Ambipolar to Unipolar Transition in Y6 Based OFETs via PFN+Br- Doping

Achieving unipolar n-type transport in Y6-based organic field-effect transistors (OFETs) is challenging due to the intrinsic ambipolar nature of Y6, which leads to low on/off current ratios, high threshold voltages, and unreliable n-type operation. Here, we report a systematic study on polarity control of Y6 OFETs through direct doping with the ionic conjugated polymer PFN+Br-. Electrical characterization reveals a clear transition from ambipolar to unipolar n-type conduction, accompanied by a substantial enhancement in charge-carrier mobility, on/off current ratios (from ∼10 to 104), and reduced threshold voltages. Complementary spectroscopic analyses, including XPS, UPS, XAS, UV–Vis absorption, and GIWAXS, provide insight into the evolution of the electronic structure and microstructure with doping. At an optimal doping concentration of ∼15%, the Fermi level shifts towards the LUMO edge, leading to efficient electron injection and effective hole blocking, while maintaining favorable molecular ordering for charge transport. Excessive doping (>20%) induces structural disorder, increased trap density, and a recovery of ambipolar characteristics, resulting in performance degradation. This work establishes a clear structure-property relationship for doping in Y6-based OFETs and demonstrates ionic polymer doping as an effective strategy to achieve high-performance unipolar n-type organic transistors.