Not Just Surface Energy: The Role of Bis(pentafluorophenoxy) Silicon Phthalocyanine Axial Functionalization and Molecular Orientation on Organic Thin-Film Transistor Performance.

Understanding the effect of surface chemistry on the dielectric-semiconductor interface, thin-film morphology, and molecular alignment enables the optimization of organic thin-film transistors (OTFTs). We explored the properties of thin films of bis(pentafluorophenoxy) silicon phthalocyanine ( F 10 -SiPc ) evaporated onto silicon dioxide (SiO 2 ) surfaces modified by self-assembled monolayers (SAMs) of varying surface energies and by weak epitaxy growth (WEG). The total surface energy (γ tot ), dispersive component of the total surface energy (γ d ), and polar component of the total surface energy (γ p ) were calculated using the Owens-Wendt method and related to electron field-effect mobility of devices (μ e ), and it was determined that minimizing γ p and matching γ tot yielded films with the largest relative domain sizes and highest resulting μ e . Subsequent analyses were completed using atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) to relate surface chemistry to thin-film morphology and molecular order at the surface and semiconductor-dielectric interface, respectively. Films evaporated on n -octyltrichlorosilane (OTS) yielded devices with the highest average μ e of 7.2 × 10 -2 cm 2 ·V -1 ·s -1 that we attributed to it having both the largest domain length, which were extracted from power spectral density function (PSDF) analysis, and a subset of molecules with a pseudo edge-on orientation relative to the substrate. Films of F 10 -SiPc with the mean molecular orientation of the π-stacking direction being more edge-on relative to the substrate also generally resulted in OTFTs with a lower average V T . Unlike conventional MPcs, F 10 -SiPc films fabricated by WEG experienced no macrocycle in an edge-on configuration. These results demonstrate the critical role of the F 10 -SiPc axial groups on WEG, molecular orientation, and film morphology as a function of surface chemistry and the choice of SAMs.