  Photosynthesis is nature's route to convert intermittent solar irradiation
into storable energy, while its use for an industrial energy supply is impaired
by low efficiency. Artificial photosynthesis provides a promising alternative
for efficient robust carbon-neutral renewable energy generation. The approach
of direct hydrogen generation by photoelectrochemical water splitting utilises
customised tandem absorber structures to mimic the Z-scheme of natural
photosynthesis. Here, a combined chemical surface transformation of a tandem
structure and catalyst deposition at ambient temperature yields photocurrents
approaching the theoretical limit of the absorber and results in a
solar-to-hydrogen efficiency of 14%. The potentiostatically assisted
photoelectrode efficiency is 17%. Present benchmarks for integrated systems are
clearly exceeded. Details of the in situ interface transformation, the
electronic improvement and chemical passivation are presented. The surface
functionalisation procedure is widely applicable and can be precisely
controlled, allowing further developments of high-efficiency robust hydrogen
generators.
