Switching N‐Type to Ambipolar Organic Electrochemical Transistors by Donor Engineering of Polymeric Mixed Conductors Based on a Novel Electron‐Deficient BITBII Building Block

ABSTRACT Ambipolar organic electrochemical transistors (OECTs) are essential for the high‐density integration of complementary logic circuits with reduced fabrication complexity. However, progress in this field remains limited by underdeveloped design principles, and the development of ambipolar single‐component organic mixed ionic‐electronic conductors (OMIECs) has lagged significantly. Herein, we design a novel electron‐deficient building block BITBII , and copolymerize it with distinct electron‐donating units to obtain two polymers ( PBITBII‐2FT, PBITBII‐F2T ). PBITBII‐F2T‐based single‐component OECTs exhibited excellent ambipolar performance (p‐type/n‐type normalized transconductances (g _{m, norm} ): 4.7/2.92 S cm ^{− 1} ; µ C ^* : 63.37/14.58 F V ^{− 1} cm ^{− 1} s ^{− 1} ). In contrast, PBITBII‐2FT exhibited only relatively weak n‐type characteristics. Through theoretical calculations and film morphology analysis, the loss of ambipolarity in PBITBII‐2FT is attributed to a more localized HOMO distribution, a wavy backbone leading to inferior intermolecular packing, and a rougher film morphology, all of which impede hole transport. Furthermore, a complementary inverter fabricated using PBITBII‐F2T achieves a high voltage gain of up to 29.1 V/V. Cytotoxicity tests confirm the good biocompatibility of PBITBII‐F2T . This work not only enriches the material library for high‐performance ambipolar OECTs but also provides valuable insights into structure–property relationships governing ambipolar functionalities, offering guidance for future ambipolar molecular engineering.