(General Student Poster Session Winner - 3rd Place) Increased Light Harvesting in Photosystem 1-Based Biophotovoltaics with Artificial Antenna
Authors: Takekuma Yuya , Nagata Morio 
Published: December 2020

Photosynthesis is a process used by algae and plants to convert light energy into chemical energy. In order to reduce the consumption of fossil fuels and curb global warming, the efficient use of sunlight by mimicking photosynthesis or directly using photosynthetic proteins is attracting attention. Photosynthetic proteins, which use highly complex and optimized processes that are impossible to reproduce, have been reportedly used in a variety of artificial applications such as biophotovoltaics, semi-artificial photosynthesis, biosensors, etc. These biomaterials are naturally abundant and pose no threat to the environment. In addition, the reaction center of a protein-pigment complex exhibits near-unity quantum efficiency attributable to spatial charge separation by multiple sequential electron-transfer pathways within the protein, which is a very attractive feature for researchers in the photoenergy-conversion field.
Photosystem 1 (PS1), a photosynthetic protein-pigment complexes with high charge separation quantum efficiency, has been used as a material for biophotovoltaics. However, the photocurrent of these biophotovoltaic devices is not high because of their low spectral response. In nature, to compensate for the low spectral response of the reaction centers of photosynthetic protein-pigments, förster resonance energy transfer (FRET) is used by the light-harvesting (LH) antennas. In this work, we have integrated an artificial LH antenna into a PS1-based biophotovoltaic in the form of a fluorescent dye, perylene di-imide derivative to expand the spectral response.
The biophotovoltaics were fabricated in the same way as dye-sensitized solar cells (DSSCs); a two-electrode sandwich cell was prepared with a stained-TiO2 photoanode, a Pt counter electrode, and a triiodide/iodide-based ionic liquid electrolyte. The fabrication of the stained-TiO2 photoanode was carried out by sequential immersion in two solutions of artificial LH antenna and PS1. The incident photon-to-current conversion efficiency (IPCE) spectra and the photocurrent-density–voltage (J–V) curve under AM 1.5 G (100 mW cm-2) were measured.
When PTCDI was used as artificial LH antenna, the magnitude of the IPCE spectrum is significantly enhanced in the range of 450–750 nm, and the photocurrent increased [1]. This range matches the absorption range of the PTCDI/PS1-assembled electrode; thus, the high-intensity part of the solar spectrum can be used to generate electric power. We measured the fluorescence spectra of the stained ZrO2 electrode. According to the fluorescence spectrum of the PTCDI/PS1-assembled ZrO2 electrode, a decrease in PTCDI fluorescent intensity at around 670 nm was observed. Time resolved fluorescence measurements show that the fluorescence of PTCDI decays faster in the presence of PS1 than in the absence. Moreover, PTCDI was previously reported to be quenched in proportion to the concentration of PS1, following the Stern–Volmer equation [2]. These results can be interpreted as evidence that the photons absorbed at PTCDI were transferred to PS1 via FRET.
The electrodes fabricated in this study are considered to be a simple stacked structure of artificial LH antenna and PS1. The advantage of this method is that there are many antennas on the electrode around PS1. Therefore, we believe that the electrode lead to more efficient not only in biophotovoltaics but also in other bio-applications.
[1] Y. Takekuma, H. Nagakawa, T. Noji, K. Kawakami, R. Furukawa, M. Nango, N. Kamiya, and M. Nagata ACS Applied Energy Materials, 2019, 2(6), 3986–3990.
[2] H. Nagakawa, A. Takeuchi, Y. Takekuma, T. Noji, K. Kawakami, N. Kamiya, M. Nango, R. Furukawa and M. Nagata Photochem. Photobiol. Sci., 2019, 18, 309–313.