Photoelectrochemical (PEC) carbon dioxide (CO ₂ ) reduction (CO ₂ R) holds the potential to reduce the costs of solar fuel production by integrating CO ₂ utilization and light harvesting within one integrated device. However, the CO ₂ R selectivity on the photocathode is limited by the lack of catalytic active sites and competition with the hydrogen evolution reaction. On the other hand, serious parasitic light absorption occurs on the front-side–illuminated photocathode due to the poor light transmittance of CO ₂ R cocatalyst films, resulting in extremely low photocurrent density at the CO ₂ R equilibrium potential. This paper describes the design and fabrication of a photocathode consisting of crystal phase-modulated Ag nanocrystal cocatalysts integrated on illumination-reaction decoupled heterojunction silicon (Si) substrate for the selective and efficient conversion of CO ₂ . Ag nanocrystals containing unconventional hexagonal close-packed phases accelerate the charge transfer process in CO ₂ R reaction, exhibiting excellent catalytic performance. Heterojunction Si substrate decouples light absorption from the CO ₂ R catalyst layer, preventing the parasitic light absorption. The obtained photocathode exhibits a carbon monoxide (CO) Faradaic efficiency (FE) higher than 90% in a wide potential range, with the maximum FE reaching up to 97.4% at –0.2 V vs. reversible hydrogen electrode. At the CO ₂ /CO equilibrium potential, a CO partial photocurrent density of –2.7 mA cm ⁻² with a CO FE of 96.5% is achieved in 0.1 M KHCO ₃ electrolyte on this photocathode, surpassing the expensive benchmark Au-based PEC CO ₂ R system.