Crosslinking OLEDs

Crosslinkable Organic Semiconductors for Use in Organic Light-Emitting Diodes (OLEDs)... 

In the following sections, we give an overview of the various attempts to fabricate crosslinked layers for use in multilayer OLEDs categorized by the reactive group used in the precursor materials. We start with the [2+2] cycloaddition of cinna-mates and the radical polymerization of acrylates and styrene derivatives. The emphasis of the chapter is on our own work, which is focused on the cationic ringopening polymerization (CROP) of oxetane-functionalized materials. Finally, we summarize the less-frequently employed synthetic routes. 

The first example of using the [2+2] cycloaddition for crosslinking of organic semiconductor layers was reported in 1997 by Remmers et al. [10]. It was a derivative of poly-p-phenylene (PP) deposited by the Langmiur-Blodgett (LB) technique (Fig. 9.5(a)). Polarized absorption and fluorescence of the films was reported, but no OLED devices were fabricated. 

Bacher et al. [15] reported on a series of triphenylene derivatives with one, two or three reactive acrylate units for crosslinking (Fig. 9.8(a)). No influence of the number of acrylate units on the degree of crosslinking was reported. Bi-layer OLEDs with the triphenylene as hole-transport agent and vapor-deposited A1Q3... 

Chen et al. [21] and Klarner et al. [22] reported on a class of triphenylamines and polyfluorenes (Fig. 9.8(d)) with reactive styrene groups for crosslinking. Curing was performed thermally at 175-200 °C for 10-60 min. Network formation within the emitter was found to inhibit subsequent excimer formation. Triplelayer OLEDs, comprising a crosslinked hole-transporter, crosslinked emitter,... 

Fig. 9.9 Performance of an OLED based on a material class shown in Fig. 9.8(c) current-voltage characteristics (left) and light output (right) for a crosslinked (solid symbols) and noncrosslinked device (open symbols adopted from Contoret et al. [18]).

Fig. 9.10 Study of UV-curable polynorbonenes (a) Chemical structures of the precursor polymers (b) efFect of UV-crosslinking of the polynorbonene-based HTL on the external quantum efficiency of OLEDs of the structure ITO/HTL/AlQ3/Mg.

Fig. 9.16 Performance of blue-emitting OLEDs based on crosslinked hole-transport layers (X-HTL) and a spirobifluorene-cofluorene derivative (PI [41]). The devices had the general structure ITO/X-HTL (x nm)/P1 (80-x nm)/Ca (100 nm), the reference device ITO/PEDOT (30 nm)/Pl (80 nm)/Ca (100 nm). Left voltage dependency of the current and the efficiency. Right Dependence of the maximum efficiency on the thickness of the X-HTL. The numbers indicate the voltage at which the maximum occurs. The chemical structure of the crosslinkable hole conductor used in this study is shown as an inset.

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]). 

The number of crosslinkable electron conductors is relatively small compared to the number of hole-conducting materials. A series of oxetane-functionalized oxa-diazoles was synthesized [51]. Unfortunately, due to the relatively strong basicity of the two nitrogen atoms in the oxadiazole ring, the reaction was strongly inhibited. An excess of more than 50wt% of the initiator was necessary to start polymerization in solution. This is of course unacceptable for application in OLEDs. 

We have considered so far only copolymerization of bifimctional vinyl monomers. However, if any of the monomers in the copolymerization is a divinyl compound or any other ole rue monomer with functionabty greater than 2, a branched polymer can result and, furthermore, the growing branches can intercormect to form an in nite crosslinked network known as gel . It is useful to be able to predict the conditions imder which such gel formation will occur. 

GRJ 11] Grigalevicius S., Zhang B., Xie Z. et al., Polycaibazole-based networks made by photo-crosslinking for hole transporting layers of OLED devices . Organic Electronics, vol. 12, no. 12, pp. 2253-2257, 2011. 

NUY 02] Nuyken O., Bacher E., Braig T. et al., Crosslinkable hole- and electron-transport materials for application in organic light emitting deviees (OLEDs) , Designed Monomers and Polymers, vol. 5, nos. 2-3, pp. 195-210, 2002. 

VOL 12] Volz D., Baumann T., Fluegge H. et al, Auto-catalysed crosslinking for next-generation OLED-design , Journal of Materials Chemistry, vol. 22, no. 38, pp. 20786-20790, 2012. 

These materials were developed in the first place for an easier OLED processing due to the multilayer capability of the crosslinked polymers. Layer after layer can... 

One of the first reports on the orientation and subsequent crosslinking of a reactive mesogen for OLED applications with polarised emission has been published by Bacher et al. [63], who synthesized the conjugated bis-stilbene with two polymerizable acrylate groups shown in Fig. 7.17. The reactive mesogen was oriented by heating into the liquid crystalline phase and subsequently thermally crosslinked at 175°C. 

The three different reactive mesogens shown in Fig. 7.20 were sequentially deposited and photochemically crosslinked using a photomask to create the red, green and blue areas of the OLED shown in Fig. 7.21. The non-crosslinked parts were removed by washing with chloroform. The green reactive mesogen has... 

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