Doped Organic Light-Emitting Diodes Based on Random Copolymers Containing Both Hole and Electron Transport Groups

Abstract

ABSTRACTDoped organic light-emitting diodes (OLEDs), such as PVK containing the electron transport molecule PBD and molecular dyes, have demonstrated three-color capabilities and efficiencies over 1%, but there are concerns about reliability due to recrystallization of the small molecule PBD. In this work we describe the incorporation of both hole and electron transport groups into a single copolymer to avoid recrystallization and phase segregation (which could occur if two separate polymers were used) as well as the application of such copolymers in dye-doped OLEDs. The polymers were synthesized through free radical copolymerization of the electron- donating monomer N-vinylcarbazole (NVK) with the electron-withdrawing monomer 2-(4-tert-butylphenyl)-5-{4-[(4'-vinyl)phenylmethoxy] phenyl}-1,3,4- oxadiazole (BVO). The radical reactivity ratios of the two monomers were estimated to be rNVK=0.052 and rBvo=12. The copolymers are transparent in the visible region, homogeneous as characterized by both GPC and DSC, and have good thermal stability. External quantum efficiencies of 0.07%, 0.3% and 0.4% were achieved in LEDs with device structures of ITO/COP:C47/Mg:Ag, ITO/COP:C6/Mg:Ag and ITO/COP:NR/Mg:Ag, respectively, where COP stands for copolymer, C47 for Coumarin 47, C6 for Coumarin 6 and NR for Nile Red. The introduction of the oxadiazole group balances the injection of holes and electrons by decreasing the hole injection and transport ability and enhancing the electron injection and transport ability of the copolymers relative to PVK.