Ultra‐thin films of organic molecules can provide new opportunities for unique device applications and guide deeper understanding of intrinsic charge transport mechanisms in these materials. In this study, we report an effective way to deposit ultra‐thin polymer films over a large area with molecular level precision using a simple wire‐wound bar‐coating method. The film thickness of poly[[2,5‐bis(2‐octyldodecyl)‐2,3,S,6‐tetrahydro‐3,6‐dioxopyrrolo[3,4‐c]pyrrole‐1,4‐diyl]‐alt‐[[2,2′‐(2,5‐thiophene)bis‐thieno(3,2‐b)thiophene]‐5,5′‐diyl]] (DPPT‐TT) as a p‐channel polymer and poly((N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)) (P(NDI2OD‐T2)) as an n‐channel polymer were precisely controlled down to 1 ∼ 2 nm, consisting of just 1 — 2 organic molecular layers, respectively. The bar‐coated ultra‐thin films not only exhibited excellent transparency up to 90% in the visible wavelength range but also showed comparable charge carrier mobilities with OFETs for the commonly used thicker films due to a high degree ofpercolation through the uniformly covered polymer nano‐fibrils. We successfully demonstrate that the bar coated ultra‐thin film acts as an active layer for high transparency large area flexible OFETs array and high sensitivity OFET‐based multi‐gas sensors.