Biophotovoltaics—Converting Sunlight to Electricity with Photosynthetic Protein Complexes

The incorporation of light-harvesting photosynthetic proteins into device architectures has attracted tremendous attention in the past years. Biophotovoltaic devices utilize the components of natural photosynthetic proteins such as Photosystem I/II (PSI/II) to generate electrical energy under illumination by exploiting light-harvesting pigments as a photosensitizer for light-induced charge separation. Photosynthetic proteins have several desirable characteristics for inclusion into biophotovoltaic devices including rapid charge separation, carbon neutral production, non-toxic, high internal quantum efficiency approaching unity, and highly abundant starting materials. In different works, we have explored several unique strategies to improve interfacial electron transport between PSI proteins and electrodes. In our first work, we demonstrated that self assembled monolayers of simple fullerene derivatives are capable of orienting phtosystem I complexes such that they form a monolayer that injects electrons into the fullerene layer when irradiated. We formed these assemblies on nano structured electrodes embedded in microfluidic channels filled with a redox couple and a liquid metal electrode to complete a dye-sensitized biophotovoltaic device. These proteins guide the photosystem to the electrode such that they are in direct contact, allowing us to compare the efficiency of charge injection with and without the mitigation of the fullerenes. We find that the fullerenes reduce recombination in the complexes, presumably by acting as selective contacts. Moreover, the unique architecture of these devices enables self-regeneration; by circulating fresh photosystem complexes through the device, inactive complexes are replaced by active complexes via self-assembly. Also, self-regeneration is a key feature of these devices because it mimics the replenishment process in biological systems using unmodified, wild-type photosystem I and does not require any encapsulation or other interventions. The performance of devices fabricated using a fullerene, phenyl-C61-butyric acid (PCBA), was superior as determined via the short circuit current, open circuit photovoltage, and power output. The PCBA linkers self-assemble onto the bottom electrodes, simultaneously directing the orientation of PSI and facilitating the collection of photo-generated electrons. In another work, we reported a power conversion efficiency to date of 0.64% in a solid-state biophotovoltaic device by tailoring the transport layers to maximize the extraction of charges from a layer of photosystem I. Specifically, we inserted complexes of gold nanoparticles into reduced graphene oxide to adjust the level alignment without sacrificing carrier mobility. We paired that layer with a polytyrosine-polyaniline hole-extraction layer. The active layer is fabricated by self-assembly, requiring only contact with a solution containing photosystem I. We achieved an overall increase in efficiency by raising the short-circuit current without sacrificing fill-factor or open-circuit voltage. This work establishes a foundation for utilizing the unique properties of graphene-based materials decorated with metal nanoparticles in future biophotovoltaic devices. Furthermore, in other research, solid-state charge transport measurements via PCBA/PSI junctions were carried out at room temperature and down to 150 kelvin. The junctions comprising PCBA/PSI demonstrated excellent stability and reproducibility on different days during around 4 months of measurements over room temperature. The efficient electron transfer through PSI protein complexes opens up possibilities for using such protein complexes as current-carrying elements in solid-state bioelectronic devices.