Probing Interfacial Halogen-Bonding Effects with Halogenated Organic Dyes and a Lewis Base-Decorated Transition Metal-Based Redox Shuttle at a Metal Oxide Interface in Dye-Sensitized Solar Cells

Abstract

Metal-free bis-cyclopentadithiophene-based molecular sensitizers with varying halogens (Cl, Br, and I) installed at a terminal position in conjugation with the dye frontier molecular orbital π-system are studied in combination with two cobalt redox shuttles (RSs): with and without a nonmetal-coordinated nitrogen atom (pyrazine or pyridine) accessible to the halide-decorated dyes. This systematic study employs UV−vis absorption, cyclic voltammetry, density functional theory calculations, and nanosecond transient absorption spectroscopy to probe the influence of possible halogen-bonding between the dyes and the pyrazine-based RS on electron-transfer reactions and effects on dye-sensitized solar cell performances. The results of this study imply a possible halogen-bonding event occurring between the halogenated dyes and the halogen binding RS with substantial effects on electrontransfer reaction rates.

Designing Self-Assembled Dye–Redox Shuttle Systems via Interfacial π-Stacking in Dye-Sensitized Solar Cells for Enhanced Low Light Power Conversion

“… Notably, the perchlorate salt of [Co­(tpy pyr ) 2 ] 2+ is known in the literature and has a nearly identical absorption spectrum to that observed in this study . Following a modified synthetic approach, treatment of [Co­(tpy pyr ) 2 ]­[(PF 6 ) 2 ] with NOBF 4 followed by a large excess of NH 4 PF 6 led to the formation of [Co­(tpy pyr ) 2 ]­[(PF 6 ) 3 ] in 92% yield similarly to the literature reported approach. Both complexes without pyrene groups, [Co­(tpy) 2 ]­[(PF 6 ) 2 ] and [Co­(tpy) 2 ]­[(PF 6 ) 3 ], have been previously reported …”

“…It is observed in the literature that dye−redox shuttle association can lead to slower rates of recombination presumably due to the dye orientation at the surface holding the oxidized redox shuttle (formed after dye regeneration) away from the TiO 2 surface. 50,70 Current dynamic measurements at varied light intensities were conducted because two diffusion-related arguments are postulated to explain the slower recombination of electrons in TiO 2 with the [Co(tpy pyr ) 2 ] 3+/2+ -based electrolyte (Figure 8).…”

“…Plausible explanations for the slower recombination rates include (1) less diffusion of the oxidized RS past the dye to the TiO 2 surface due to the increased size of the [Co­(tpy pyr ) 2 ] redox shuttle and (2) a favorable π-stacking interaction between the dye and the redox shuttle that slows diffusion of the oxidized RS past the dye to the TiO 2 surface, thus diminishing the rate of recombination. It is observed in the literature that dye–redox shuttle association can lead to slower rates of recombination presumably due to the dye orientation at the surface holding the oxidized redox shuttle (formed after dye regeneration) away from the TiO 2 surface. , …”

“…TAS films (3 μm thickness) were fabricated according to a previously reported literature procedure. 50 TAS films were immersed in 0.3 mM SS1 dye solution in THF:EtOH (1:4) for 16 h. The films and electrolyte were sealed with a glass coverslip in place of the typical counter electrode position.…”

Preferential Direction of Electron Transfers at a Dye–Metal Oxide Interface with an Insulating Fluorinated Self-Assembled Monolayer and MgO

“…We stress that assigning absolute regeneration efficiencies is challenging with several approaches noted in the literature in addition to the commonly used approach above. − We note that this comparison likely overestimates the absolute regeneration efficiency as has been noted in the literature, but this method does allow for a simple comparison of regeneration efficiencies within this study …”