Polymorphism in the 1:1 Charge-Transfer Complex DBTTF-TCNQ and Its Effects on Optical and Electronic Properties.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Organic Electronics

Background:

• Organic charge-transfer (CT) complexes are crucial for organic electronics.
• Polymorphism in organic materials can significantly alter their properties.
• Dibenzotetrathiafulvalene - 7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ) is a well-studied CT complex.

Purpose of the Study:

• To investigate the crystallization behavior of DBTTF-TCNQ.
• To characterize the structural and optoelectronic properties of different DBTTF-TCNQ polymorphs.
• To understand the relationship between crystal structure and charge transport in DBTTF-TCNQ.

Main Methods:

• Physical vapor transport for crystal growth.
• Selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and polarized IR spectroscopy for structural and elemental analysis.
• Raman spectroscopy and organic field-effect transistor (OFET) fabrication for optoelectronic property and charge transport characterization.

Main Results:

• Two polymorphs of DBTTF-TCNQ, α and β, were identified with unique unit cells but identical 1:1 stoichiometry.
• The α-polymorph shows a charge transfer of ~0.5e⁻ and electron-dominant transport, while the β-polymorph is nearly neutral and exhibits hole-dominant transport.
• Significant differences in optoelectronic properties were observed between the two polymorphs.

Conclusions:

• The discovery of the β-polymorph expands the understanding of DBTTF-TCNQ crystallization.
• Structural variations in DBTTF-TCNQ polymorphs directly influence their charge transfer degree and transport characteristics.
• The findings highlight the importance of controlling polymorphism for tuning the performance of organic electronic devices.