DPVBI

Another important class of electron-transporting emitter is the distyryl-arylenes. These have been explored most extensively by workers at Idemitsu Kosan [31, 32], with the bulk of the published data focusing on a compound designated as DPVBi (see Table 13-2). This class of materials may be considered as small molecule analogs of the PPV polymers, with distyrylbenzene and its derivatives ]33] as prototypical examples. Because of the short conjugation length they tend to be blue emitters. 

SCHEME 3.56 Chemical structures of DSA-host DPVBI and dopants BCzVB and BCzVBI. 

It can be seen from Figure 3.10a and d that the emission spectra of the neat BCzVB and DPVBi doped with BCzVB are essentially the same and can be attributed to an energy transfer process. The emission spectrum of CBP doped with BCzVB is quite different it comes from emission contributed from both CBP and BCzVB molecules owing to both charge trapping as well as a partial energy transfer process. 

Ueno studied a series of different triaryl- or tetraaryl-benzene host materials and found that using triarylbenzene (TPB3, 190) (Scheme 3.58) as a host material with a DSA-amine (Ide 102, 191) doped OLED showed a peak luminance of 142,000 cd/m2 at 12 V. This device also showed a luminescent efficiency of 6.0 lm/W at 5 V and 820 cd/m2 and an external efficiency of 2.4%. The lifetime of the device is also better than that of a device with DPVBI as the host. 

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. 

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]. 

Without using Alq3 as the ETM, BCP has been used as an ETL and was demonstrated in DPVBI-based blue OLEDs [347], BCP has superior electron transport properties and its electron mobility is around 5.2 x 10-4 cm2/(V s) (5.5 x 105 V/cm) as measured by the TOF method [348], The concept of using BCP has been extended into doped OLEDs. 

In order to round-off our overview on the symmetric compounds of this class, we add the stilbene-like molecules TTPAE (20) (Tg = 111°C) [27], the bis-(styryl)anthracene (BSA, 21) [64], and DPVBi (22) [65] (Fig. 3.9). Compounds 20 and 21 are characterized by a rigid structure leading to a large electronic delocalization through the center of the molecules. Compound 22 is more flexible due to the biphenyl bond. The outer phenyl rings are twisted by steric repulsion, leading to a nonplanar structure of the stilbene units. 

Bazan and co-workers [93] coupled four branches of stilbene chromophores to a central C atom, leading to tetrastilbenylmethane [C(STB)4, 46a] tetrakis (4-ferf-butylstyryIstiIbenyljmethane [C(f-BuSSB)4, 46b], and the higher homolo-gue tetrakis 4-[4 -(4"-terf-butyl-styryl)styryl]stilbenyl methane [C(4R—f-Bu)4, 46c]. For 46b, Tg is 190°C, and for 46c it is as high as 230°C. Tetrakis(4,4 -2,2-diphenyl-vinyl)-l,l -biphenyl]methane [C(DPVBi)4, 47a] and the cyano derivative 47b exhibit the glass transition at 142 and 174°C, respectively. Similar compounds to 46a and 46b have also been synthesized with silicon and adaman-tane as the tetrahedral center. 

Enhancement in the performance of OLEDs can be achieved by balanced charge injection and charge transport. The charge transport is related to the drift mobility of charge carriers. Liu et al. [166] reported blue emission from OLED based on mixed host structure. A mixed host structure consists of two different hosts NPB and 9,10-bis(2 -naphthyl)anthracene (BNA) and one dopant 4,4 -bis(2,2-diphenylvinyl)-l,l -biphenyl (ethylhexyloxy)-l,4-phenylene vinylene (DPVBi) material. They reported significant improvement in device lifetime compared to single host OLEDs. The improvement in the lifetime was attributed to the elimination of heterojunction interface and prevention to formation of fluorescence quenchers. Luminance of 80,370 cd/m2 at 10 V and luminous efficiency of 1.8 cd/A were reported. 

FIGURE 1.3. The photoluminescence (PL) and electroluminescence (EL) spectra of some representative 7r-conjugated films and OLEDs, respectively (a) EL of blue aminooxadia-zole fluorene (AODF) and green Alq3 OLEDs,9 (b) PL and EL of PPV films and PLEDs, respectively,10 (c) PL of m-LPPP films, (d) EL of DPVBi (solid line) and DPVBi/Alq3 (dashed line) OLEDs,11 and (e) PL of CBP films and EL of CBP OLEDs.12... 

These generally blue-emitting materials were studied extensively by Hosokawa and coworkers.73 Among them, 4,4 -bis(2,2 -diphenylvinyl)-l,l -biphenyl (DPVBi) (see Figs. 1.1 and 1.3) has proven to be a particularly promising material for blue OLEDs. The degradation of OLEDs based on this material is apparently due to its crystallization, which results from its relatively low Tg 64° C. Indeed, the related spiro-DPVBi, with Tg 100° C, yields considerably more stable devices.70... 

The major efforts to increase Tg include two noteworthy innovative approaches (i) Synthesis of starburst molecules, suitable for HTLs, whose patently nonplanar structure inhibits recrystallization,71 and (ii) synthesis of novel molecules in which familiar luminescent molecules are synthesized around a spiro-bifluorene core.70 As shown by Spreitzer et al.,70 Tg of these spiro derivatives, such as spiro-DPVBi, is considerably higher than that of the parent molecules, yet their PL and EL spectra are essentially identical. As expected, the lifetime of the OLEDs fabricated from the spiro derivatives is considerably higher than that of the parent-compound-based devices, both at room and at elevated temperatures. 

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