Blue light electroluminescent devices based on a copolymer derived from fluorene and carbazole

Thin layers of poly(dihexylfluorene-co-N-hexylcarbazole) (poly(DHF-co-NHK) sandwiched between indium tin oxide (ITO) and aluminium electrodes show sta ble photoluminescence (PL) and electroluminescence (EL) at room temperature and ambient atmosphere. I(V) and EL vs. voltage curves present a much lowe r threshold than device made up of poly(DHF) homopolymer. The behaviour of both materials has been quantitatively investigated through a Fowler-Nordhe im analysis allowing the determination of the energy offsets between the Fe rmi level of the ITO electrode and the HOMO of polymers. When using the cop olymer, the barrier height for hole injection is reduced to a value close t o 0.17 eV compared to 0.83 eV for the homopolymer. This difference in offse ts has been confirmed by electrochemical investigations. (C) 1999 Elsevier Science S.A. All rights reserved.