Fabrication of a semi-transparent flexible humidity sensor using kinetically sprayed cupric oxide film

Abstract

In this study, a cupric oxide (CuO) flexible humidity sensor was fabricated using a kinetic spray process. After deposition of a 200-nm-thick Cu film, the as-deposited Cu layer was annealed at 250 °C, a relatively low temperature, in air conditions to create a CuO layer. N-type CuO (carrier concentration: −23.08 × 1014 cm−3) formed after reacting with oxygen during the annealing process in air via oxygen vacancy formation. The semi-transparent properties of the CuO film were characterized by measuring visible light transmittance. Stable sensing signals of 0.16–0.18 were obtained after the 1000th bending test. Scanning electron microscopy was used to examine the surface morphology of the CuO layer after the bending test; despite the presence of a few microcracks, delamination was not observed. Thus, a CuO-based humidity sensor was successfully fabricated through simple particle spray deposition and low-temperature annealing processes; the sensor maintained its performance under repeated bending stress (up to 1000 cycles).