Mitigating Contact-Induced Leakage Degradation in n-Channel P(NDI2OD-T2) Thin-Film Transistors via Interfacial Energetic Control

The operational stability of n-channel organic thin-film transistors (OTFTs) is a critical requirement for organic logic circuits. This study investigates the origin of instability in P(NDI2OD-T2) TFTs, focusing on the role of source/drain (S/D) contact energetics. We demonstrate that the face-on orientation of P(NDI2OD-T2) facilitates efficient initial charge injection but effectively masks underlying interfacial instabilities caused by the high Schottky barrier (0.66 eV) at Au contacts. Under operation, this energetic mismatch promotes localized charge accumulation, leading to a two-order-of-magnitude increase in off-state leakage current without a threshold voltage shift. In contrast, Ni electrodes significantly mitigate this degradation. The spontaneous formation of a native NiO x layer induces an interfacial dipole that lowers the injection barrier to 0.26 eV. Low-frequency noise analysis confirms that the Ni interface reduces trap state density, preventing parasitic charge buildup at the S/D junctions. Our findings underscore that contact-mediated charge dynamics are as vital as the gate-dielectric interface for device reliability. This work provides a key design rule for electrode selection, emphasizing that suppressing the Schottky barrier is essential for the long-term stability of high-performance n-channel OTFTs.