Self-Powered CuO/Cu 2 O Heterojunction Photodetector for Blood-Oxygen Monitoring Towards Exercise Fitness Assessment

In the field of sports training and physical fitness monitoring, noninvasive and real-time blood oxygen concentration detection serves as a critical approach to evaluating athletes’ aerobic metabolic capacity, exercise fatigue level and training load adaptability. However, conventional devices generally suffer from insufficient the requirement of external power supply, and poor spectral matching with blood oxygen detection, which restrict their practical application in real sports settings. In this work, a CuO/Cu 2 O layered heterojunction thin film was fabricated on Cu foil via a facile one-step air annealing method, and a graphene/CuO/Cu 2 O/Cu heterojunction photodetector was further constructed by transferring monolayer graphene as the top transparent electrode. The phase composition, chemical valence states, layered microstructure and interfacial characteristics of the heterojunction were systematically characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. The results confirm that the annealed Cu foil forms a sandwich structure consisting of an outer CuO layer (∼3 μm), an intermediate Cu 2 O layer (∼10 μm) and a Cu substrate. The constructed Cu 2 O/CuO heterojunction generates a built-in electric field. Driven by this built-in electric field, a self-powered photodetector is achieved, which delivers excellent spectral selectivity for 600–900 nm light, a responsivity up to 80 mA/W for 880 nm, and a response speed at the microsecond scale. Our device offers a new strategy for the iteration of conformable devices for practical blood oxygen detection.