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Interfacial halogen bonding boosts the photovoltage of the dye-sensitized solar cell Simon, Sarah June Charlene

Abstract

A series of donor-bridge-acceptor (D-π-A) compounds, differing only by the identity of two halogen atoms substituted on the triphenylamine (TPA, donor), were synthesized and characterized for insight into the regeneration reactions within dye-sensitized solar cells (DSSCs) [Dye-X⁺/TiO₂(e-) + I− → Dye-X/TiO₂ + I₂•−]. The structures of each series conformed to a molecular scaffold bearing a TPA donor, thiophene spacer, and acrylic acid unit as the anchoring group. In Chapter 2, each Dye-X (X = F, Cl, Br, and I) was immobilized on a TiO₂ surface to investigate how the halogen substituents affect the reaction rate between the light-induced charge-separated state, TiO₂(e−)/Dye-X⁺, with iodide in solution. Transient absorption spectroscopy showed progressively faster reactivity towards nucleophilic iodide with more polarizable halogen substituents: Dye-F < Dye-Cl < Dye-Br < Dye-I. In Chapter 3, it is reported that the Dye-X series show a higher photovoltage for DSSCs where intermolecular halogen bonding interactions exist between a nucleophilic electrolyte species (I−) and a photo-oxidized dye immobilized on a TiO₂ surface. Given that all other structural and electronic properties for the series are held at parity, with the exception of an increasingly larger electropositive σ-hole on the heavier halogens, the differences in dye regeneration kinetics for Dye-Cl, Dye-Br, and Dye-I are ascribed to the extent of halogen bonding with the nucleophilic solution species. The open-circuit voltages (VOC) tracked these rate of regeneration (kreg) values and provides incontrovertible evidence that the DSSC photovoltage is sensitive to kreg and halogen bonding. This study also provides the first direct evidence that halogen bonding interactions between the dye and the electrolyte can bolster DSSC performance.

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