Thin film transistors (TFTs), especially organic TFTs (OTFTs), play a key role in enabling the next-generation flexible electronics. Nevertheless, challenges remain on developing TFTs with low power consumption. A common solution to reduce power consumption is to reduce operation voltage by increasing the capacitance of a gate insulator material, because capacitance is inversely related to the voltage. To this end, ionic liquid gel (ion-gel) based solid-state electrolytes are promising replacement for traditional inorganic or organic high-k dielectric materials, because they have much higher capacitance than that of traditional dielectric materials (e.g., 1-10 µF cm-2 versus 0.1 µF cm-2). The high capacitance of ion-gel originates from the formation of electrical double layers (EDLs) at gate/ion-gel and ion-gel/semiconductor interfaces. However, the capacitance of an ion-gel film generally reduces rapidly at switching frequencies higher than 10-100 KHz, preventing its application in megahertz (MHz) TFTs. The high frequency limitation stems from the overall slow polarization/diffusion rate of ions within the insulating layer. The past researches, pioneered by Frisbie and Lodge groups, focused on maximizing the ionic conductivity to enhance the high frequency capacitance. On the other hand, reducing the thickness of an ion-gel insulating layer (e.g., to sub-micron) should, in theory, improve its frequency response too. But, it is extremely challenging to prepare pinhole-free sub-micron ion-gel layers via solution-based techniques. Herein, we employed a solvent-free technique, initiated chemical vapor deposition (iCVD), to first deposit ultrathin dry polymer films, followed by injecting ionic liquid into these swellable polymer films to form ion-gel films as thin as 20 nm. These ultrathin pinhole-free ion-gel films can retain a capacitance greater than 1 µF cm-2 at a frequency up to 1 MHz, making them promising gate insulator materials for low-voltage MHz TFTs. In addition, ion-gel films obtained from this new strategy is soft substrate (e.g., plastics) compatible, and patternable, which are beneficial for fabricating soft electronics.