A series of well-defined oligomers were synthesized to provide insight on the effect of the position of cyano substituents on the electronic properties of conjugated molecules and polymers. Two copolymers PF3CNP1 and PF1CNP1 composed of 9,9-di-n-hexylfluorene and 2,5-dicyanobenzene have been synthesized based on this model study. By attaching two electron-withdrawing cyano groups onto the phenylene ring, the electron affinity and electron transporting ability of polyfluorenes are greatly enhanced. The devices employing a conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT) as the hole injecting layer, and the cyano- containing copolymers as the emitting and electron-transporting layer showed a much improved electroluminescence (EL) efficiency and brightness compared to that of the homopolymer, poly(9,9-di-n-hexylfluorene) (PHF) with the same device configuration. It is ascribed to the smaller energy barrier for electron injection and higher electron mobility due to increased electron affinity of the polymers through the cyano substituents. The device based on PF3CNP1 has the best performance with a low turn-on voltage (3.4 V), bright blue emission (5430 cd/m2 at a bias voltage of 7.4 V), and a maximum external quantum efficiency of 0.50 % due to both optimized electron affinity and charge carrier mobility.

© 2001 Optical Society of America

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