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Claudia Villarreal1 3 David Rangel4 Duyen Pham2 Alejandro Martínez-Brenes3 Senthil Prasad7 Venkatesan Renugopalakrishnan6 5 Ashok Mulchandani4 1

1, University of California Riverside, Riverside, California, United States
3, Costa Rican Institute of Technology, Catago, , Costa Rica
4, Univeristy of California, Riverisde, Riverside, California, United States
2, University of California, Riverside, Riverside, California, United States
7, Institute of Microbial Technology, Chandigarh, , India
6, Havard Meidcal School, Boton, Massachusetts, United States
5, Northeastern University, Boston, Massachusetts, United States

This work contributes to the development of bio-sensitized solar cells (BSSC) by studying the photoactive protein bacteriorhodopsin (BR) as the photoanode sensitizer to replace the common expensive and toxic Ru dyes. Substrates of ZnO nanoparticles (NPs) spread over fluorione-doped tin oxide (FTO)-coated glass or graphene-coated glass are used to immobilized BR by three different techniques: dropcasting, pheniltriethoxysilane (PTES) chemical functionalization, and electrophoretic sedimentation. The photoactive functionality of the protein after immobilization is assessed through ultraviolet visible absorption spectroscopy (UV-VIS). The performance of the photoanodes is determined by chronopotentiometry, linear sweep voltammetry and electrochemical impedance spectroscopy (EIS) under illumination. Preliminary results show a high charge transfer resistance of the photoanodes in the iodide electrolyte extracted from EIS provided relative high values (105 Ω.cm2) for the three methods, in comparison with the values commonly reported for photoanodes prepared with Ru-based dyes (102 Ω.cm2). The molecular layer of PTES has been found to reduce the time required to impregnate BR, but increased the resistance and slowed down the kinetics of charge transfer, thus it is not appropriate for the desired application. The photoanodes prepared by electrophoretic method provide higher photovoltage than those prepared by dropcasting, indicating that the orientation of the protein on ZnO is determinant in the photovoltaic performance. In electrophoresis the electric field applied causes the protein molecules to orient uniformly according to their electric dipole, but with the dropcasting technique, the orientation is random and uncontrollable, resulting in counteracting of individual charge transfer processes. We are working on reducing the large charge transfer resistance, for example by using different electrolyte system, optimize a monolayer formation and increasing the surface area coated with BR.

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