Forward and back electron transfer at dye−semiconductor interfaces are pivotal events in dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. Here we introduce self-assembled bilayers as a strategy for manipulating electron transfer dynamics at these interfaces. The bilayer films are achieved by stepwise layering of bridging molecules, linking ions, and dye molecules on the metal oxide surface. The formation of the proposed architecture is supported by ATR-IR and UV−vis spectroscopy. By using time-resolved emission and transient absorption, we establish that the films exhibit an exponential decrease in electron transfer rate with increasing bridge length. The findings indicate that self-assembled bilayers offer a simple, straightforward, and modular method for manipulating electron transfer dynamics at dye−semiconductor interfaces. ■ INTRODUCTIONElectron transfer from a photoexcited chromophore to a semiconducting metal oxide surface is a critical event in dyesensitized solar cells and dye-sensitized photoelectrosynthesis cells. 1−3 Considerable efforts have been dedicated to manipulate electron transfer rates at these interfaces by reducing the electronic coupling between the dye and semiconductor which in turn slows the electron transfer rates. 4−8 Spatial separation of the dye and semiconductor is arguably the most common method of reducing electronic coupling and is achieved by two primary strategies: (1) atomic layer deposition (ALD) and (2) synthetic modification of the dye.ALD of "insulating" layers between the dye and semiconductor effectively increases their separation and is effective in manipulating electron transfer rates. 9−11 However, this method requires dedicated, high vacuum instrumentation that is less than ideal for large scale production of dye-sensitized devices.The second strategy involves covalently modifying the dye molecules with rigid bridging moieties terminated in a surfacebinding group. 12−15 Despite their elegant design, many of these elongated bridge-dyes lie down on the surface, negating the desired effect. Recently, Meyer and co-workers demonstrated thatwith at least some bridge-dyesincreasing bridge lengths can be effective in slowing electron transfer rates. 16 However, the covalent interaction between the dye and bridge influences the electrochemical and photophysical properties of the dye, making it difficult to differentiate the distance dependence from other variables. 8 Additionally, the multistep synthesis required for bridge-dye molecules limits the generalizability of these systems.In this report, we introduce an alternative strategy, selfassembled bilayers, as a means of controlling the distance between dye and the metal oxide surface. This approach, based on the work of Mallouk and Haga, 17,18 provides a simple and modular method for the self-assembly of bilayer structures on metal oxide surfaces via metal ion linkages. This stepwise assembly method has been successfully implemented with chromophore−catalyst and chromophore−chromophore assemblie...

Current high efficiency dye-sensitized solar cells (DSSCs) rely on the incorporation of multiple chromophores, via either codeposition or preformed assemblies, as a means of increasing broad band light absorption. These strategies have some inherent limitations including decreased total light absorption by each of the dyes, low surface loadings, and complex synthetic procedures. In this report, we introduce an alternative strategy, self-assembled bilayers, as a simple, stepwise method of incorporating two complementary chromophores into a DSSC. The bilayer devices exhibit a 10% increase in J, V, and η over the monolayer devices due to increased incident photon-to-electron conversion efficiency across the entire visible spectrum and slowed recombination losses at the interface. Directional energy and electron transfer toward the metal oxide surface are key steps in the bilayer photon-to-current generation process. These results are important as they open the door to a new architecture for harnessing broadband light in dye-sensitized devices.

Stiff polymers with tunable self-healing and mechanical properties show promising potential in electrochemical devices.