Simultaneous Interactions of Ru Dye with Iodide Ions and Nitrogen-Containing Heterocycles in Dye-Sensitized Solar Cells

Abstract

Iodide ions and nitrogen-containing heterocycles, including 4-t-butylpyridine, are fundamental in the electrolyte solution of dye-sensitized solar cells as a redox and a promotive additive, respectively. Although discrete interaction of iodide ions and of heterocycles with the sensitizing dye is known, their simultaneous interaction with the dye and its effect on dye-sensitized solar cell performance is still an open question. In this paper, we have studied simultaneous interactions of iodide ions and N-containing heterocycles with cationic Ru(2,2′-bipyridine-4,4′-carboxylic acid) 2 (NCS) 2 dye (N3) by means of density functional theory (DFT). The DFT results reveal that the dye interacted with both the iodide ions and the heterocycles at different sites simultaneously. The dye formed a S-I bond between the NCS ligand and I -, while concurrently forming two hydrogen bonds between the carboxyl group of its ligand and the heterocycles. However, the interaction of the heterocycles weakened the interaction of iodide ions, suppressing the regeneration of the oxidized dye, which was after electron injection to the TiO 2 conduction band, by an I -/I 3 -redox couple, and thereby reducing the short-circuit photocurrent density of the dye-sensitized solar cell.

Non‐classical Design of High‐Efficiency Sensitizers for Dye‐Sensitized Solar Cells

“…They also proposed that by using bipyridyl anchoring groups having both pyridine groups bind to TiO 2 anode so that the dye molecule stands perpendicular to the TiO 2 surface, opening the binding sites for reacting with the iodide ions. These studies, however, did not clearly show what the regeneration sites of the oxidized dye were in general, or how to enhance the interaction of the iodide ion with the regeneration sites in particular.…”

“…(7)), which is probably the minor dye regeneration step. However, even though have been many experimental 9,[11][12][13][14]19,20 and theoretical 8,17,[21][22][23][24][25][26][27][28][29][30][31] studies focusing on the dye regeneration process, detailed structures of the involved intermediates and the regeneration sites of the dye cation have not been fully understood. Consequently, strategies for increasing the dye regeneration rate by molecular structure design to improve the photovoltaic performance of the dye molecules are scarce.…”

“…Moreover, many studies [36][37][38][39][40] have shown the mismatch between the dye regeneration rate and the magnitude of the driving force, indicating that the kinetics of dye regeneration depends also on some sophisticated factors other than the regeneration driving force. Kusama et al 26 suggested that increasing the interaction of I − with the S atom of the thiocyanate ligand on the oxidized dye can enhance dye regeneration. Jeon et al 24 suggested that dye regeneration could be enhanced by replacing the thiocyanate ligand with a ligand having a more delocalized electronic structure.…”

“…Here, instead of using complicated quantum calculations, 8,17,[21][22][23][24][25][26][27][28][29][30][31] we propose a simple and straightforward way to identify the dye regeneration sites using the concept of resonance structures. 41,42 By considering the chemical softness and spatial steric effect of the regeneration sites identified from the resonance structures of an oxidized dye molecule, the relative reactivity of the oxidized dye toward the reductant (I − ) in the electrolyte (and thus the kinetics of the dye regeneration) can be understood.…”

Highly stable dye-sensitized solar cells with quasi-solid-state electrolyte based on Flemion

“…These phenomena suppress electron transfer from the conduction band of TiO 2 to I À 3 (Huang et al, 1997;Rijnberg et al, 1998;Hara et al, 2004;Shi et al, 2005), and increase the V oc . Kusama et al (2010) suggested that TBP can directly bind to the carboxyl groups in Ru dye via two hydrogen bonds. If the carboxyl groups in Flemion similarly bind to TBP, many of the TBP molecules would be trapped within the Flemion matrix, thereby lowering the concentration of adsorbed TBP at the TiO 2 surface and reducing the V oc relative to non-Flemion-based electrolytes.…”