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Janna Domenico1 Michael Foster2 Erik Spoerke3 Mark Allendorf4 Karl Sohlberg1

1, Department of Chemistry, Drexel University, Philadelphia, Pennsylvania, United States
2, Department of Materials Physics, Sandia National Laboratories, Livermore, California, United States
3, Department of Electronic, Optical, and Nano Materials, Sandia National Laboratories, Albuquerque, New Mexico, United States
4, Department of Energy and Transportation Technology Center, Sandia National Laboratories, Livermore, California, United States

The efficiency of dye-sensitized solar cells (DSSCs) is strongly influenced by dye molecule orientation and interactions with the substrate. Therefore, understanding the factors controlling the surface orientation of sensitizing organic molecules will aid in the improvement of both traditional DSSCs and other devices that integrate molecular linkers at interfaces. Herein, we employ DFT calculations and ab initio molecular dynamics simulations to investigate the effect of substrate, solvent, and protonation state on the orientation of linker molecules relevant to DSSCs. In the absence of solvent, we predict that most carboxylic acid-functionalized molecules prefer to lie flat (parallel) on the surface, due to van der Waals interactions, as opposed to binding at a tilted orientation with respect to the surface that is favored by covalent bonding of the carboxylic acid group to the substrate. Once solvation effects are considered, however, most molecules are predicted to orient perpendicular to the surface. This approach can be extended to help understand and guide the orientation of metal–organic framework (MOF) thin-film growth on various metal–oxide substrates. Finally, a two-part analytical model is developed that predicts the binding energy of a molecule by chemical and dispersion forces on rutile and anatase TiO2 surfaces, and quantifies the dye solvation energy for two solvents. The model is in good agreement with the DFT calculations and enables rapid prediction of dye molecule and MOF linker binding preference on the basis of the size of the adsorbing molecule, identity of the surface, and the solvent environment. Results suggest that linker orientation can be controlled by choice of adsorbate, substrate, and solvent; this novel approach can be used to achieve a desired linker orientation and, by extension, MOF growth orientation in a MOF-based DSSC.

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