Template and Temperature Controlled Polymorph Formation in Squaraine Thin Films

TL;DR

This study investigates how substrate type and deposition temperature influence the formation of different polymorphs in squaraine thin films, affecting their opto-electronic properties for photovoltaic applications.

Contribution

It demonstrates the controlled formation of specific polymorphs in squaraine thin films through substrate choice and deposition conditions, advancing material design for organic electronics.

Findings

Polymorph formation depends on substrate and temperature.

Crystalline substrates induce in-plane alignment.

Vapor deposition yields surface-templated orientations.

Abstract

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.