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Controlling the polymorph formation in organic semiconductor thin films bythe choice of substrate and deposition temperature is a key factor for targeteddevice performance. Small molecular semiconductors such as the quadrupolardonor-acceptor-donor (D-A-D) type squaraine compounds allow both solution andvapor phase deposition methods. A prototypical anilino squaraine with branchedbutyl chains as terminal functionalization (SQIB) has been considered forphotovoltaic applications due to its broad absorption within the visible todeep-red spectral range. Its opto-electronic properties depend on the formationof the two known polymorphs adopting a monoclinic and orthorhombic crystalphase. Both phases emerge with a strongly preferred out-of-plane and ratherrandom in-plane orientation in spincasted thin films depending on subsequentthermal annealing. Upon vapor deposition on dielectric and conductivesubstrates, such as silicon dioxide, potassium chloride, graphene and gold, thepolymorph expression depends on the choice of growth substrate. In all casesthe same pronounced out-of-plane orientation is adopted, but with a surfacetemplated in-plane alignment in case of crystalline substrates. Combining X-raydiffraction, atomic force microscopy, ellipsometry and polarizedspectro-microscopy we identify the processing dependent evolution of thecrystal phases, correlating morphology and molecular orientations within thetextured SQIB films.

Template and Temperature Controlled Polymorph Formation in Squaraine Thin Films