Biophotovoltaics: converting sunlight to electricity with photosynthetic protein complexes

Biophotovoltaics has emerged as a promising technology for generating renewable energy because it relies on living photosynthetic microorganisms as inexpensive, self-repair building blocks to produce electrical current from an abundant sunlight resource. Photosystem I (PSI) is a transmembrane, multi-subunit protein-chlorophyll complex, and its attractive features such as its ability to capture photonic energy, undergo charge separation, and transfer electrons efficiently make PSI an excellent candidate for utilizing PSI as an active layer in biophotovoltaic devices. The results presented in this Ph.D. project have demonstrated the versatility of PSI, a naturally abundant and highly functional photoactive protein complex for integration into biophotovoltaic devices.

In chapter 1, an introduction to the primary steps of photosynthesis is given. Moreover, the most important biological photoactive microorganisms involved in electron transfer steps in photosynthesis are highlighted. These multiprotein complexes are able to harvest light and perform charge separation. The efficiency of biophotovoltaic devices, however, has remained significantly lower than that of traditional photovoltaics. Extracting energy from light-harvesting photosynthetic complexes and generating practical efficiencies poses many challenges. In Chapter 1, further improvements in the development of high-performance biophotovoltaics which requires strategies to minimise these challenges have been discussed.

In chapter 2, the techniques used to fabricate biophotovoltaic devices studied in this Ph.D. project are described. These include the extraction of PSI complexes from cyanobacteria and plants such as spinach leaves. In this chapter, a detailed description of the fabrication of soft, stretchable biophotovoltaic devices using fabricated polydimethylsiloxane (PDMS) microchannels, as well as a design of solid-state biophotovoltaic devices have been explained. Additionally, this chapter includes a description of the types of equipment for the different measurements of BPV devices.

In chapter 3, we demonstrated that the photocurrent production from monolayers of PSI on nanostructured gold electrodes can be improved by controlling the orientation of the PSI complexes. We compared the degree of orientation on self-assembled monolayers of phenyl-C61-butyric acid (PCBA) and a custom peptide which are able to direct PSI trimers on gold surfaces. In order to enhance the quantity of useful electron transfer, the PSI trimers were oriented with the stromal side down to the electrode. We used a soft-lithography approach to prepare our devices in which a nanostructured gold electrode was embedded in microfluidic channels filled with a redox couple and a liquid metal electrode to complete a dye-sensitized biophotovoltaic device. Biophotovoltaic devices fabricated with the C61 fulleroid exhibit significantly improved performance and reproducibility compared to those utilizing the peptide. By using PCBA and custom peptide linkers, we could control the orientation of the protein complexes. Moreover, the unique architecture of BPV devices enables self-regeneration; by circulating fresh photosystem complexes through the devices, inactive complexes are replaced by active complexes via self-assembly, which we demonstrate by measuring the photocurrent generation over a period of two weeks.

In chapter 4, PSI was incorporated into solid-state biophotovoltaic cells. PSI was used as the photoactive component and was immobilized on polytyrosine-polyaniline as a hole extraction layer on ITO electrodes. The PSI monolayer was covered with an electron-extraction layer of reduced graphene oxide with gold nanoparticles (rGO-Au). The unique band alignment between PSI, electron, and hole transfer layers in solid-state biophotovoltaic cells facilitates the extraction of photo-generated electrons without sacrificing other parts of the power conversion process, and therefore charges could efficiently pass through the BPV devices. The combination of reduced graphene oxide with gold nanoparticles caused to tailor the electronic structure and aligns the energy levels while also increasing the electrical conductivity of the devices. The best-performing device reached an external power conversion efficiency of 0.64%. This work establishes a foundation for utilizing the unique properties of graphene-based materials decorated with metal nanoparticles in future biophotovoltaic devices.

In chapter 5, we compared large-area SAM-based tunneling junctions of PSI which are self-assembled on the surface of mica substrates via PCBA linkers using two different PSI protein complexes extracted from cyanobacteria and spinach leaves. The stability measurements of the PSI complexes from the thermophilic cyanobacteria Thermosynechococcus elongatus to PSI complexes extracted from spinach leaves over room temperature are compared. Moreover, EGaIn measurements, from SI{130}{kelvin} up to close to the protein’s denaturation temperature, all show temperature-independent electron transport behaviour for PCBA//PSI junctions. The comparison between these junctions using two different PSI complexes show that they do not suffer from leakage currents, enable temperature variable measurements down to 130 Kelvin, have excellent current retention characteristics, and are stable for at least 4 months.