Novel fully transparent and flexible humidity sensor

A novel fully transparent and flexible impedance-type humidity sensor was fabricated by depositing a gallium-doped ZnO (GZO) thin film on a flexible substrate (polyester film; PET) as a transparent pair of comb-like electrodes, and then coating the PET substrate with a copolymer of methyl methacrylate and [3-(methacrylamino)propyl] trimethyl ammonium chloride (MAPTAC) (poly-MMA/MAPTAC) as a sensing film. The sensor had high transmittance and good flexibility. This flexible humidity sensor also had acceptable sensitivity and a highly linear (Y = −0.0221X + 6.9620; R2 = 0.9976) between logarithmic impedance (log Z) and RH in the range 30–90% RH, low hysteresis (at 4.8% RH), a good response time (70 s), a recovery time (90 s), and long-term stability (55 days at least), measured at 1 V, 1 kHz and 25 °C. Other sensing properties such as the effects of applied voltage, frequency and ambient temperature were also studied.