With the emergence of environmental, biomedical, and medical monitoring technologies, development of flexible and lightweight sensors is ongoing. This work presents a flexible lightweight bilayer (BL) film (polycarbonate/polycrystalline layer of crystalline (BEDT-TTF)xBr(H2O)y salts [BEDT-TTF = bis (ethylendithio)tetrathiafulvalene]) as a promising material for humidity sensing which may be applied in a number of monitoring scenarios. X-ray structural characterization revealed the presence of three different crystal types in the crystalline layer of the BL film, i.e., [phase I, [(BEDT-TTF)5(Br4(H5O2))]; phase II, [(BEDT-TTF)2Br(H2O)3]; and crystals of neutral BEDT-TTF compound], showing that phase I is highly sensitive to humidity. The humidity testing of the BL film showed that it is capable of monitoring relative humidity (RH) levels from 15 up to 90% with a well-defined and reproducible electrical signal. Electrical …