We have investigated crystalline structures in organic semiconducting thin films with regioregular poly(3-alkyl
thiophene)s, pentacene, and oligofluorene-thiophene derivatives, using grazing-incidence X-ray diffraction and atomic
force microscopy. We found that crystalline morphologies and molecular orientation in these thin films strongly affect
electric characteristics of these film-based organic thin-thin transistors (OTFTs). Specifically, in solution- and vacuumdeposited
films, preferentially perpendicular orientation of π-conjugated crystalline planes with respect to dielectric
substrates require results in high-performance charge mobility in the OTFT devices.
We studied how the underlying grain boundary morphology in the first pentacene monolayer affects the "macroscopic" mobility of the 40 nm thick OTFT film. Through manipulating surface properties of a SiO2 dielectric layer using self assembled monolayers (SAMs), we controlled the crystalline domain morphology of pentacene films that have thicknesses ranging from sub-monolayer to 40 nm. Atomic force microscopy (AFM) and two-dimensional grazing incidence X-ray diffraction are employed to characterize the morphology and crystalline structure of pentane films. In addition, the spatial variation of charge carriers in the first few layers was investigated using conducting AFM (C-AFM). In particular, faceted or dendritic island morphology has been observed in the first pentacene layer mainly depending on surface morphology of hydrophobic SAMs, and C-AFM supported that the faceted islands showed larger current flow than the dendritic islands. This C-AFM current tendency correlates with the "macroscopic" charge mobility in OTFT. Because the faceted morphology should represent the single crystal-like pentacene island, faceted islands have fewer internal crystal defects and the higher current flow than the dendritic islands.