While solar cells are becoming increasingly more efficient, intermittency remains a key issue. A potential solution to this problem is the green hydrogen economy, in which solar power is used to convert water into molecular hydrogen H2, which can be stored as a chemical fuel. While promising, the efficiency, stability and cost of solar water splitting devices remain significant challenges. To attain higher efficiencies, the III-V semiconductor alloys InGaAsP and AlGaAs were investigated as narrow-gap and wide-gap materials, respectively, for high-performance tandem cells, with both materials demonstrating very useful photoelectrochemical properties. To improve the stability of immersed solar water splitting devices, earth-abundant cocatalyst foils were used to construct decoupled photoelectrodes, facilitating the integration of earth-abundant cocatalysts with chemical-sensitive photoabsorbers. This method was applied to Si and GaAs artificial leaves, which were highly efficient and remained stable for several days. Cocatalyst foils were also applied to a triple-junction device with multiple cells and electrolysers. By adjusting the ratio of cells to electrolysers, a remarkable solar-to-hydrogen efficiency of over 20% was achieved.