AbstractConverting sunlight into a storable form of energy by spontaneous water splitting is of great interest but the difficulty in simultaneous management of optical, electrical, and catalytic properties has limited the efficiency of photoelectrochemical (PEC) devices. Herein, we implemented a decoupling scheme of light harvesting and electrocatalysis by employing a back-buried junction (BBJ) PEC cell design, which enables >95% front side light-harvesting, whereas the electrochemical reaction in conjunction with carrier separation/transport/collection occurs on the back side of the PEC cell. The resultant silicon BBJ-PEC half-cell produces a current density of 40.51 mA cm⁻² for hydrogen evolution by minimizing optical, electrical, and catalytic losses (as low as 6.11, 1.76, and 1.67 mA cm⁻², respectively). Monolithic fabrication also enables three BBJ-PEC cells to be connected in series as a single module, enabling unassisted solar water-splitting with a solar-to-hydrogen conversion efficiency of 15.62% and a hydrogen generation rate of 240 μg cm⁻² h⁻¹.