Hole-transporting polyimide for organic electroluminescent display

The thermal stability of newly synthesized hole-transporting polyimide, pol y[N,N'-diphenyl-N,N'-bis(4-aminobiphenyl)-(1,1'-biphenyl)-4,4'-diamine pyro mellitimide] (PMDA-DBABBD PI), via vapor deposition polymerization was inve stigated with the aid of the capacitance-temperature (C-T) measurement tech nique. Prior to the examination of the complete organic electroluminescent device (OELD), the single layer devices with the individual materials inclu ding tris(8-hydroxyquinolinato) aluminum (Alq(3)), N,N'-diphenyl-N,N'-bis(3 -methylphenyl)-( 1,1'-biphenyl)-4,4'-diamine (TPD), N,N'-diphenyl N,N'-bis( 1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB), copper phthalocyanine (CuPc ), and PMDA-DBABBD PI were subjected to the C-T measurement. The relaxation temperatures of the single laver devices with Alq(3), TPD, NPB, CuPc, and PMDA-DBABBD PI were 180, 76, 110, 125, and more than 200 degrees C, respect ively. The OELD with PMDA-DBABBD PI and Alq(3) as a hole-transporting layer and emissive layer was not relaxed up to 150 degrees C, while those contai ning CuPc/TPD and NPB thin films were catastrophically damaged at ca. 76 an d 110 degrees C, respectively. The OELD with the small organic hole-transpo rting molecule has almost the same relaxation temperature as the single lay er device with the respective molecules. The rectifying and electroluminesc ent characteristics disappeared for the annealed OELD with the small organi c hole-transporting molecules, whereas the OELD with the hole-transporting polyimide did still show the rectifying behavior with the green light emiss ion even though the current density and the light intensity became largely reduced. (C) 2000 Elsevier Science S.A. All rights reserved.