Balanced Ambipolar OECTs through Tunability of Blend Microstructure

Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronics, bridging ionic biological signals and electronic circuits. Ambipolar OECTs, capable of both n- and p-type charge transport, are highly desirable for versatile bioelectronic applications, offering a simplified circuit design and enhanced sensing capabilities. However, achieving a balanced ambipolar performance remains a materials science challenge. Here we show that blending judiciously selected unipolar p-type and n-type organic mixed ionic-electronic conductors (OMIECs), and most importantly, tuning film microstructure through composition and thermal treatment, can be leveraged to attain balanced ambipolar OECTs with near-equal n-type and p-type performances, both in the transconductance and in symmetrical threshold voltages. This is demonstrated by studying two blends based on a p-type OMIEC polymer and two n-type OMIEC fullerene derivatives with distinct miscibility and self-assembly tendencies that direct completely different blend organizations. Employing comprehensive electrochemical and microstructural characterization, we were able to correlate microstructure features, such as phase separation, domain continuity, and crystallinity, with volumetric capacitance and charge mobility, and hence overall device performance in both polarities. Based on the insights gained in this study, we propose a set of design rules and a rational framework for realizing balanced ambipolar OECTs using the blend approach. These rules emphasize informed material selection and precise microstructure control of phase continuity and order, attainable through composition and thermal annealing. The blend strategy offers a facile and versatile pathway for advancing next-generation bioelectronic devices with multifunctional OECT performance through rational microstructure engineering.