Microfractures in TiO2 Enhance the Photoelectrochemical Properties of Photosystem I Bio-Sensitized Solar Cells

Climate change, caused by the increasing energy demand from fossil fuels due to population growth and economic expansion, triggers the need for clean, renewable, and sustainable energy sources. The solar energy contributes to approximately 99.9% of the Earth's energy. Therefore, photovoltaic technology is considered a possible substitute for fossil fuels owing to its abundance, but the current silicon and thin film photovoltaic (PV) technologies generate greenhouse gas emissions (GHE) during manufacturing and use scarce and polluting materials. Bio-sensitized solar cells (BSSCs) offer a promising solution using simple, lower energy-demand manufacturing processes and abundant, non-toxic materials. BSSCs are derived from dye-sensitized solar cells (DSSCs) technology by replacing the synthetic dye with biological molecules, such as photosystem I (PSI), into a thin film device. We report a BSSC using PSI as a sensitizer, TiO2 as a wide-bandgap semiconductor, Co-bipyridine compound as a redox mediator in the electrolyte, and PEDOT-CNT composite as the counter electrode. This study investigates how microfractures on the TiO2 thin film induced during thermal sintering affect the photoelectrochemical performance of the BSSCs. Electrochemical characterization of the PSI-BSSCs was performed under a solar simulator using linear sweep voltammetry (LSV), open-circuit voltage decay (OCVD), chronoamperometry, and electrochemical impedance spectroscopy (EIS). SEM micrographs reveal fissures in the photoanode surface induced by the fast heating and cooling during thermal treatments. Surprisingly, the microfractures improved the performance of the solar cells and will be discussed regarding the accessibility of PSI and the mediator to penetrate the nanostructured electrode and enhance the photocurrents generated. The BSSCs improvement due to the microfractures is ascribed to the enabled diffusion of the PSI sensitizer through the nanoporous TiO2, given the significantly larger size of this biological macromolecule than traditional synthetic dyes used in DSSCs.