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

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 degrees C, a relatively low temperature, in air conditions to create a CuO layer. N-type CuO (carrier concentration: -23.08x10(14) 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).