Novel Photovoltaic Applications of Photosystem I Multilayer Films

Photosystem I (PSI) is a ~500 kDa photocatalytic membrane protein complex contained within the thylakoid clusters of plant chloroplasts. Boasting a 1.1 V light-induced charge separation, PSI provides high-energy electrons needed for NADPH production within the cells of plants and cyanobacteria. The ubiquity and unique optoelectronic properties of PSI have made it an interesting prospect as a nanoscale photoactive component in nature-inspired “biohybrid” solar cell devices.

More recently, multilayer films of PSI have been applied to surfaces for significant improvements in photovoltaic efficiency over monolayer based systems. Purified solutions of PSI proteins are drop cast atop a surface, resulting in a dense, ion-permeable film of stabilized macromolecules. Each protein unit contains a redox-active reaction center internally photo-sensitized by tightly bound core chromophores. The basis for enhancement is rapid photocatalytic cycling of redox-active ions between reduced and oxidized states, culminating in a large and prolonged charge asymmetry within the protein film. When coupled to an electroactive substrate, heterogeneous charge equilibration results in a net exchange of carriers and sustained photocurrent. The enhancement phenomenon is preserved over periods exceeding 180 days (beyond the normal growing season of plants) and increases with the amount of protein deposited.