Operation Limits of Integrated Photo-Rechargeable Batteries.

Monolithic photo-rechargeable batteries (PRB) are attractive solution for powering off-grid autonomous systems. However, the fundamental effects limiting the light-charging process in such devices are not well understood. Herein, we present an integrated PRB design that can be fully charged under a range of illuminances. We use it as a model system to correlate the decay in photo-charging current with photo-induced charge kinetics. Our results indicate that light-induced hole transport gradually deteriorates with increasing state of charge, which is attributed to the anion-coupled hole accumulation in the cathode layer. Furthermore, our device reveals that the potential gap between the hole transport layer and the cathode is critical for driving a photo-induced delithiation of the cathode. If the cathode has a more positive delithiation potential than the hole transport level, the photo-charging current rapidly decays despite the photo-cells providing sufficient voltage to charge the battery. These findings demonstrate that the device physics of PRBs vary greatly from that of separately coupled solar cells and batteries, thus providing new insights in their working mechanism and their future design guidelines.