Polymer Interfacial Layer with High Glass Transition Temperature for the Improvement of Bias Stability in Organic Field-Effect Transistors

We investigated the bias stability of organic field-effect transistors (OFETs) with a gate dielectric modified with a polymer with a high glass transition temperature, which is a cyclic olefin copolymer (COC). With the COC modification, the surface of the SiO₂ gate dielectric became significantly more hydrophobic and exhibited low surface energy, which yielded an enhanced crystallinity of the organic semiconductors. As a result of a reduced surface trap density and enlarged grain size that was obtained by employing COC, the bias stability and electrical performance of a pentacene OFET were remarkably improved. After two hours of biasing, the bias-stress-induced threshold voltage shift (ΔV _TH) was significantly reduced from 10 V to 2 V, and the relaxation time (τ) extracted from a stretched exponential fit was extended from 2.7 × 10⁴ sec to 4.7 × 10⁶ sec. We also applied this surface modification method to n-type OFETs and determined that the COC modification is applicable to other types of FETs to improve the bias stability.