New Materials Interfaces and Modeling for Photosystem I-Based Biohybrid Solar Cells
Robinson, Maxwell Thatcher
Photosystem I (PSI) is a naturally abundant protein photodiode that sustains a 1.1 V light-induced reduction potential with nearly perfect internal quantum efficiency. Due to its abundance, stability, and optoelectronic properties, PSI has been extracted from photosynthetic organisms—including plants and cyanobacteria—and paired with various redox chemistries to form a biohybrid solar cell. Recently, researchers have sought to improve the performance of PSI-based biohybrid solar cells through use of three-dimensional assemblies of PSI that offer increased absorption cross-section. This dissertation offers mechanistic insights into photocurrent production of PSI multilayer films on electrode surfaces and describes two new materials systems designed to augment performance. First, while the photocatalytic effect of PSI multilayers has been theorized as an electrolyte-mediated mechanism, no comprehensive, first-principles study has been presented. Here, an electrochemical reaction-diffusion model is developed and optimized to simulate the significant electrochemical, physicochemical, and transport processes that underpin photocurrent development of a PSI multilayer film. The model is used to provide strong evidence that PSI’s terminal co-factors rapidly exchange electrons with diffusible mediators and stimulate photocurrent principally due to alteration of mediator concentrations at a solution-electrode interface as governed by Butler-Volmer kinetics. Once fitted to experimental data, the model accurately simulates photocurrent trends with variable applied bias and PSI multilayer film thickness. Second, a new design for natural dye-sensitized solar cells is presented. A stacked assembly consisting of a PSI multilayer film atop a natural dye-sensitized photoanode permits reduced recombination kinetics to provide for a more than two-fold increase in cell photovoltage relative to the unmodified photoanode. PSI and the natural dye used have complementary absorbance and thus, this cell architecture expands utilization of the solar spectrum. Finally, a gentle method for incorporating intrinsically conductive polymers (ICPs)—including poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy)—into as-prepared PSI multilayer films is presented; the Friedel-Crafts catalyst, FeCl3, is added to PSI solutions used in drop casting, allowing for vapor-phase polymerization of a wide library of ICPs within the assembled film. We utilize IR spectroscopy to demonstrate negligible degradation of the protein’s secondary structure during preparation and polymerization steps. Additionally, photoelectrochemical analysis demonstrates that the assembly method does not disrupt the photocatalytic activity of a PSI multilayer, but rather boosts it for thicker films relative to PSI control films. We demonstrate that the PEDOT within PSI:vpPEDOT films shuttles holes from photoexcited PSI to permit unidirectional photocurrent production not possible for unmodified PSI films.