Cooperative effect of adsorbed cations on electron transport and recombination behavior in dye-sensitized solar cells

• Disclose the mechanism of cooperative effects of adsorbed cations in DSCs.
• Characterize the influence of adsorption of Im+s on photoinduced electron density.
• The effect of Li+ is orderly enhanced in DSCs with increasing alkyl chain length.
• The DSCs efficiencies are relatively depended on the trade-off between Jsc and FF.

Lithium ion (Li+) and imidazolium cations (Im+s) had been reported to have competitive effects on the photoinduced electrons in TiO2-electrolyte systems. Herein, a further investigation about their cooperative effect in dye-sensitized solar cells (DSCs) using organic liquid electrolyte is developed by altering alkyl chain length. Imidazolium iodides (Im+I−s) with different alkyl chain length (3, 6, and 12) were synthesized and used as iodide sources. The adsorption amount of Im+s onto TiO2, band edge shifts, trap states distribution, electron recombination/transport processes and ion transport within the electrolyte for DSCs were detected. It is found that the multilayered adsorption of Im+s can induce a lower photoinduced electron density. In-depth characterizations indicate that this negative effect can be reduced as the adsorption amount decreased with increasing alkyl chain length and the effect of Li+ is consequently strengthened in varying degrees. The decisive role of Li+ in cation-controlled interfacial charge injection process finally contributes an ordinal increase of short-circuit photocurrent density Jsc for DSCs with increasing alkyl chain length because of the increasing charge injection efficiency ηinj. Additionally, a large power dissipation in ions transport process is induced by the long alkyl chain of Im+s. Overall, the cell efficiencies are relatively dependent of the trade-off between Jsc and FF, which is essentially related to the cooperative effect of adsorbed cations.