Doping‐Free Polymer Nanoparticle Engineering of n‐Type Organic Mixed Ion–Electron Conductors for Enhancement‐Mode Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) offer unique advantages for bioelectronic and neuromorphic applications. The development of n-type enhancement-mode devices is essential for creating complementary circuits and low-power, bidirectional bioelectronic platforms. However, progress in this area has been hindered by the challenges associated with processing suitable n-type polymers. Here, we present a nanoparticle-based processing strategy for the ladder polymer poly(benzimidazobenzophenanthroline) (BBL). This n-type mixed conductor is soluble only in strong acids, which hinders straightforward fabrication. BBL nanoparticles are obtained by reprecipitation with an anionic surfactant, and systematic analysis reveals a particle-number-controlled scaling law, in which surfactant concentration and the polymer-to-surfactant ratio govern stabilization. Films prepared from these dispersions further underscore the importance of nanoparticle assembly: spray-coating yields dense, interconnected networks with markedly higher electrochemical activity than the porous films obtained by the filtration-transfer method. The spray-coated BBL films operate in enhancement mode, exhibiting efficient switching in the subthreshold regime while remaining non-conductive at zero gate bias. This work establishes a scalable route to n-type enhancement-mode OECTs, thereby broadening the foundation for next-generation bioelectronic and neuromorphic systems.