From Event: SPIE Organic Photonics + Electronics, 2015
We present the structural, optical, and electronic properties of two polymorphs of the charge-transfer complex dibenzotetrathiafulvalene-7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ). The structures were resolved using electron diffraction. One is a rectangular-shaped platelet and exhibits ambipolar transport with gold contacts in organic field-effect transistors (OFETs). The second polymorph grows as a thin, circular platelet and shows p-type transport within the same OFET structure. The difference in electrical characteristics originates from two factors. First, the position of the HOMO level of the rectangular polymorph lies 0.07 eV below that of the circular polymorph, as determined by ultraviolet photoelectron spectroscopy (UPS). Second, using X-Ray photoelectron and Raman spectroscopies we find that the degree of charge transfer between the donor DBTTF and acceptor TCNQ varies from 0.4e ± 0.1e for the rectangular crystals to 0.1e ± 0.1e for the circular crystals. Additional differences between the two polymorphs arise when measuring the absorbance of the crystals. First, the peak corresponding to intermolecular charge transfer is offset by 0.05 eV between the two polymorphs, in agreement with UPS measurements. Additionally, the polarization angle dependence of the intermolecular CT band for the rectangular crystal is shifted by 60deg with respect to the angle dependence of the intramolecular transition. In contrast, this shift was only 20deg for the circular polymorph. These angle dependences allowed us to couple the anisotropy of the electrical characteristics to the optical measurements. Our results demonstrate how slight modifications in crystal structure yield drastic differences in optical and electrical properties in organic semiconducting materials.
Katelyn P. Goetz, Jun ya Tsutsumi, Sujitra Pookpanratana, Jihua Chen, Tatsuo Hasegawa, and Oana D. Jurchescu, "Polymorphism in the organic charge-transfer complex dibenzotetrathiafulvalene-7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ) and its effect on optical and electrical properties (Presentation Recording)," Proc. SPIE 9568, Organic Field-Effect Transistors XIV; and Organic Sensors and Bioelectronics VIII, 95680E (Presented at SPIE Organic Photonics + Electronics: August 11, 2015; Published: 5 October 2015); https://doi.org/10.1117/12.2187069.4519371854001.
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