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 stable photoluminescence (PL) and electroluminescence (EL) at room temperature and ambient atmosphere. I(V) and EL vs. voltage curves present a much lower threshold than device made up of poly(DHF) homopolymer. The behaviour of both materials has been quantitatively investigated through a Fowler–Nordheim analysis allowing the determination of the energy offsets between the Fermi level of the ITO electrode and the HOMO of polymers. When using the copolymer, the barrier height for hole injection is reduced to a value close to 0.17 eV compared to 0.83 eV for the homopolymer. This difference in offsets has been confirmed by electrochemical investigations.