  Ethyl-hexyl substituted polyfluorene (PF) with its high level of molecular
disorder can be described very well by one-carrier space-charge-limited
conduction for a discrete set of trap levels with energy $\sim$ 0.5 eV above
the valence band edge. Sweeping the bias above the trap-filling limit in the
as-is polymer generates a new set of exponential traps, which is clearly seen
in the density of states calculations. The trapped charges in the new set of
traps have very long lifetimes and can be detrapped by photoexcitation. Thermal
cycling the PF film to a crystalline phase prevents creation of additional
traps at higher voltages.
