The overlayers were characterized through SEM measurements, observing changes in both the morphology of the back contacts (Supplementary Fig. ) and the front layers (Supplementary Fig. ). The thickness and deposition of each layer were further verified by cross-sectional transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. The cross-sectional TEM showed conformal layers of the 15 nm Ga2O3, 100 nm TiO2, 150 nm ITO on the back contact (Fig. ), and the 100 nm Au and 50 nm Ni layers were also well-deposited on the front layer of the Cu2O sheet (Fig. ). The 1 nm Al2O3 layer was too thin to be observed directly (Fig. ), but its presence was confirmed through X-ray photoelectron spectroscopy (XPS) analysis. In the XPS analysis, we identified the Al 2p doublet on the Cu2O electrode with the 1 nm Al2O3 layer at 74.7 eV and 77.1 eV (Supplementary Fig. ). Additionally, when the 100 nm Au and 50 nm Ni layers were further deposited, the Au 4f and Ni 2p doublets appeared, while the peaks of the underlying layers disappeared (Supplementary Fig. ). In the XPS measurements of the back contact layers, Ga 2p (1118.1 eV), Ti 2p (458.4 eV and 464.2 eV), and In 3d (444.4 eV and 452.0 eV) peaks corresponding to Ga2O3, TiO2, and ITO were also observed (Supplementary Fig. ).