Oxadiazoles transport emitters

As compared with hole-transporting materials, fewer electron-transporting materials have been reported. A well-known green emitter, Alq3, has been used as a good electron transporter. Other reported electron-transporting materials include oxadiazole derivatives, a dendrimer-type oxadiazole, a triazole derivative, tris(phe-nylquinoxaline), silole derivatives, benzimidazole derivative, and boron-containing compounds (Table 7.3). 

Figure 3. Structural formula of a copolymer containing pendant oxadiazole segments for charge-transport and distyrylbenzene (PPV oligomers) as blue emitters. Adapted from Li et al.

Figure 7.4 shows the structures of typical electron transport organic semiconductors. Alq3, which can act both as an electron conductor and green emitter, was used by Tang in his first OLEDs and is still in use today. Besides metal complexes electron deficient heterocycles are frequently used as electron transport materials. The triazine derivative TPT [14, 15], the oxadiazole PBD and its spiro analogue spiro-PBD [16, 17] and the benzimidazole TPBI serve as typical examples. 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) has a very low HOMO level and is often used as a hole blocking layer. 

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