Polyelectrolyte interlayer for ultra-sensitive organic transistor humidity sensors

We demonstrate low-voltage, flexible, transparent pentacene humidity sensors with ultrahigh sensitivity, good reliability, and fast response/recovery behavior. The excellent performances of these devices are derived from an inserted polyelectrolyte (poly[2-(methacryloyloxy)ethyltrimethylammonium chloride-co-3-(trimethoxysilyl)propyl methacrylate] (poly(METAC-co-TSPM)) interlayer, which releases free Cl^- ions in the electrolyte dielectric layer under humid conditions and boosts the electrical current in the transistor channel. This has led to extreme device sensitivity, such that electrical signal variations exceeding 7 orders of magnitude have been achieved in response to a 15% change in the relative humidity level. The new sensors exhibit a fast responsivity and a stable performance toward changes in humidity levels. Furthermore, the humidity sensors, mounted on flexible substrates, provided low voltage (<5 V) operation while preserving the unique ultrasensitivity and fast responsivity of these devices. We believe that the strategy of utilizing the enhanced ion motion in an inserted polyelectrolyte layer of an OFET structure can potentially improve sensor technologies beyond humidity-responsive systems.