The absolute hole injection efficiency from pre-treated indium tin oxide electrodes into poly(9,9-dioctylfluorene) and its copolymers

If conduction in a polymer light emitting diode is by trap-free, space-charge-limited current (TFSCLC), the injecting contact must be ohmic. Such an ohmic contact is, by definition, the perfect injecting electrode. The absolute injection efficiency of any specific contact can therefore be quantified by the ratio of the actual current density, J, to that of the ideal TFSCLC current density, JTFSCLC. In this work we report on the injection efficiency of positive carriers from pretreated indium tin oxide (ITO) and metal electrodes into poly(9,9-dioctylfluorene) (PFO) and two representative copolymers. The ITO was either washed only in solvents, exposed to an oxygen plasma or coated in a film of poly(ethylenedioxythiophene)/polystyrenesulphonic acid blend. Time-of-flight photocurrent measurements were used to find the positive carrier mobility, μp, so that JTFSCLC could be calculated. Dark injection current transient measurements were used to confirm the presence or absence of TFSCLC conduction. Steady-state, current density-voltage measurements were then used to compare the actual current to JTFSCLC, thus allowing the absolute injection efficiency, J/JTFSCLC, to be calculated.