Cyanophenoxy-Substituted Silicon Phthalocyanines for Low Threshold Voltage n-Type Organic Thin-Film Transistors

Silicon phthalocyanines (SiPcs) are a class of n-type or ambipolar organic semiconductors that have been incorporated into organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs). Despite a relatively large catalogue of previously reported SiPc materials, fabricated OTFTs with these materials typically have threshold voltages (VT) above 10 V, limiting their usage in commercial devices due to exceedingly high power consumption. Recent studies have suggested that the VT can be reduced in OTFTs prepared from phenoxy-substituted SiPcs by introducing electron-withdrawing groups onto the phenoxy moieties. Herein, we report the synthesis and characterization of three SiPcs with phenoxy axial substituents containing nitrile and fluorine functional groups. These SiPcs, along with 3,5-difluorophenoxy SiPc were evaluated as candidate materials for n-type OTFTs. We found that further increasing the electron-withdrawing character of the pendant phenoxy groups of the SiPc resulted in a significant decrease in average VT with the lowest reported value being 4.8 V, the lowest VT reported for a phenoxy-SiPc-based OTFT exceeding the previous record low of 7.8 V attributed to F10-SiPc. This decrease in VT could be directly correlated to the Hammett parameter of the axial functional groups. Furthermore, it was noted that dewetting occurred when the phenoxy pendant group of the SiPc was substituted at the para position with a nitrile group combined with ortho- or meta-substituted fluorines, which was attributed to interactions at the semiconductor/dielectric interface. Depositing these SiPcs on silane-terminated poly(styrene) brush modified substrates improved long-term stability, demonstrated by a minimal change in surface morphology according to atomic force microscopy (AFM) images.