Hole-limiting conductive vinyl copolymers for AlQ3-based OLED applications

A series of soluble conductive vinyl copolymers containing a hole-transporting N-(4-methoxyphenyl)-N-phenylnaphthalen-1-amine (MeONPA) moiety and an electron-transporting/hole-blocking 2,5-diphenyl-1,3,4-oxadiazole (OXA) moiety at different composition ratios were synthesized and characterized. The copolymers were applied as the hole-transporting layer (HTL) for a series of heterojunction Organic Light-emitting Diodes (OLEDs) employing the commonly used green emitter tris(8-hydroxyquinolinato)aluminum (AlQ₃) as the electron-transporting layer. AlQ₃ is known to have inferior electron mobility compared to most typical hole-transporting materials. As a result, oxidative degradation of the AlQ₃ emitters caused by the excessive holes accumulated at the interface led to deterioration of the device over time. From the measurement of hole current only devices using electron blocking gold as cathode (ITO/PEDOT:PSS/copolymer/Au), it was found that the hole current for the copolymers reduced as the OXA composition increased. Optimum performance for the AlQ₃-based OLED (ITO/PEDOT:PSS/copolymer/AlQ₃/Ca/Al) was achieved for a 82/18 (molar ratio) (MeONPA/OXA) copolymer. The maximum current efficiency and luminance were 4.2 cd/A and ca 24,000 cd/m² respectively for the charge-balanced copolymer compared to 3.5 cd/A and 6600 cd/m² for similar device employing a homopolymer P(MeONPA) as the HTL.