The photooxidation of iodide in dye-sensitized solar cells (DSSCs) is not optimized. Under illumination, unwanted charge recombination between the injected electron and the oxidized dye molecule occurs. In this study halogen bonding, a recently reinvigorated non-covalent association, was utilized to facilitate iodide-dye association at a semiconductor interface to increase the iodide oxidation rate. Halogen bonding is an interaction between a Lewis base and an electropositive σ-hole on a bound halogen atom (R-X●●●I-). The sigma hole increases with the polarizability and quantum number of the halogen. Very few studies have examined halogen bonding at interfaces. A series of four donor-π-acceptor dyes were synthesized in which a halogen on a triphenylamine donor unit was varied between F, Cl, Br, and I (Dye-X, X = halide), and the effect of halogen bonding ability at the surface was investigated. Density functional theory showed evidence for a σ-hole on the Cl, Br, and I containing dyes that increased after oxidation of the dyes. Titrations with Lewis bases at the sensitized TiO2 semiconductor interface were examined by steady-state spectroscopy. Upon light excitation of the sensitized interfaces and electron injection (TiO2/Dye-X* -&gt; TiO2(e-)/Dye-X+), iodide oxidation regenerated the dyes (TiO2(e-)/Dye-X+ + I- -&gt; TiO2(e-)/Dye-X●●●I). Dye-I, which had the largest σ-hole, was found to have the largest rate constant for regeneration, even though the driving force was not the most optimal. The knowledge garnered from this study is applicable to DSSCs and will be utilized in designing new dyes that take advantage of the favorable halogen bonding interaction.

Promoting interfacial photoinduced iodide oxidation: Halogen bonding at the interface

251st National Meeting (2016)

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