In this work, staggered top-gate n-type organic thin film transistors (OTFTs) with evaporated PDIF-CN2 semiconducting layers, spin-coated Cytop™ dielectric barriers and channel lengths ranging from 100 to 2 μm were fabricated on polyethylene-naphtalate (PEN) substrates. Hexamethyldisilazane (HMDS) treatment of the PEN surface was successfully tested as an effective strategy to achieve flexible devices with improved electrical response. Following this approach, maximum field-effect mobility (μFE) values exceeding 0.4 cm2/V⋅s were observed in air. Moreover, the self-encapsulating features of the investigated top-gate configuration, employing the highly hydrophobic Cytop™ dielectric films, allowed getting considerable performances in terms of un-sensitivity to hysteresis and bias stress phenomena.