*Article* **Electric Double Layer Doping of Charge-Ordered Insulators** α**-(BEDT-TTF)2I3 and** α**-(BETS)2I3**

**Yoshitaka Kawasugi 1,2,\*, Hikaru Masuda 1, Jiang Pu 3, Taishi Takenobu 3, Hiroshi M. Yamamoto 2,4, Reizo Kato 1,2 and Naoya Tajima <sup>1</sup>**


**Abstract:** Field-effect transistors based on strongly correlated insulators are an excellent platform for studying the electronic phase transition and simultaneously developing phase transition transistors. Molecular conductors are suitable for phase transition transistors owing to the high tunability of the electronic states. Molecular Mott transistors show field-induced phase transitions including superconducting transitions. However, their application to charge-ordered insulators is limited. In this study, we fabricated electric double layer transistors based on quarter-filled charge-ordered insulators α-(BEDT-TTF)2I3 and α-(BETS)2I3. We observed ambipolar field effects in both compounds where both electron and hole doping (up to the order of 10<sup>13</sup> cm<sup>−</sup>2) reduces the resistance by the band filling shift from the commensurate value. The maximum field-effect mobilities are approximately 10 and 55 cm2/Vs, and the gate-induced conductivities are 0.96 and 3.6 *e*2/*h* in α-(BEDT-TTF)2I3 and α-(BETS)2I3, respectively. However, gate-induced metallic conduction does not emerge. The gate voltage dependence of the activation energy in α-(BEDT-TTF)2I3 and the Hall resistance in α-(BETS)2I3 imply that the electric double layer doping in the present experimental setup induces hopping transport rather than band-like two-dimensional transport.

**Keywords:** organic conductor; charge-ordered insulator; electric double layer transistor; organic field-effect transistor
