Highly sensitive and fully printable humidity sensor on a flexible substrate based on a zinc oxide and polyethylenimine composite

We report a highly sensitive and fully printable capacitive humidity sensor based on a zinc oxide (ZnO) and polyethylenimine (PEI) composite. The sensor has a simple structure, consisting only of a layer of the ZnO:PEI composite, coated using an ethanol solution, on a layer of silver inter-digital electrodes that have been pre-printed on a polyethyleneterephthalate substrate. The sensor with ZnO:PEI in the ratio of 2:1 by volume exhibits a response of 43 907 000% at maximum humidity, with a detection range of 15%–95% relative humidity, higher than other sensors fully made by wet-coating processes. Fourier transform infrared spectroscopy, atomic force microscopy, and scanning electron microscopy measurements suggest that the high response likely arises from the use of a hydrophilic polymer with a high dipole moment which facilitates dipole-dipole interactions with water molecules and from the highly granular morphology of the composite which leads to a high surface-to-volume ratio and more-numerous water adsorption sites. The fabricated sensor also demonstrates short response/recovery times (5 s/3 s), good repeatability over multiple humidification and desiccation cycles, and only 5% loss in response after being kept in the ambient for three weeks.