Comparative interface metrics for metal-free monolayer-based dye-sensitized solar cells

The first quantitative comparison between self-assembled monolayers of homologous carboxylate-and phosphonate-terminated organic dyes that are of use in dye-sensitized solar cells (DSSCs) is reported. (Cyanovinyl)phosphonate- terminated oligothiophenes and (cyanovinyl)carboxylate-terminated oligothiophenes were synthesized on TiO₂ thin film electrodes. Structurally analogous organics were compared for the effect of the anchoring groups on photochemical properties in solution as measured by UV/vis spectroscopy and for reactivity with the electrode surface. Monolayers were grown on the TiO₂ electrodes either by "tethering by aggregation and growth" (T-BAG) or by solution dipping. Surface roughness and homogeneity, elemental composition, and thickness of the monolayers were evaluated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ellipsometry. Molecular loadings for each monolayer on TiO ₂ were quantified by quartz crystal microgravimetry (QCM), and the stability of bonding between each class of dyes and the TiO₂ was evaluated by measuring desorption, also by QCM; the carboxylates underwent significant dissociation in aqueous media but the phosphonates did not. DSSCs were prepared from each congener and from simple oligothiophene phosphonates to determine the effect of the cyanovinyl group on device behavior; all DSSCs were studied under irradiation from a AM 1.5G solar light source; the effect of cyanovinyl group termination was comparable to that of adding a thiophene moiety, and the DSSC using a self-assembled monolayer of (sexithiophene) phosphonate (6TP) had total power conversion efficiency (η) of ca. 5%.