The photoelectrochemical carbon dioxide reduction reaction (PEC-CO 2 RR) allows us to convert solar power to chemical energy with photosensitive materials. Semiconductor-based, plasmon-assisted, and dye-sensitized systems have been extensively investigated in the PEC-CO 2 RR. Beyond the remarkable progress in materials science, it is expected to realize new systems for satisfying the needs of both research discoveries and industry applications. In this Perspective, we summarize the latest progress in the field of the PEC-CO 2 RR, focusing on enhancing efficiencies via matching photoelectron flux, charge transfer rate, and mass transport rate. Based on the principles for the state-of-the-art PEC-CO 2 RR, new designs of the system engineering strategies on photoelectrode fabrication, reactor design, electrolyte optimization, and membrane selection are proposed to enhance selectivity and stability of the PEC-CO 2 RR.

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