WFU Physics Colloquium

Organic charge-transfer complexes are ordered combinations of charge-donating (D) and charge-accepting (A) compounds. The proximity of the D and A units results in a partial degree of ionicity and a band structure that is a hybrid of the parent compounds, allowing the complex to exhibit novel physical characteristics. These include metallicity, ambipolar semiconductivity, photoconductivity, ferroelectricity, and more. The focus of this work is to identify the structure-function relationships in charge-transfer complexes pertaining to organic field-effect transistors (OFETs). The materials of focus in these studies are perylene- 7,7,8,8-tetracyanoquinodimethane (perylene-TCNQ), dibenzotetrathiafulvalene-TCNQ (DBTTF-TCNQ), and stilbene-F4TCNQ.

We found that the perylene-TCNQ crystallizes in three D:A ratios . 1:1, 2:1, and 3:1. Single crystal X-ray diffraction revealed that each compound exhibits a mixed-stack structure with slight differences in D/A overlap. Though the differences are small, the charge-transfer was found to increase with amount of donor. Fabrication of OFETs revealed that the 1:1 crystal was n-type, the 2:1 was ambipolar, and the 3:1 p-type. DBTTF-TCNQ grows in two 1:1 polymorphs, one of which, the .-polymorph, is known and the other, the .-polymorph, was grown here for the first time. We found that the former exhibits a degree of charge transfer of 0.5e, while the latter is nearly neutral. OFET measurements revealed that with the same device structure, the .-polymorph is ambipolar with electron-dominant transport, while the .-polymorph exhibits hole-dominant ambipolar transport. The investigation of electrical properties of stilbene-F4TCNQ revealed that in this complex, the transport is thermally activated above 235 K, and temperature independent at low temperatures. The transition correlates with a freezing-in of orientational disorder caused by a mobile moiety on the donor. Together, these results provide insight into the interplay between crystal structure, ionicity, and charge transport.