Nanometer-Thick Films of Cobalt Phthalocyanine/Indium-Gallium-Zinc Oxide Heterojunctions for Ultraviolet-Assisted Recoverable NO₂ Gas Sensors

Nitrogen dioxide (NO₂) is a major source of air pollution and is known to cause severe respiratory diseases and lung cancer. A high-accuracy and reliable gas sensor is, therefore, essential for effectively tracking and managing human exposure to NO₂. Herein, we demonstrate highly sensitive and NO₂ selective gas sensors operating at room temperature under ultraviolet (UV)-light illumination. The gas sensor, based on a p-n heterojunction structure, was fabricated by vertically stacking a nanometer-thick layer of p-type cobalt phthalocyanine (CoPc) on a solution-processed n-type indium-gallium-zinc oxide (IGZO) film. The upper CoPc layer interacts directly with the gas molecules, causing changes in the charge distribution and depletion region at the CoPc/IGZO interface. This interaction significantly contributes to the sensing properties of the sensor. With UV activation, the CoPc/IGZO sensor demonstrated high sensitivity to NO₂ gas with fast response and full recovery behaviors. At an optimized UV intensity of 20 mW/cm², the sensor achieved a high sensing response of 66.52% upon exposure to 5 ppm NO₂ gas, with a low detection limit of 150 ppb. Additionally, the sensors exhibited long-term stability and robustness under high-temperature and humidity conditions. Furthermore, high selectivity to NO₂ gas was observed, attributed to the strong interaction between CoPc and NO₂ gas molecules, as investigated using density functional theory. The electron withdrawal by the NO₂ molecule facilitates the depletion region at the CoPc/IGZO interface, resulting in a decrease in the sensing current. In contrast, electron donation from ammonia and hydrogen sulfide narrows the depletion region, leading to a higher conductivity channel and an increase in current. These results demonstrate that the CoPc/IGZO sensor is a promising platform for NO₂ detection in environmental applications.