Copolymer OLEDs

Figure 30 Block copolymer OLEDs are spun-coated on glass/ITO/PEDOT PSS and then capped with LIF/AI electrodes (a). The block copolymer demonstrates superior external quantum efficiency (b), brightness as a function of current density (c), and /-I/characteristics (d) than comparable pure homopolymer PPV and PPV/POX blends. Reprinted with permission from Tao, Y. Ma, B. Segalman, R. A. Macromolecules 200S, 41,7152-7159, Copyright 2008 American Chemical Society. ...

Dow used the Suzuki coupling process to synthesize a variety of fluorene-based homopolymers and copolymers that are able to emit the three primary LED colors red, green, and blue. Structural representations of these polymers are shown in Figure 11.20. The U.S. patents granted to Dow Chemical on OLED emitting materials are listed in Table 11.4. 

The photocopolymerization of the N- vin y lin d ole/FN system yielded an alternating copolymer with partial addition of FN at the electron-rich 3-position of the indole ring [27] ... 

Fig. 5 CIE coordinates of OLED devices fabricated from copolymers 31 [39]. Reproduced by permission of the Royal Society of Chemistry...

The first oxetane-functionalized emitter were pyrene derivatives (Fig. 9.19(b), [46]), however, with very limited performance in OLEDs. Later, a poly(pheny-lene-fluorene)-copolymer was reported, however, without any electroluminescence data (Fig. 9.19(c), [47]). The breakthrough came with a recent report on a crosslinkable class of state-of-the-art spirofluorenes (Fig. 9.19(a), [48, 49]). By incorporating a green- and red-emitting comonomer, it was possible to generate the three primary colors (RGB) necessary for color-display applications. More recent concepts use sterically hindered 9,9 -diarylfluorene blocks to link conjugated oligomers (Fig. 9.19(d), [50]). 

In blends of PVK with PBD and in random copolymers with carbaz-ole and oxadiazole groups attached as side chains, the active groups have different mobility, or are subjected to different topological constraints, respectively. In the blends, exciplexes emerge, and in the copolymers, electroplexes are effective. Both types of complexes shift the EL spectra to red in comparison to pure PVK homopolymer. The red-shift is significantly greater for the electroplex. 

Copolymers having the carbazole group and the oxadiazole group directly in the backbone, i.e., poly[3,6-A-Ethylcarbazole-l,3,4-oxadiaz-ole-2,5-diyl], can be prepared from A-ethylcarbazole-3,6-dicarboxylic acid and hydrazine hydrochloride.The reaction runs at 140°C with PPA. The synthesis is shown in Figure 10.4. 

H. C. Bach. Novel process for the preparation of fiber of arylene oxadiaz-ole/arylene V-alkylhydrazide copolymer. US Patent 4115 503, assigned to Monsanto Company (St. Louis, MO), September 19, 1978. 

Fluorene polymers and copolymers (either obtained by electrochemical or chemical processes ) are proven to be among the most promising materials in the field of organic light-emitting devices (OLEDs). 

The copolymers have been synthesized in a statistical copolymer approach (Scheme 5.11). The resulting copolymers show weak emission bands related to the phosphor in solid state PL measurements. However, the EL spectrum is dominated by the emission of the Pt-salen phosphors the emission of the PF chromophore is negligible. This may be due to an additional pinning of negative charges at the phosphorescent metal complexes. The pure copolymer showed the occurrence of aggregate-related EL bands. A dilution of the phosphors in a PF2/6 matrix resulted in a suppression of these aggregation-related bands and allowed for a distinct increase of the OLED device efficiency up to 6 Cd/A. 

Block copolymers containing terthiophene units alternating with polyesters 181 and 182, and polyamide 183 spacers have been proposed for OLEDs fabrication [310,311]. These materials associate good film-forming properties to pure color of the emitted light [312]. Miller and coworkers have shown that quaterthiophene-based polyester 184 could be used to form a LED [313] as well as a photodiode [312]. ... 

It has been shown that insertion of phenylene group in the PT backbone improves the PL efficiency of the polymer. A green OLED based on 191 gave an external EL quantum efficiency of 0.1% [336]. Thienylene-phenylene copolymers such as 192 and 193 with ethylene oxide side chains also exhibit electroluminescent properties [337,338]. In the case of the paracyclophane derivative 193, a color variable light-emitting device generating two independent colors was demonstrated red under forward bias and green under reverse bias [338]. 

Several groups have prepared block copolymer systems containing both p- and n-type functional blocks for OLEDs... 

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