Spectroscopic detection of halogen bonding resolves dye regeneration in the dye-sensitized solar cell

Abstract

The interactions between a surface-adsorbed dye and a soluble redox-active electrolyte species in the dye-sensitized solar cell has a significant impact on the rate of regeneration of photo-oxidized dye molecules and open-circuit voltage of the device. Dyes must therefore be designed to encourage these interfacial interactions, but experimentally resolving how such weak interactions affect electron transfer is challenging. Herein, we use X-ray absorption spectroscopy to confirm halogen bonding can exist at the dye-electrolyte interface. Using a known series of triphenylamine-based dyes bearing halogen substituents geometrically positioned for reaction with halides in solution, halogen bonding was detected only in cases where brominated and iodinated dyes were photo-oxidized. This result implies that weak intermolecular interactions between photo-oxidized dyes and the electrolyte can impact device photovoltages. This result was unexpected considering the low concentration of oxidized dyes (less than 1 in 100,000) under full solar illumination.

π covalency in the halogen bond

“…The triphenylamine moiety is not an electron-withdrawing group in the neutral form and consequently does not facilitate significant halogen bonding interactions. Oneelectron oxidation of the molecules to the corresponding radical cation states Dye-I •+ and Dye-Br •+ greatly increases the electronegativity of the triphenylamine core and thereby promotes halogen bonding 37 . The triphenylamine core stabilizes the radical cation sufficiently to allow for spectroscopic studies 43,44 .…”

“…We experimentally resolve this π-covalent component by studying a homologous pair of triphenylamine-based XB-donors denoted as Dye-X (where X = Br or I), along with the non-halogen bonding analog, Dye-F ( Fig. 2a) [35][36][37] . When oxidized by one electron to form the corresponding radical cation, the singly occupied molecular orbital (SOMO) of the open-shell Dye-X •+ is a πsymmetric MO with lobes oriented orthogonally to the halogen bond axis 36 .…”

Supramolecular strategies in artificial photosynthesis

“…30). 123 Furthermore, preorganization of Li + -ions close to the dye increases the local concentration of I À , and results in decreased recombination. 124 Supramolecular antennae as discussed in Section 3 are applied in n-type DSSC.…”

“…It is then quickly regenerated and as a result back electron transfer is inhibited. Used with permission from ref 123…”

Electrochemical activation of halogen bonding

“…Up to date, we are aware of only one example exploiting this potentiality for the regeneration of a photo-oxidized halogenated dye via iodide oxidation, thus impacting the open-circuit voltage of a solar cell ( Figure 4C) [41]. Parlane et al [42] used X-ray absorption spectroscopy to directly detect interfacial XB between a homologous soluble halide species (Cl À ) and the activated dye cation demonstrating that these interactions occur after electron injection into TiO 2 .…”