Hole-transport properties

Further developments in this area have included the neparation of several additional N,N -diaryl indolo[3,2-h]carbazoles with substituents such as m-tolyl, ffi-anisoyl, or triarylamine-containing species. Like 221, these compounds, possessing excellent hole-transport properties, also occurred in stable amorphous states and displayed high glass-transition temperatures. LED devices involving these systems were also constructed and showed promising characteristics [OOSMO11-112)42]]. 

However, a phase separation problem in the above two-component system may affect the device stability. To overcome the problem, Mullen and coworkers [322] introduced triphenylamine groups as side chains at the PF backbone. The triphenylamine substituents simultaneously improve the hole-transport properties of PF (facilitating the injection of holes),... 

Copolymerization of fluorene with triarylamine compounds was shown to increase the hole-transport properties of the polymers. Several copolymers of triarylamine and fluorene (246-250) synthesized by Suzuki coupling were reported by Bradley et al. [347,348], The hole s... 

While retaining much of the substituted PT character (e.g., good hole-transport properties and stability), these materials exhibit significantly improved fluorescence efficiency in the solid state (cl>Pi up to 29%) that leads to (hllof UP to 0.1% for ITO/453/Ca PLED (Table 2.6). Other widely studied thiophene copolymers with aromatic 9,9-disubstituted fluorene units were already described above in Section 2.3. 

An interesting series of high Ts HTMs based on novel indolo[3,2-b]carbazoles has been discovered by the Xerox group [85]. These compounds not only showed the desired hole transport properties and high Ts of 164°C but also display an unusual atropisomerism with two discrete trans- and U.v-rotational isomers (Scheme 3.14), which greatly improves their tendency to form stable amorphous glasses. 

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]. 

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). 

Due to its direct relation to Section VI, we begin with a cell in which mainly the hole transporting properties of molecular glasses are used the solid-state dye sensitized cell. 

Mizusaki et al. (5) prepared triphenylamine copolymers, (VI), which had hole transport properties that were used in organic electroluminescent devices. 

Although effective as electroluminescent polymers, fluorene-containing polymers have limited hole transporting properties because of the tendency of fluor-ene units to aggregate. To impede aggregration Mckieman et al. (3) prepared diindenofluorene monomers, ( ), which were readily processable and had excellent electrical and optical properties. 

Bach et al. have successfully introduced the concept of a solid p-type semiconductor (heterojunction), with the amorphous organic hole-transport material 2,2, 7,7 -tetrakis (, V, V-di-/ -methoxyphcnyl-aminc)9,9 -spirobifluorenc [96]. This hole-conducting material allows the regeneration of the sensitizers after electron injection due to its hole-transport properties. Nevertheless, the incident photon-to-current conversion efficiencies using complex 22 as a charge-transfer sensitizer... 

Arylamines display electronic properties that are favorable for materials science. In particular, arylamines are readily oxidized to the aminium form, and this leads to conductivity in polyanilines, hole-transport properties in triarylamines, stable polyradicals with low energy or ground-state, high-spin structures, and the potential to conduct electrochemical sensing. The high yields of the palladium-catalyzed formation of di- and triarylamines has allowed for ready access to these materials as both small molecules and discrete oligomeric or polymeric macromolecules. 

Table 8 Hole Transport Properties of N,N -bis(2,2-diphenylvinyl)-N,N/-diphenylbenzidine (ENA-B) Doped into a Series of Polymers. The ENA-B Concentration was 45% (After Siniaropi et al. 1996a)...

Table 10 Hole Transport Properties of a Series of Vapor-Deposited Enamine Glasses and Enamine Doped Polymers. For the Doped Polymers, the Polymer was PS and the Enamine Concentration was 45% (After Sinicropi et al. 1997)...

Table 11 Hole Transport Properties of DAS Doped PS (After Borsenberger etal. (1997)...

After a systematic study of the terbium pyrazolonate complexes, we demonstrated that Tb(ip-PMP)3(TPPO)2 (compound 36) had very good electron transport properties and the recombination zone was confined mainly to the TPD layer, while Tb(ip-PMP)3(H20)(Et0H) (compound 37) showed only a hole-transport property and the recombination zone was confined mainly to the AIQ layer (Eigure 11.24). To adjust the carrier-transport properties and make the recombination zone easily confined to the terbium complex layer, we synthesized... 

A maximum luminance value of about 90000 cd/m at 1300 mA/cm was reached for 13 and of about 90000 cd/m at a similar current density for 2. The good performance of the devices was likely to be related to a much better balance of electron- and hole-transport properties than that achieved with linear or V-shaped oligothiophene-S,S-dioxides. 

Tested molecules having hole-transporting properties are shown in Fig. 2. 

Bathocuproine (BCP) has been used as a hole-blocking material. However, it tends to form exciplexes with a number of emitting materials with hole-transporting properties, e. g., TPD and m-MTDATA, and emits exciplex emission in the longer wavelength region [51]. Its morphological and thermal stability have not been made clear. 

A further interest in dendrimers for EL applications is the possibility of assigning different functional properties to the individual components of the macromo-lecular structure. For instance, it is conceivable to attach moieties with hole-transporting properties to the core region and electron-transporting properties to the dendrons, or even to integrate bipolar transport functionality in one molecule by virtue of asymmetric dendronization [41, 70]. Many of the fundamental aspects relating to the physical structure and architecture of dendrimers can be mapped... 

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