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

TL;DR

This study demonstrates that ethyl-hexyl substituted polyfluorene exhibits space-charge-limited conduction with discrete trap levels, and reveals how bias and thermal treatment influence trap formation and charge dynamics.

Contribution

It introduces polyfluorene as a model system for space-charge-limited conduction and details how bias and thermal cycling affect trap states and charge trapping.

Findings

Polyfluorene shows one-carrier space-charge-limited conduction with specific trap levels.

Bias above trap-filling creates new exponential traps with long-lived trapped charges.

Thermal cycling to crystalline phase prevents additional trap formation.

Abstract

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.