Polyfluorene as a model system for space-charge-limited conduction

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 similar to 0.5 eV above the valence-band edge. Sweeping the bias above the trap-filling limit in the as-is polymer generates a set of exponential traps, which is clearly seen in the density of states calculations. The trapped charges in this set of traps have very long lifetimes and can be detrapped by photoexcitation. Thermal cycling the PF film to a crystalline phase prevents the creation of additional traps at higher voltages.