On the Sensing Mechanisms of a Hydroresistive Flexible Film Based on an Organic Molecular Metal

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 resistance measurements revealed that the crystalline conducting layer can absorb moisture reaching equilibrium at constant RH as reflected in a stable relative resistance response. The structural response of the BL film to variations of RH clearly demonstrated that crystallite interlayer spacing (d) of phase I is strongly affected, exhibiting a reversible metal–nonmetal transition, while phase II was insensitive to humidity. An overview of mechanical and humidity sensing properties of the developed BL film corroborates that it can be used as flexible hygrometer as well as moisture sensing units on the board of low-cost electronic sensing devices.