Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer.

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface. The choice of molecules, metal oxide, and the nature of the assembly method can have a profound influence on the mechanism, rate, and efficiency of photoinduced energy and electron transfer events at the interface. Owing to their diversity and high level of control, these interfacial assemblies are of interest for numerous applications including solar energy conversion, photoelectrosynthesis, photo-writable memory, and more. Although these assemblies are generated with very different goals in mind, they rely on similar surface binding motifs and molecular structure-property relationships. Therefore, the goal of this review is to summarize the various strategies (i.e. co-deposition, axial coordination, metal ion linkages, electrostatics, host-guest interactions, etc.) for assembling chromophores, hosts, electron donors/acceptors, and insulating co-adsorbent molecules on mesoporous metal oxide substrates. The assembly, synthesis, and characterization, as well as subsequent photoinduced events (i.e. cross-surface energy/electron transfer, interchromophore energy transfer, electron injection, and others) are discussed for the various assembly strategies.

Increasing the Open-Circuit Voltage of Dye-Sensitized Solar Cells via Metal-Ion Coordination.

Considerable efforts are dedicated to increasing the open-circuit voltage (Voc) of dye-sensitized solar cells (DSSCs) by slowing charge recombination dynamics using atomic layer deposition, alkyl-substituted dyes, coadsorbents, and other strategies. In this report, we introduce metal-ion coordination to a metal oxide bound dye as an alternative means of increasing Voc. Metal-ion coordination has minimal influence on the photophysical and electrochemical properties of the N3 dye, but presumably because of increased steric hindrance at the interface, it slows charge recombination kinetics and increases Voc by upwards of 130 mV relative to the parent N3 DSSC. With respect to the nature of the metal ion, the trend in decreasing short-circuit current (Jsc) and increasing Voc correlates with the charge of the coordinated metal ion (MIV → MIII → MII). We attribute this trend to electrostatic interactions between the metal cation and I- or I3-, with the more highly charged cations maintaining a higher concentration of mediator anions in proximity to the surface and, as a result, increasing the regeneration and recombination rates.

Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions.

Self-assembled bilayers on nanocrystalline metal oxide films are an increasingly popular strategy for modulating electron and energy transfer at dye-semiconductor interfaces. A majority of the work to date has relied on ZrII and ZnIV linking ions to assemble the films. In this report, we demonstrate that several different cations (CdII, CuII, FeII, LaIII, MnII, and SnIV) are not only effective in generating the bilayer assemblies but also have a profound influence on the stability and photophysical properties of the films. Bilayer films with ZrIV ions exhibited the highest photostability on both TiO2 and ZrO2. Despite the metal ions having a minimal influence on the absorption/emission energies and oxidation potentials of the dye, bilayers composed of CuII, FeII, and MnII exhibit significant excited-state quenching. The excited-state quenching decreases the electron injection yield but also, for CuII and MnII bilayers, significantly slows the back electron transfer kinetics.

Metal ion mediated electron transfer at dye-semiconductor interfaces.

Here we demonstrate that the incorporation of Cu2+ linking ions into self-assembled bilayer films significantly perturbs the electron transfer rate at dye-semiconductor interfaces. Despite near unity quenching of the excited state of the dye by the Cu2+ ions, a charge separated state with slowed recombination is observed.

Energy and Electron Transfer Cascade in Self-Assembled Bilayer Dye-Sensitized Solar Cells.

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 Jsc, Voc, 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.

Inhibiting Interfacial Recombination Events in Dye-Sensitized Solar Cells using Self-Assembled Bilayers.

The rate and efficiency of electron transfer events at the semiconductor-dye-electrolyte interface is of critical importance to the overall performance of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells. In this work, we introduce self-assembled bilayers composed of a metal oxide electrode, bridging molecules, linking ions, and dye as an effective strategy to manipulate interfacial electron transfer events at the photoanode of DSSCs. Spectroelectrochemical measurements including current-voltage, incident photon-to-current efficiency, and electrochemical impedance spectroscopy are used to quantify interfacial electron transfer and transport events with respect to the length of the bridging molecules. The general trend in increased lifetime and diffusion length in TiO2 as well as an increase in open circuit voltage with bridge length indicate that the bilayer is an effective strategy in inhibiting the TiO2(e(-)) to redox mediator recombination events. However, the increased separation between the dye and the semiconductor also reduces the electron injection rate resulting in a decrease in photocurrent as the bridge length increases. The observed enhancement in open circuit voltages are far outweighed by the significant decrease in photocurrent and thus overall device performance decreases with increasing bridge length.