We report on the electrical conductivity and Seebeck coefficient of an electric-double-layer transistor based on an organic Mott insulator. The measurements were performed along the two in-plane crystallographic axes (a and c) of the same device. While the Seebeck coefficient along the a-axis was decreased by electron or hole doping, the value along the c-axis was increased by hole doping. This is in contrast to the general trade-off relation between the conductivity and the Seebeck coefficient. The simultaneous enhancement of the conductivity and the Seebeck coefficient is attributed to pseudogap formation in the hole-doped state, where a steep slope of the density of states emerges at the chemical potential because of the electron interaction.

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