Spontaneous splitting of water into hydrogen and oxygen is of great attention for a clean and storable form of chemical fuels. Silicon, the most investigated material has a great potential to be employed as both the single photoelectrode and as a bottom absorber in a tandem photoelectrochemical (PEC) system. However, a major inadequacy in the simultaneous achievement of the optical, electrical management, and bandgap engineering limits the efficiency. Herein, to achieve concurrent enhancement of light-harvesting, surface protection and catalytic reactions, we implement a novel photon decoupling scheme to fabricate back buried junction photoelectrochemical (BBJ-PEC) cells by n-type crystalline Si (c-Si). The light decoupling scheme enables the maximum light-harvesting from the top surface without any metal contacts that avoids the shading effect, while the electrochemical reaction happens at the bottom side of the PEC cell. The resultant single-junction Si-based BBJ-PEC cell archives the current density of 41.76 mA/cm² for hydrogen evolution the best reported so far, with the open-circuit potential of 0.62 V and the half-cell solar-to-hydrogen conversion efficiency (SHCE) of 11.44 %. The outstanding results achieved here sets a record SHCE efficiency for single-junction Si-based PEC cells. By connecting the three BBJ-PEC cells in series, we have also realized an unassisted photoelectrochemical water-splitting with the solar-to-hydrogen conversion efficiency (STH) of 15.62 %, with 240 μg.cm^-2h^-1 of hydrogen. Finally, we have developed a model to understand the loss mechanisms involved in the PEC cell, with the comprehensive analysis on the absorptance, quantum efficiencies and absorbed photon-to-current efficiencies (APCE).