ABSTRACT
Stabilization and reduction of surface recombination opened the way to efficient and direct photoelectrochemical cells for producing hydrogen. The most efficient photoelectrodes for conversion of solar energy to hydrogen are made from high-quality, defect-free semiconductors with non-leaking barriers and with surface recombination passivated by a controlled semiconductor surface chemistry. Thus, a catalysed hydrogen-generating photocathode is most efficient when it is enveloped by hydrogen; a catalysed oxygen- or halogen-evolving photoanode is most efficient when its products reactively eliminate traces of hydrogen from the catalyst. In photoanode-based cells hydrogen is evolved at their non-illuminated, catalyst-activated counter-electrodes. The only commercial product-oriented engineering attempt at electrochemical solar cells in the United States utilized a pair of silicon p–n and n–p junctions. For efficient solar-to-hydrogen conversion, the photogenerated charge carrier reaction rates must be high relative to the rates of recombination and of leakage across the barrier.
