4,4 ,4"—tris , hole transport

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]. 

Monomers, (III) and (IV), having pendant tri ary I methane groups were prepared by Herron et al. (3) and then free radically polymerized and used as hole transport polymers. 

Table 3 [45] compares EL characteristics of the devices fabricated using P-6. As alluded above, existence of a hole-transporting layer (copper phtahlocyanin CuPc) [46,47] and an electron-transporting layer (tris(8-hydroxyquino-lino)aluminum jAlqj) [48,49] in the device and utilization of the low work function Li (Al Li alloy) [50] cathode can indeed improve the LED efficiency. The Alq3 layer, however, tends to increase the threshold electric field. 

Figure 6.7 Plot of the relative intensity (A.U.) of polarised electroluminescence against wavelength (nm) of a bilayer OLED consisting of an electron-transport and emission layer represented by a nematic network formed by polymerising compound (86) with isotropic UV light and a combined hole-transport and a coumarin non-contact alignment layer doped with 4.4,4-tris(naphthylyl)-N-(phenylamino) triphenylamine.

Magin et al. (1995) and Gruenbaum et al. (1996) measured hole and electron mobilities of binary and ternary mixtures containing 4H-l,l-dioxo-4-dicyanomethylidene-2-/ -tolyl-6-phenylthiopyran (PTS), tri-p-tolylamine (TTA), and a polyester (PE). PTS is an acceptor while TTA is a donor. Electron transport in PTS polymers and hole transport in TTA doped polymers have been described previously. For the ternary mixtures, the photocurrent transients were nondispersive over a wide range of fields, temperatures, and concentrations. Values of W were between 0.45 and 0.50 for both electron and hole transients. The presence of the TTA donor did not affect W for electron transients, nor did the presence of the PTS acceptor influence W for hole transients. Figure 40 shows the field dependencies of electron and hole mobilities for a ternary mixture of PTS, TTA, and PE containing 40% PTS and TTA. The temperature was 296 K. The results show that electron and hole mobilities are comparable and show similar field dependencies. The presence of the TTA donor has no significant affect on the electron mobilities, nor does the PTS acceptor have any affect on the hole mobilities. 

Figure 4.46. Molecular structures of commonly used OLED/PLED materials. Shown are (a) Alq3 (tris(quinoxalinato)Al (III)) used as an electron-transport material (b) DIQA (diisoamylquinacridone) used as an emissive dopant (c) BCP (2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline) used as an exciton/ hole blocking agent (d) NPB (l,4-bis(l-napthylphenyl amino)biphenyl) (e) PFO (9,9-dioctylfluorene) used as an emissive polymer in PLEDs (f) PEDOT-PSS (poly-3,4-ethylenedioxythiophene-polystyrene sulfonate) used as a hole transport material in PLEDs.

Figure 29. Representative time-of-flight signals for hole transport, a) tri-p-tolylamine (TTA) (30 wt.%) in polystyrene. (Reprinted with permission from Ref. [60b].) Two operational definitions of the transit time are indicated by t, and t /2. b) p-Diethylamino-benzaldehyde diphenylhydrazone (DEH) (30 wt. /o) in bisphenol-A polycarbonate. (Reprinted with permission from Ref. [60i].) c) A polysiloxane with A-alkylcarbazole pendant groups. (Reprinted with permission from Ref. [72g].)...

Figure 32. Field-dependence parameter 5 vs. temperature T), plotted linearly in T for hole transport in tri-p-tolylamine (TTA) (40 wt.%)-doped bisphenol-A polycarbonate. (Adapted from Ref. [73r].)...

It is easy to tell when transport is essentially nondispersive—the current transients approach the ideal, rectangular shape. It is more difficult to tell whether apparently dispersive transport is an intrinsic property of a material. For example, an early study of triphenylamine (TPA)-doped polycarbonate found that the transport was highly dispersive [73q], but later studies of tri-/ -tolylamine (TTA)-doped polymers revealed relatively nondispersive transport [57b, 73r]. Commercial triphenylamine is typically contaminated with A,A,A, A -tetraphenylbenzidine, which contributes very deep sites for hole transport (hole traps) [73s], whereas as-received TTA is apparently free of such impurities. This is probably the reason why the two, very similar materials appear to have very different transport properties. In fact, when TPA is purified adequately, the transport properties approximate those of TTA [73tj. 

Another approach to molecular assembly involves siloxane chemistry [61]. In dais method, die electrically or optically active oligomers are terminated with tri-chlorosilane. Layers are built up by successive cycles of dip, rinse, and cure to form hole transport, emissive, and electron transport layers of the desired thicknesses. Similar methods have also been used to deposit just a molecular mono-layer on the electrode surface, in order to modify its injection properties. 

The phosphorescent organic light emitting diodes (PHOLEDs) based on Ir(dmp>py)3 complexes were fabricated by the vacuum deposition technique with the following configuration ITO/copper phthalocyanine (CuPc, 10 nm) as hole injection layer/4,4 -bis[(l-naphthyl)(phenyl)-amino]-l,l -biphenyl (NPD, 40 nm) as hole transport layer/CBP Ir(dmppy)3 (8%) (20 nm) as emissive layer/2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline (BCP, 10 nm) as a hole blocking layer/ tris-(8-hydroxyquinoline)aluminum (Alqs, 40 nm) as an electron transport layer/LiF (1 nm) as electron injection layer/ A1 (100... 

To evaluate hole-transporting ability of newly synthesized phenylnaphthyldiamine derivative HIM 2, we fabricated the hole-dominant device using HTM 2 as a hole-transporting material with structures as follows 1TO/2-TNATA/HTM 2/EML/Al (device 11). On TTO substrate, 4, 4"-tris(N-(naphth-2-yl)-N-phenyl-amino)tri- phenylamine (2-TNATA) was previously deposited as a hole-injecting material. IDE 215 doped with 3 % of IDE 118 (host and dopant materials by Idemitsu Co., LTD) was used as blue emitting layer. 

O. 32 wt%), a green emitting material of tris(2-phenylpyridinato) iridium (Ir(ppy)3 0.06 wt%), and a hole-transporting host material of PVK (0.34 wt%) into mixed solvents of 1,2-dichloroethane and chloroform (mixing weight ratio 3 1), which have different volatilities. 

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