It is widely recognized that the effect of doping into a Mott insulator is complicated and unpredictable, as can be seen by examining the Hall coefficient in high Tc cuprates. The doping effect, including the electron–hole doping asymmetry, may be more straightforward in doped organic Mott insulators owing to their simple electronic structures. Here we investigate the doping asymmetry of an organic Mott insulator by carrying out electric-double-layer transistor measurements and using cluster perturbation theory. The calculations predict that strongly anisotropic suppression of the spectral weight results in the Fermi arc state under hole doping, while a relatively uniform spectral weight results in the emergence of a non-interacting-like Fermi surface (FS) in the electron-doped state. In accordance with the calculations, the experimentally observed Hall coefficients and resistivity anisotropy correspond to the pocket formed by the Fermi arcs under hole doping and to the non-interacting FS under electron doping.

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.

Heavily doped semiconductors are well investigated and widely used in inorganic electronics. However, controlled heavy doping of organic crystalline semiconductors is yet to be studied because of the lack of suitable methods. This article reports on doping by electrolyte gating combined with bandwidth control by uniaxial stress using a bilayered nonstoichiometric κ–β″‐type charge‐transfer salt, in which the β″ layer exhibits competition between metallic and charge‐ordered insulating states. A change from insulating‐like to metal‐like conduction with a positive temperature coefficient of resistance is induced by the simultaneous application of a negative gate voltage and compressive stress applied by bending the substrate. The simultaneous heavy doping and bandwidth‐control technique presents a novel approach for investigating nonstoichiometric doping of organic semiconductors for novel electronic functions using metal–insulator transitions and superconductivity of correlated electron systems.

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