Transient and steady-state space-charge-limited currents in polyfluorene copolymer diode structures with ohmic hole injecting contacts

We report detailed measurements on diode structures containing the electrol uminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-bis-N,N'-(4-m ethoxyphenyl)-bis-N,N'-phenyl-1,4 phenylenediamine). Ohmic injection of hol es is achieved with an oxygen plasma cleaned indium tin oxide (ITO) electro de, untreated ITO coated with a film of poly(ethylenedioxythiophene)/polyst yrenesulphonic acid (PEDOT/PSS) and plasma cleaned ITO with PEDOT/PSS. Tran sient dark injection and time-of-flight mobility measurements and steady st ate current density versus voltage measurements are then entirely consisten t with a positive carrier, trap-free, space-charge-limited current. Injecti on limited behavior is observed, however, for untreated ITO without PEDOT/P SS and for evaporated Au contacts. (C) 2000 American Institute of Physics. [S0003-6951(00)04211-X].