To address the challenge in sustainable global development, considerable effort has been made to produce fuels from renewable resources with photocatalysts and photoelectrochemical cells (PECs) by harvesting solar energy. The solar energy conversion efficiency of photocatalysts and PECs is strongly dependent on the light absorption, charge separation, charge migration, charge recombination processes and (electro)catalytic activity in photoactive semiconductors. This perspective article describes the barrier, progress and future direction of research on the correlation of the chemical constituent, size, dimensionality, architecture, crystal structure, microstructure and electronic band structure of photocatalysts (or photoelectrodes) with five vital processes including light absorption, charge separation, migration and recombination as well as surface redox reactions. This article deals with both single materials and composites such as co-catalysts on photoelectrodes/photocatalysts, dye-sensitized or plasmon-enhanced photocatalysts, semiconductor–semiconductor heterostructures, semiconductor–carbon hybrids as wells as Z-scheme and tandem cells. This article also highlights the application of representative photocatalysts and PECs in solar water splitting.