Copper oxide-based transparent and flexible humidity sensors with laser-induced single-layer integration

This study presents an innovative application of the laser-induced reductive sintering process for fabricating CuO x -based transparent flexible humidity sensors under ambient conditions. The laser process applies localized thermal energy to the CuO x nanoparticle (NP) thin film deposited on a flexible polyethylene terephthalate (PET) substrate, inducing direct reductive sintering at the CuO x /PET interface to form conductive Cu electrodes, and indirect reductive sintering at the Cu/CuO x interfaces to create a porous, Cu₂O-rich sensing channel between the electrodes. As a result, conducting electrodes and a semiconducting sensing channel are simultaneously formed within a single layer. Additionally, the current collecting Cu layers are patterned into unidirectional line arrays to enhance transparency. The humidity sensors reliably operate in a relative humidity range of 25‒80 %, with remarkably high sensitivity (7.04 × 10 4 ) and rapid response and recovery times (0.35 s and 0.80 s). Moreover, the sensor exhibits remarkable optical transparency (96.1 % at 550 nm) and mechanical flexibility, as confirmed by bending, twisting and cyclic bending tests. X-ray photoelectron spectroscopy analysis reveals a significant increase in Cu + in the CuO x sensing channel by the indirect reductive sintering process, which enhances water adsorption and proton conduction at the CuO x /water molecule interfaces under humid conditions. Direct and indirect laser-induced reductive sintering of a CuO x thin film enable the fabrication of a transparent, flexible humidity sensor with a fully integrated single-layer Cu–CuO x –Cu architecture.