Charge Balance Conductive Vinyl Polymers for Homojuncton Organic Light Emitting Diodes (OLEDs)

Several series of bipolar small molecules have been synthesized and studied as charge balancing materials for homojunction OLED applications.(1-3) The bipolar molecules consisted of several components including a hole transporting arylamine moiety, an electron transporting oxadiazole moiety and an emitting polycyclic aromatic moiety. The achievement of charge balance was confirmed via time-of-flight (TOF) mobility measurements and/or optimum device performances. The same arylamine, oxadiazole, and polycyclic aromatic moieties have been converted to vinyl monomers via a Vilsmeyer formylation and then a Wittig coupling reaction.(4) The different vinyl monomers were then copolymerized to produce a range of conductive vinyl copolymers. The transport and emission properties of the copolymers were found relating to the chemistry of the respective monomers and overall copolymer compositions. The achievement of charge balance and their applications in homojunction OLEDs have been investigated. In this paper, several series of conductive vinyl polymers shall be discussed including the followings. (i) The first is a series of hole transporting poly(N-phenyl-N-styryl-1-naphthylamine) (PNPA) with different substituted group on the benzene ring.⁴ Their holes mobilities were measured to be 0.5-2 × 10^-4 cm² V^-1s^-1. (ii) The second series is copolymers of hole transporting N-(4-methoxyphenyl)-N-(4-vinylphenyl) naphthalen-1-amine (MeONPA) and electron transporting 2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (OXA) which has been applied as a hole limiting layer for devices containing oxidative degradating Alq₃.⁵ At the optimum charge balance composition with Alq₃ as the emitting layer, the optimum current efficiency and luminance were 4.2 cd/A and 24,000 cd/m² with extented device life-time. (iii) The third series is copolymers prepared from MeONPA and 1-vinylpyrene (py) at dif- erent compositions.⁶ Homojunction devices were prepared with highest luminance and current efficiency at 2,270 cd/m² and 1.32 cd/A. The emission color, however, shifted from greenish to bluish pending on the composition of the π-stackable and exciplexes forming pyrene moieties. (iv) The fourth copolymer series is terpolymers containing perylene as the emitters, and 4MeONPA and OXA as the charge transporting moieties. A greenish luminance of 760 cd/m² was achieved for the terpolymer with only 0.6 mol % perylene contents.