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

Abstract

We report detailed measurements on diode structures containing the electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-bis-N,N′-(4-methoxyphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine). Ohmic injection of holes is achieved with an oxygen plasma cleaned indium tin oxide (ITO) electrode, untreated ITO coated with a film of poly(ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT/PSS) and plasma cleaned ITO with PEDOT/PSS. Transient dark injection and time-of-flight mobility measurements and steady state current density versus voltage measurements are then entirely consistent with a positive carrier, trap-free, space-charge-limited current. Injection limited behavior is observed, however, for untreated ITO without PEDOT/PSS and for evaporated Au contacts.