Structural Modifications for Tuning Performance and Operational Modes in n-Type Organic Electrochemical Transistors

Organic mixed ionic–electronic conductors (OMIECs) are crucial in defining the operational modes and performance of organic electrochemical transistors (OECTs). However, studies on the design and structure–performance correlations of small-molecule n-type OMIECs remain scarce. Herein, we designed and synthesized a series of naphthalene diimide (NDI)–based n-type small molecules by extending π-conjugation and increasing the number of electron-withdrawing groups, achieving performance optimization and even changes in operational modes through structural regulations. OECTs based on 4Br-NDI-3EG exhibit a low threshold voltage of −0.022 V, which is the lowest reported for n-type channel materials to date. NDI-DTYA-3EG, synthesized through π-expansion of 4Br-NDI-3EG, maintains a low threshold voltage of −0.041 V and achieves 2 orders of magnitude improvement in electron mobility (1.04 × 10 –2 cm 2 V –1 s –1 ) owing to its mixed edge-on and face-on orientation. Specifically, by further increasing the number of electron-withdrawing groups, NDI-DTYM-3EG attains a sufficiently low LUMO energy level (−4.51 eV), enabling a spontaneous reduction in 0.1 M NaCl solution without external bias, thereby achieving self-doping. Consequently, it exhibits n-depletion-mode characteristics with a transconductance value of 287 μS. Moreover, devices made with NDI-DTYM-3EG show exceptional stability, retaining 98% of the initial drain current after 150 min operation. These results provide insights into the understanding and design of n-type mixed ionic–electronic conductor materials.