEM as a blue host material, the device shows a maximum luminance of 7526 cd/m2 and a luminance of 408 cd/m2 at a current density of 20mA/cm2. The maximum efficiency is 1.45 lm/W with CIE (0.14,0.21). A half-life of over 1000 h at initial luminance of 100 cd/m2 has been achieved. The authors also compared the device performance using DPVBI as a host, which gave them a less stable device. 

By introducing the hole transport arylamine as an end cap for an anthracene backbone, Lin et al. designed a series of novel materials (207-212) (Scheme 3.65) [247]. The aim of these dual function materials is to combine the emitting property of the blue anthracene lumino-phore with the hole transport property of the triarylamine to simplify the device fabrication steps. Though the introduction of the arylamino moieties produces moderate QE (f 20%) for these materials, the OLEDs using them as emitters as well as HTMs demonstrate only moderate EL performance with a maximum luminance of 12,922 cd/m2 and 1.8 lm/W with CIE (0.15, 0.15). 

OLED ITO/NPD/Ph3Si(PhTPAOXD)/Alq3/Mg Ag emits pure blue light with an EL emission band centered at 460 nm (FWHM 75 nm) and a CIE (0.17, 0.17). The maximum luminance exceeds 20,000 cd/m2 at 15 V, with an EQE of 1.7% and a power efficiency of 0.9 lm/W. Later, the same group optimized the device structures by introducing a HTL of an organosilicon compound and achieved a much higher performance [261], An optimized OLED... 

Yu et al. synthesized two methyl-substituted Alq3, named tris(2,3-dimethyl-8-hydroxy-quinoline) aluminum complex (Alm23q3, 237) (Scheme 3.72) [264]. This compound emits blue color with an emission peak centered at 470 nm and FWHM of 90 nm. OLEDs with a structure of ITO/TPD/Alm23q3/Mg Ag emit blue light and the luminous efficiency is 0.62 lm/W with a maximum luminance of 5400 cd/m2 at 19 V. 

Beryllium chelates such as bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Beq2, 86) (Scheme 3.74) emit pure blue light with an emission peak centered at 465 nm [269]. ITO/NPD/Bepp2/LiF/Al exhibited a maximum luminance of 15,000 cd/m2 and amaximum luminescent efficiency of 3.43 lm/W (3.8 cd/A). The emission color may have contributions from both NPD and Bepp2 as stated by the authors. 

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. 

The estimated luminance values (L(iisp ay and L,.mi ior) at maximum luminous flux are about 50 and 120 cd/m2, respectively. These values can be increased similarly through optimization of a-Si H TFTs AM-PLED design and fabrication methods. 

As the dependence of the property on components is described adequately by the second-order regression equation, the possibility presented itself to find optimal conditions through the use of nonlinear programming. Subject to the restrictions of Eq. (3.94), the conditions providing the maximum luminance are found to be ... 

Table 3 shows that the ITO/CuPc/P-6/Alq3/Al Li device exhibits an external quantum efficiency of 2000 times the ITO/PPV/Al device and 40 times the ITO/P-6/A1 device. The maximum emitted light intensity presented as maximum luminance in the table is 1600 cd/m2 for the best device. Because CuPc absorbs over 500-800 nm, a part of emitted light from the emitting layers is reabsorbed by the CuPc. This is the reason why the external quantum efficiency is not improved even when the CuPc layer was coated onto the ITO electrode... 

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