Effect of substituents on the properties of indolo[3,2-b]carbazole-based hole-transporting materials

Three new compounds based on indolo[3,2-b]carbazole,2,8-bis(4-diphenylaminophenyl)-5,11-di-n-octylindolo[3,2-b]carbazole (BTOICZ), 2,8-bis(9,9′-di-n-butylfluorenyl)-5,11-di-n-octylindolo[3,2-b]carbazole (BFOICZ) and 2,8-bis[N-(n-butyl)carbazolyl]-5,11-di-n-octylindolo[3,2-b]carbazole (BCOICZ), as hole-transporting materials have been synthesized by Suzuki coupling reaction. The effects of substituents on the optical, thermal and electrochemical properties of indolo[3,2-b]carbazole derivatives have been studied carefully. Electroluminescent (EL) devices using these compounds as hole-transporting layer in combination with Alq3 as electron-transporting and emitting layer have been fabricated and characterized. The devices based on BTOICZ or BFOICZ showed green emission at 526 nm, while the device based on BCOICZ emitted very weak light. The turn-on voltages are 4.1 and 5.4 V for BTOICZ and BFOICZ, respectively. The maximal luminance efficiencies are 0.738 and 0.767 lm/W for BTOICZ and BFOICZ, respectively. The difference of the EL performances of these compounds reveals that the substituent effects have great effect on the hole-transporting performance of the indolo[3,2-b]carbazole-based compounds.