Charge mobilities and luminescence characteristics of blue-light emitting bent carbazole trimers connected through vinylene linkers - effect of nitrile substituents

Abstract

Two new blue-light emitters, 3,6-bis(4-carbazolylstyryl)-N-4-methylphenylcarbazole (Cz(3)) and 3,6-bis[4-(3,6-dicyanocarbazolyl)styryl]-n- N-4-methylphenylcarbazole (Cz(3)(CN)(4)), were found to form high glass-transition temperature (T-g), amorphous organic glasses, when vacuum deposited. Cz(3)(CN)(4) exhibited a much higher T-g value (242 degreesC) than Cz(3) (163 degreesC). It was found that Cz(3) is an excellent hole transporter (mu(h+) = 1.3 x 10(-4) cm(2)/V s, mu(e-) = 2.8 x 10(-8) cm(2)/V s) while Cz(3)(CN)(4) shows a faster electron mobility (mu(e-) = 3.1 X 10(-6) cm(2)/V s) than hole mobility (mu(h+) = 2.0 x 10(-8) cm(2)/V s), which is perceived to tell us that the presence of the electron-attracting nitrile (C=N) groups facilitates the electron mobility. The single layer electroluminescence (EL) device (ITO/EML/Li:Al) of Cz(3)(CN)(4) performed much better (external quantum efficiency similar to3.0 x 10(-2)%) than that of Cz(3) (external quantum efficiency similar to1.2 x 10(-3)%).