Template and Temperature-Controlled Polymorph Formation in Squaraine Thin Films.

Area of Science:

• Materials Science
• Organic Electronics
• Crystallography

Background:

• Controlling polymorph formation in organic semiconductor thin films is essential for achieving desired device performance.
• Anilino squaraine (SQIB) is a small molecule semiconductor with broad absorption, suitable for photovoltaic applications, typically processed as amorphous films.

Purpose of the Study:

• To investigate the influence of processing parameters on the polymorph formation and molecular orientation of SQIB thin films.
• To correlate the identified crystal phases with film morphology and optical properties.

Main Methods:

• Thin film deposition via spin-casting and vapor deposition on various substrates (SiO2, KCl, graphene, gold).
• Characterization using X-ray diffraction, atomic force microscopy, ellipsometry, and polarized in situ spectro-microscopy.
• Analysis of polarized optical response to identify H-type and J-type aggregates and Davydov splitting.

Main Results:

• Spin-casted SQIB films exhibit monoclinic or orthorhombic polymorphs with preferred out-of-plane orientation, controllable by thermal annealing.
• Vapor-deposited SQIB films show substrate-dependent polymorphs with out-of-plane orientation and surface-templated in-plane alignment.
• Amorphous films from vapor deposition are stable and cannot be crystallized by post-annealing, unlike spin-casted films.

Conclusions:

• Processing methods significantly dictate the polymorph, orientation, and morphology of SQIB thin films.
• Substrate choice is critical for polymorph control in vapor-deposited films, while annealing is key for spin-casted films.
• Understanding these structure-property relationships is vital for optimizing SQIB-based organic electronic devices.