The performance of dye-sensitized solar cells (DSSCs) is partly governed by dye molecule orientation and molecular interaction with the substrate. Therefore, being able to control the orientation of such molecules can improve traditional DSSCs and, further, allow the growth direction of thin-film metal-organic frameworks (MOFs) on substrates to be controlled as well. In principle, MOFs can be used in photovoltaic applications as the active layer to harvest light, impose order, and prevent aggregation. Herein, we employ DFT calculations and MD simulations to investigate the effect of substrate, solvent, and protonation state on the orientation of dye molecules and MOF building blocks relevant to DSSCs. On dry substrates, it is predicted that most carboxylic-acid-containing molecules prefer to bind parallel to the surface, as opposed to perpendicular where the carboxylic-acid group is covalently bonded to the substrate. Once solvation effects are considered, however, the molecules are predicted to orient perpendicular to the surface. To aid in prediction of MOF linker and dye molecule orientation, a model was developed that reasonably predicts a binding preference based on the size of the adsorbing molecule and the surface/solvent environment. From this model, the size limit for which parallel binding begins to dominate over perpendicular binding can be predicted. 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.

Effect of solvent and substrate on dye molecule orientation for DSSC applications

254th National Meeting (2017)

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