The solar energy conversion efficiency of photoelectrochemical (PEC) devices is usually limited by poor interface energetics, limiting the onset potential, and light reflection losses. Here, a three-pronged approach to obtain excellent performance of an InP-based photoelectrode for water reduction is presented. First, a buried p-n+ junction is fabricated, which shifts the valence band edge favorably with respect to the hydrogen redox potential. Photoelectron spectroscopy substantiates that the shift of the surface photovoltage is mainly determined by the buried junction. Second, a periodic array of InP nanopillars is created at the surface of the photoelectrode to substantially reduce the optical reflection losses. This device displays an unprecedented photocathodic power-saved efficiency of 15.8% for single junction water reduction. Third, a thin TiO2 protection layer significantly increases the stability of the InP-based photoelectrode. Careful design of the interface energetics based on surface photovoltage spectroscopy allows obtaining a PEC cell with stable record performance in water reduction. The efficiency of photoelectrochemical (PEC) water splitting devices is usually limited by interface energetics mismatch between the semiconductor and the electrolyte and insufficient light absorption. The highest photocathodic power-saved efficiency of 15.8% reported so far for single junction PEC water reduction is obtained through interface energetics design and photon management of an InP-based photoelectrode.