Electron Transfer at Photosystem I - Electrode Interfaces: Porous & Translucent Indium Tin Oxide Cathodes

Abstract

Biohybrid photovoltaics leverage naturally occurring photo-active protein complexes to convert solar energy into electricity; offering a unique route to lowering the cost and environmental impact of solar energy conversion. Photosystem I (PSI), a protein complex found in higher plants, algae, and cyanobacteria, produces photoexcited excitons over a distance of approximately 10 nm with a 1.1 V photopotential. PSI has been utilized in a variety of biohybrid photovoltaics with success; however, improvements in performance are necessary to bring PSI – based devices to a technologically viable state. The electrons and holes produced within a PSI biohybrid photovoltaic must pass through multiple interfaces before being harnessed in an electrical circuit. Therefore, the total current, potential, and efficiency are dependent on losses incurred at each interface and could be vastly improved by optimizing individual electron transfer steps. Many researchers are looking toward direct electron transfer (DET) between the active sites of PSI and electrodes to reduce these losses. Unfortunately, achieving DET is difficult due to the necessity of proper PSI orientation and the close proximity (<1 nm) of the active sites to the electrode material for electron tunneling. Alternatively, mediated electron transfer (MET) can be employed through the use of soluble redox mediators to shuttle charge between the active sites and electrodes. MET can benefit from the use of high surface area electrodes and increased loadings of PSI. Our group has studied both DET and MET within PSI bioelectrodes of various architectures. In this presentation, the incorporation of PSI into porous and translucent indium tin oxide (ITO) cathodes to achieve significant improvements in MET will be highlighted. Additionally, the limitations and challenges of DET, directions for improving MET in porous electrodes, and future prospects for improving charge transfer at PSI – electrode interfaces will be discussed.