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

Photoelectrochemical (PEC) water splitting offers a sustainable route for direct solar-to-hydrogen conversion, but its efficiency is often constrained by limited light harvesting and inefficient charge transport in photoanodes. For α-Fe2O3 and BiVO4, increasing film thickness improves light harvesting but lengthens carrier-transport pathways and promotes recombination, whereas wide-bandgap TiO2 is constrained primarily by limited spectral coverage. Stacked photoanodes provide a device-level strategy by arranging multiple individual photoanodes sequentially along the incident light direction and connecting them in parallel, thereby enabling transmitted-light reuse while allowing each photoanode to retain an independently optimized thickness. This review summarizes recent progress in stacked photoanodes for PEC water splitting, focusing on α-Fe2O3, BiVO4, and TiO2 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.