Water Adsorption‐Induced Color Sensor: Insight Into the Sensing Mechanism and Interfacial Engineering for Improved Responsiveness

ABSTRACT Color‐sensitive photodetectors are at the forefront of next‐generation light‐ and image‐sensing research. In this context, a photovoltaic device employing SbSI:Sb 2 S 3 and TiO 2 as the visible light absorber and electron transport material, respectively, exhibits a unique wavelength‐dependent photovoltage effect and its reversible output in response to changing humidity levels. However, the specific functions of the individual layers of the device and their interaction with water remain largely unknown. In this study, we perform in situ X‐ray absorption spectroscopy (XAS) of thin films under humidity‐controlled conditions to reveal the interaction between water and the photovoltaic layers. The oxygen K‐edge XAS data reveal the adsorption of water molecules onto TiO 2 . Ultraviolet (UV) irradiation of TiO 2 can trigger photochemical reactions of the adsorbed water at the interface, which is considered to contribute to changes in the photovoltage. Based on this finding, we introduce hydrophilic layers between TiO 2 and SbSI:Sb 2 S 3 to promote the accumulation of water in the devices, which successfully enhance both the wavelength sensitivity and response speed. Finally, we demonstrate color recognition with various irradiation intensities, thereby broadening the scope and applications of unique wavelength‐responsive single‐cell photovoltaics.