  Controlling the polymorph formation in organic semiconductor thin films by
the choice of substrate and deposition temperature is a key factor for targeted
device performance. Small molecular semiconductors such as the quadrupolar
donor-acceptor-donor (D-A-D) type squaraine compounds allow both solution and
vapor phase deposition methods. A prototypical anilino squaraine with branched
butyl chains as terminal functionalization (SQIB) has been considered for
photovoltaic applications due to its broad absorption within the visible to
deep-red spectral range. Its opto-electronic properties depend on the formation
of the two known polymorphs adopting a monoclinic and orthorhombic crystal
phase. Both phases emerge with a strongly preferred out-of-plane and rather
random in-plane orientation in spincasted thin films depending on subsequent
thermal annealing. Upon vapor deposition on dielectric and conductive
substrates, such as silicon dioxide, potassium chloride, graphene and gold, the
polymorph expression depends on the choice of growth substrate. In all cases
the same pronounced out-of-plane orientation is adopted, but with a surface
templated in-plane alignment in case of crystalline substrates. Combining X-ray
diffraction, atomic force microscopy, ellipsometry and polarized
spectro-microscopy we identify the processing dependent evolution of the
crystal phases, correlating morphology and molecular orientations within the
textured SQIB films.
