A framework for understanding efficient diurnal CO₂ reduction using Si and GaAs photocathodes

Integrated solar fuels and photoelectrochemical (PEC) CO₂ reduction (CO₂R) are promising pathways toward producing value-added chemicals from CO₂. However, improvements are needed in activity and selectivity as well as in fundamental understanding of device behavior to engender wide deployment. We report two single-junction, integrated photocathodes for PEC CO₂R based on TOPCon Si and GaAs substrates, which achieve −10 mA cm⁻² at −0.33 V vs. RHE with 41% selectivity to C₂₊ products and at −0.03 V vs. RHE with 27% selectivity to C₂₊ products, respectively. We investigated the viability of a light-mediated strategy to direct selectivity in buried-junction PEC devices and confirmed that these devices could be optimized independently and described by the physics-based models of the individual components. Finally, we designed a framework to assess operational modes for PEC CO₂R devices and demonstrated this framework under continuous galvanostatic control and variable illumination conditions.