AbstractPhotoelectrochemical CO₂ reduction reaction flow cells are promising devices to meet the requirements to produce solar fuels at the industrial scale. Photoelectrodes with wide bandgaps do not allow for efficient CO₂ reduction at high current densities, while the integration of opaque photoelectrodes with narrow bandgaps in flow cell configurations still remains a challenge. This paper describes the design and fabrication of a back-illuminated Si photoanode promoted PEC flow cell for CO₂ reduction reaction. The illumination area and catalytic sites of the Si photoelectrode are decoupled, owing to the effective passivation of defect states that allows for the long minority carrier diffusion length, that surpasses the thickness of the Si substrate. Hence, a solar-to-fuel conversion efficiency of CO of 2.42% and a Faradaic efficiency of 90% using Ag catalysts are achieved. For CO₂ to C₂₊ products, the Faradaic efficiency of 53% and solar-to-fuel of 0.29% are achieved using Cu catalyst in flow cell.