What Makes a Photobattery Light-Rechargeable?

The demand for autonomous off-grid devices has led to the development of "photobatteries", which integrate light-energy harvesting and electrochemical energy storage in the same architecture. Despite several photobattery chemistries and designs being reported recently, there have been few insights into the physical conditions necessary for charge transfer between the photoelectrode and counter electrode. Here, we use a three-electrode photobattery with a dye-sensitized TiO₂ photoelectrode, triiodide (I^-/I₃ ^-) catholyte, and anodes with varying intercalation potentials to confirm that photocharging is only feasible when the conduction band quasi-Fermi level (E_Fc) is positioned above the anode intercalation/plating potential. We also show that parasitic reactions after the battery is fully charged can be accelerated if the voltage of the battery and solar cell are not matched. The integration of multiple anodes in the same photobattery ensures well-controlled measurement conditions, allowing us to demonstrate the physical conditions necessary for charge transfer in photobatteries, which has been a topic of controversy in the field.