Stacked photoanodes for solar water splitting: decoupling light harvesting and charge transport through light reuse and spectral complementarity

as representative metal-oxide systems that illustrate distinct motivations for stacking and have been experimentally demonstrated in stacked configurations. Stacked configurations are classified into homo-type stacks using the same semiconductor and hetero-type stacks combining different semiconductors. Homo-type stacks reuse transmitted light but show diminishing photocurrent gains because of front-layer shading and optical losses. Hetero-type stacks combine complementary absorbers to extend spectral utilization and enable photocurrent summation without the strict band-alignment requirements of conventional heterojunctions. Key performance factors include individual photoanode transparency, optical losses, and the balance between front absorption and rear transmission. Large-area scaling further introduces current-collection and fabrication challenges. Future opportunities lie in absorber selection, optical management, long-term stability, and scalable device engineering.