We report on the link between the structure of n-octylphosphonic acid (C8PA) monolayer implemented in low-voltage organic thin-film transistors (OTFTs) based on aluminium oxide/C8PA/pentacene and the kinetics of the transistor bias-induced degradation. Structural changes in the vapour-deposited C8PA monolayer, studied by Fourier Transform Infrared (FTIR) spectroscopy, are induced by annealing. Changes in the threshold voltage, subthreshold slope, field-effect mobility, and the transistor on-current are measured as functions of the bias stress time and fitted with stretched exponential functions. The presence of C8PA molecules physisorbed to the monolayer and/or the increased disorder between the aliphatic tails results in substantial degradation of the subthreshold slope and faster reduction in normalized mobility, while slowing the degradation of the threshold voltage. The removal of all physisorbed molecules and improved order between aliphatic tails achieved via optimized post-deposition annealing leads to an improved, microscopically-less-varied interface between C8PA and pentacene. Consequently, the degradation of the subthreshold slope becomes negligible, the reduction in normalized mobility becomes smaller and the degradation of the threshold voltage dominates. The equilibrium value of the normalized on-current after prolonged bias stress is ~ 0.16 regardless of the disorder in C8PA monolayer, indicating that even though the structure of the monolayer affects the kinetics of the transistor degradation process, the same bias stress condition ultimately leads to the same relative drop in the on-current.
- self-assembled monolayer (SAM)
- alkyl phosphonic acid
- FTIR measurement
- organic thin-film transistor