Tandem Cascade Photoelectrochemical Devices

Cascade photoelectrocatalysis (PEC) is a possible method to improve the selectivity of solar-driven CO₂ reduction (CO₂R). This concept can be realized by coupling different CO₂R catalysts to different subcells in a multijunction photovoltaic (PV) stack. Efficient implementation will require careful tuning of the photocurrents and design of the photovoltages provided by the subcells to the CO₂R catalysts in such a way as to facilitate the target reaction. Here, we outline the design principles of the tandem PEC approach using two-step conversion of CO₂ to ethylene in aqueous, via a CO intermediate as a model system and use the open-source circuit simulator, SPICE, to simulate operating conditions. Cascade PEC can be realized in a three-terminal tandem (3TT) configuration using III-V-semiconductor based subcells coupled to Au (produces CO intermediate) and Cu (converts CO to ethylene). We identify conditions under which a 3TT configuration can have a higher solar to chemical conversion efficiency compared to a two-terminal two-junction tandem (2T 2J) with the same absorbers and a Cu catalyst only and demonstrate that 3TT PEC devices can be less sensitive to variations in catalyst activity compared to 2T devices. Three terminal tandem devices also allow for light and potential activated independent control of each subcell. To demonstrate this control, we made a GaInP/GaAs 3TT solar cell with different catalysts coupled to each site, each driving different reactions who’s respective <i>E</i>⁰ values differ (one produces the intermediate). Using different colored LED light sources and varying the potential applied while tracking the product current with online mass spectroscopy, we show independent and cascade control of the reduction sites on the 3TT PEC device.