wileyonlinelibrary.comand portable devices, the device performance must be substantially improved. To this end, high-performance organic gate dielectric layers with high mechanical stability and large-area processability are urgently requested. [ 1 ] However, OTFTs made of polymer gate dielectrics are suffering from high operating voltage, typically exceeding 20 V, [ 3,4 ] due to the gate dielectrics with the thickness often greater than 100 nm to ensure low gate leakage current. [ 4 ] Thermally crosslinked polymers including poly(4vinylphenol) (PVP), [ 5 ] Cytop, [ 6 ] benzocyclobutene (BCB), [ 7 ] and polyimides (PIs) [ 8 ] have been adopted for gate dielectrics, but only few of them could successfully reduce the operating voltage of OTFTs down to 10 V by reducing the thickness of dielectric layer. Moreover, the high annealing temperature required to induce the crosslinking of polymer gate dielectrics is also problematic, which may cause damage to OTFTs and limit their application to thermally vulnerable substrates.Besides reducing the operating voltage of OTFTs, controlling the interface between dielectric and semiconductor is another critical factor to optimize the performance of OTFTs. [ 9,10 ] Applying selfassembled monolayers (SAMs) [ 10 ] or plasma treatment [ 11 ] had been widely applied to optimize the fi lm morphology of semiconductors on the surface of gate dielectrics. However, most of these SAM treatment procedures require specifi c surface coupling reaction, and had been applied mostly onto inorganic dielectric layers rather than polymer dielectrics, mostly due to the lack of target surface functionalities on most of the polymer dielectrics and/or a reliable, damage-free coupling reaction applicable onto the polymer dielectrics.Recently, we have proposed initiated chemical vapor deposition (iCVD) as a new deposition method to form ultrathin (<10 nm) crosslinked polymer dielectrics with high density. [ 12 ] iCVD is a well-established dry process, which can deposit highly pure polymer thin fi lms in mild process temperature (10-40 °C) and pressure (in the order of 100 mTorr). [ 13 ] Especially, an organosilicon polymer, poly(1,3,5-trivinyl-1,3,5-trimethyl cyclotrisiloxane) (pV3D3), exhibited an extremely low leakage current (less than 10 −9 A cm −2 at 3 MV cm −1 ) even with ultralow thickness (≈6 nm), and chemical/mechanical robustness. Nevertheless, Tailoring the surface of the dielectric layer is of critical importance to form a good interface with the following channel layer for organic thin fi lm transistors (OTFTs). Here, a simple surface treatment method is applied onto an ultrathin (<15 nm) organosilicon-based dielectric layer via the initiated chemical vapor deposition (iCVD) to make it compatible with organic semiconductors without degrading its insulating property. A molecular-thin oxide capping layer is formed on a 15 nm thick poly(1,3,5-trimetyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) by a brief oxygen plasma treatment. The capping layer greatly enhances the thermal stability of the diel...