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Subhabrata Das1 Bernardo Barbiellini-Amidei2 Xingyu Gao3 Steve Harvey4 Shinichiro Muramoto5 Kai Zhu7 Ponisseril Somasundaran1 Venkatesan Renugopalakrishnan6

1, Columbia University, New York, New York, United States
2, Lappeenranta University of Technology, Lappeenranta, , Finland
3, Chinese Academy of Sciences, Shanghai, , China
4, Materials Science Center | National Renewable Energy Laboratory, Golden, Colorado, United States
5, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
7, National Renewable Energy Laboratory, Golden, Colorado, United States
6, Northeastern University, Boston, Massachusetts, United States

Bacteriorhodopsin (bR) has been deposited on TiO2 and gold substrates separately. An ultraviolet photoemission experiment (UPS) reveals the energy-level alignment and the Fermi energy of the bR/TiO2 system while combined x-ray absorption measurements (XANES) and X-ray photoemission spectroscopy (XPS) indicate that in bR the lowest unoccupied molecular orbital (LUMO) is located about 2 eV above the highest occupied molecular orbital (HOMO). A separate Time of Flight Secondary In Mass Spectrometry (TOF-SIMS) study, conducted to characterize the attachment of bR to Au substrate, confirm successful Thiol-Gold bond formation with films thicker than 10 nm. Principal component Analysis (PCA) of TOF-SIMS data further separated the samples based on ratio peak intensities of C2H3+, C2H5+ to that of Ca2+, Cu2+.The HOMO-LUMO gap of retinal was spectroscopically determined to be 2.49 eV. For comparison, we also performed DFT calculations to determine the HOMO-LUMO gap of free retinal and Spiro OMe-TAD. Using the G-311G basis set, the calculated HOMO-LUMO gap was 2.69 eV for Retinal and 1.1 eV for Spiro respectively. The chromophore can be stabilized in the bR protein by the HOMO-LUMO interaction with the protein environment. Based on these spectroscopy results and DFT calculations, new solar cell architectures combining perovskite and bR are proposed and discussed.

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