Electrolyte gated single-crystal organic transistors to examine transport in the high carrier density regime

Recent advances in understanding electronic charge transport in organic semiconductors are motivated by the fast growth of organic electronics. In particular, organic single crystals provide an ideal test bed for systematic studies of charge transport, with rapid progress in single-crystal-based field-effect transistors in the past few years. Charge densities induced in crystals by the field-effect have been in the low limit regime (10¹⁰ cm^-2 to 10¹³ cm^-2) mainly due to the difficulties of boosting gate dielectric capacitance. Consequently, the transport physics of organic crystals in the high-charge-density regime has not been systematically explored. With the emergence of the electrolyte gating technique, ultrahigh charge densities (10¹³ cm^-2 to 10¹⁵ cm^-2) can be achieved. In this article, we first discuss the general methodologies of applying electrolyte gating to organic crystals. We then review several recent research highlights, including the maximization of charge density and improvement of carrier mobility, enhanced understanding of the mobility-charge density relationship, and observations of ambipolar transport and a novel conductivity peak that occurs only at high charge densities. These recent achievements are extremely important for ongoing efforts to realize novel transport behavior in organic crystals, such as superconductivity and the insulator-to-metal transition.