Fully transparent and flexible humidity sensors fabricated by layer-by-layer self-assembly of thin film of poly(2-acrylamido-2-methylpropane sulfonate) and its salt complex

A novel fully transparent and flexible impedance-type humidity sensor was fabricated by the LBL self-assembly of poly(2-acrylamido-2-methylpropane sulfonate) (PAMPS) polymer electrolyte and PAMPS thin films that were doped with salts (NaCl and K2CO3) on a flexible substrate (polyester film; PET) substrate with a pair of comb-like aluminum-doped ZnO (AZO) transparent electrodes. The electrical properties of the LBL self-assembled PAMPS polymer electrolyte thin films were studied in detail as functions of relative humidity (RH), to elucidate the effects of the deposition concentration of PAMPS and the number of PAMPS multilayers on the sensing properties. NaCl and K2CO3 were simultaneously doped into PAMPS polyelectrolyte to improve the sensing properties (sensitivity and linearity) of the transparent and flexible impedance-type humidity sensors. The transparency, flexibility, hysteresis, ambient temperature, response and recovery times and long-term stability were also studied.