Influence of Cation on Charge Recombination in Dye-Sensitized TiO₂ Electrodes

Abstract

The reaction of a dye cation recombining with an electron in TiO(2), in the presence of Li(+), Ca(2+), and TBA(+) cations, was studied with laser-induced transient absorption measurements. The active cations, Li(+) and Ca(2+), shorten the dye cation lifetime on sensitized TiO(2) but not ZnO electrodes. By combining the absorbance measurements of the dye cation with simultaneous measurements of the current transient, the contribution of the recombination reaction to the current is identified. Furthermore, classical porous electrode theory is used to quantify the behavior of the heterogeneous electrode, and in doing so, the processes contributing to photoinduced current are identified as Helmholtz layer charging, porous electrode charging, recombination reactions, and surface diffusion of the active cations. The rate of charge recombination is proportional to the concentration of initially deposited active cations. The effect of water on the recombination rate and the current is also observed.

A surface science perspective on TiO2 photocatalysis

“…For example, low coverages of water on TiO 2 have been shown to promote electron injection from excited dyes and sensitizers [637,658,1324], presumably by favorably modifying the electrostatics at the injection site. Water, as a solvent, can provide hydrogen-bonding networks that assist in: stabilizing surface species important in O 2 photoreduction [820], minimizing depletion of important surface species (such as O 2 ) that result from photodesorption [201], or stabilizing excited electrons in surface states [192][193][194][195][196].…”

“…In some cases, the ability of adsorbed metal cations to scavenge CB electrons had a negative effect on O 2 photoreduction, particularly when the reaction products of O 2 photoreduction were needed to promote indirect oxidation processes [230,1288,1461]. Adsorbed cations have been shown to inhibit back-electron transfer in DSSC situations [1324,1462,1463]. The presence of cationic species (e.g., Mg 2+ , Li + , Na + and K + ) at the surface of TiO 2 stabilized I − at or near the surface, enhancing its rate of oxidation (by a photoionized dye) in DSSC applications [1463].…”

Effect of electrolyte constituents on the motion of ionic species and recombination kinetics in dye-sensitized solar cells

“…Most of these charge transport processes occur at the semiconductor/ electrolyte solution interface, and it was reported that the constituents in the electrolyte solution such as cation and redox species affect various processes and the performance of DSSCs as a result. [9][10][11][12][13][14] It is generally known that the constituents in an electrolyte solution have an influence on the yield of electron injection, open-circuit voltage, electron diffusion coefficient and the rate of dye cation regeneration. 4,6 As for the effect of additives, by putting 4-tert-butyl pyridine (TBP) in an electrolyte solution, the open circuit voltage increases, while the electron injection yield decreases.…”

Influences of cation charge density on the photovoltaic performance of dye-sensitized solar cells: lithium, sodium, potassium, and dimethylimidazolium

“…For example, through downward shifting of the conduction band edge of the titania film, the lithium cations increase the injection yield [5][6][7][8][9][10] and 1,2-dimethyl-3-propylimidazolium (DMPIm + ) and Li + decrease the open-circuit voltage. [11][12][13][14][15][16][17] Furthermore, the ambipolar diffusion coefficients 18,19 and the rate of dye cations reduction 20 are either affected by the concentration or the species of cations. Therefore, to design the semiconductor/dye/ electrolyte interfaces for highly efficient solar cells, it is important to elucidate and understand the influence of the cations in the electrolyte on the cell performance.…”