How can the bulky octadecyltriethoxysilane molecule promote single-crystal growth?
The function of the octadecyltriethoxysilane monolayer in this case is not to promote epitaxial growth but to reduce the surface energy of the dielectric layer (SiO2), on top of which the semiconductor (pentacene/rubrene/etc.) is about to be deposited. The reduced surface energy allows the crystal to grow "comfortably".

Organosilanes are typical compounds for modifying oxide layers (Pernstich et al., 2004):