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

Abstract

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 (F10-SiPc) evaporated onto silicon dioxide (SiO2) surfaces modified by self-assembled monolayers (SAMs) of varying surface energies and by weak epitaxy growth (WEG). The total surface energy (γtot), dispersive component of the total surface energy (γd), and polar component of the total surface energy (γp) were calculated using the Owens-Wendt method and related to electron field-effect mobility of devices (μe), and it was determined that minimizing γp and matching γtot yielded films with the largest relative domain sizes and highest resulting μe. 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 μe of 7.2 × 10-2 cm2·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 F10-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 VT. Unlike conventional MPcs, F10-SiPc films fabricated by WEG experienced no macrocycle in an edge-on configuration. These results demonstrate the critical role of the F10-SiPc axial groups on WEG, molecular orientation, and film morphology as a function of surface chemistry and the choice of SAMs.