Electro-modulation (EM) is used to measure interfacial hole densities in operating OLEDs. This in-situ
optical probe has provided us with a measure of relative hole and electron currents in a wide range of
OLED structures. Insight to the dynamics of interfacial hole densities allows an assessment of the longterm
stability of electron-hole balance, and therefore helps greatly with the identification of
electroluminescence (EL) degradation mechanisms. Here, we limit the investigation to NPB/AlQ3 OLEDs,
but note that the technique is versatile, and can be readily adapted to a wide range OLED structures,
especially those comprising arylamine -based hole transport materials and interlayers.
We have studied the temperature-dependent photoluminescence (PL) characteristics of oxidised PFO thin films at temperatures above 298K. We find the relative strength of the green emission band (g-band) to increase greatly at temperatures corresponding to the onset of crystallisation. Based on the proposal that the g-band arises from a fluorenone-based excimer, this finding would seem to indicate that a close approach of neighbouring fluorenone-containing segments may be energetically favourable. Finally, by successfully identifying diffusion-limited and dynamic equilibrium regimes within the temperature dependence of the ratio R=ID/IM, we extract an activation energy for excimer formation of ~ 0.05 eV and an excimer binding energy of ~ 0.51 eV. This is a relatively high binding energy for an excimer and lends credibility to the notion of an energetically favourable fluorenone:fluorenone coupling configuration, perhaps as a result of the considerable ground state dipole moments associated with the ketone group.
We report systematic measurements of the evolution of the emission characteristics of PFO whilst undergoing photo-oxidation. Pure PFO and highly diluted PFO/polystyrene blended films were prepared for the studies by spin-coating. Each film was oxidized by exposure to the 351 nm line of a cw Ar+ laser. Both the kinetics of the various spectral components and the photoluminescence intensity for each film was monitored as a function of oxidation time and their respective behaviors were compared. Our results demonstrate that there is a strong tendency for singlet intrachain excitons initially created on pristine PFO segments to migrate to the fluorenone moieties produced by photo-oxidation. However, we conclusively show that emission from states localized at these defect sites cannot account for the appearance of the broad green emission band (g-band) that is well-known to occur in degraded polyfluorenes. Instead, it is shown that the g-band must emanate from interchain states that are formed after energy has been transferred to the fluorenone moieties (either via energy transfer form non-defective PFO segments or by direct excitation).