A liquid crystalline copper phthalocyanine derivative for high performance organic thin film transistors

Abstract

Bottom-gate, bottom-contact organic thin film transistors (OTFTs) were fabricated using solvent soluble copper 1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine as the active semiconductor layer. The compound was deposited as 70 nm thick spin-coated films onto gold source–drain electrodes supported on octadecyltrichlorosilane treated 250 nm thick SiO₂ gate insulators. The performance of the OTFTs was optimised by investigating the effects of vacuum annealing of the films at temperatures between 50 °C and 200 °C, a range that included the thermotropic mesophase of the bulk material. These effects were monitored by ultraviolet-visible absorption spectroscopy, atomic force microscopy and XRD measurements. Device performance was shown to be dependent upon the annealing temperature due to structural changes of the film. Devices heat treated at 100 °C under vacuum (≥10⁻⁷ mbar) were found to exhibit the highest field-effect mobility, 0.7 cm² V⁻¹ s⁻¹, with an on–off current modulation ratio of ∼10⁷, a reduced threshold voltage of 2.0 V and a sub-threshold swing of 1.11 V per decade.