Polymer-Assisted Polymorph Transition in Melt-Processed Molecular Semiconductor Crystals

Area of Science:

• Materials Science
• Crystallography
• Organic Electronics

Background:

• Organic semiconductors like 5,11-bis(triisopropylsilylethynyl)anthradithiophene (TIPS ADT) exhibit polymorphism, affecting their electronic properties.
• Melt processing offers an alternative to solution processing for fabricating organic electronic devices, but can lead to different crystalline structures.
• The influence of polymer blends on the crystallization behavior of organic semiconductors during melt processing is not fully understood.

Purpose of the Study:

• To investigate the crystallization of TIPS ADT in blends with medium density polyethylene (PE) via melt processing.
• To identify and characterize new polymorphs of TIPS ADT formed under these conditions.
• To understand the role of PE in the phase transitions and crystal growth of TIPS ADT.

Main Methods:

• Melt processing of TIPS ADT blended with 16 ± 1 wt % PE.
• Crystallization from the melt followed by cooling and thermal annealing at 100 °C.
• Characterization of crystalline phases using techniques sensitive to crystal structure and morphology (implied).

Main Results:

• A novel polymorph of TIPS ADT, designated Form II, was crystallized from melt-processed TIPS ADT/PE blends.
• Melt-processed TIPS ADT/PE initially formed a metastable polymorph (Form IV) with a brickwork packing motif.
• Annealing induced a transition from Form IV to Form II, with Form II growth influenced by initial spherulite structure; this transition was absent in neat TIPS ADT films.

Conclusions:

• Medium density polyethylene (PE) facilitates the formation of a new TIPS ADT polymorph (Form II) during melt processing and subsequent annealing.
• The mobile PE phase enhances TIPS ADT diffusion and nucleation rates, promoting the Form IV → II transition.
• Form II crystals exhibit different electronic properties (less conductive, less emissive) compared to the metastable Form IV crystals.