The thermopowers of single-crystalline anilinium tetrathiafulvalene-2-carboxylate (TTFCOONH3Ph) and its deuteriated form (TTFCOOND3Ph) have been examined at room temperature, and a significant isotope effect was found: 102 μV/K for TTFCOONH3Ph, and 48 μV/K for TTFCOOND3Ph, which amounts to a difference of a factor of around two. The origin of the difference, for example, X-ray crystal structure and doping level, was examined based on chemical and physical characterizations, and it was found that very low ion transport drastically enhances the thermopower of holes, giving rise to the large isotope effect. A large isotope effect of thermopower has been found for semiconducting TTFCOONH3Ph and its deuteriated form, TTFCOOND3Ph. The origin of the isotope effect has been investigated based on chemical and physical characterizations using high-quality single crystals.
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