Narrow-gap semiconductors with high conductivity and mobility are an important class of materials for various applications, especially for thermoelectric and optical device applications. Herein, we designed and synthesized novel organic narrow-gap semiconductors, which are modified forms of the main skeleton of a single-component pure organic metal, tetrathiafulvalene-extended dicarboxylate (TED). Molecular design of the TED derivatives with substituent groups on the skeleton led to highly-conducting semiconductors even when powder crystalline samples were used. Their thermopower is greater than that of metallic TED without a substituent group, demonstrating the successful tuning of carrier concentration in the TED system by molecular design. Near-/middle-infrared (IR) diffuse reflectance measurements revealed each band gap, and optical parameters extracted from the spectra evaluated the carrier concentration and mobility of the TED derivatives with fitting calculations on the basis of a Drude-Lorentz dielectric function.
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