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

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 J(sc), for DSCs with increasing alkyl chain length because of the increasing charge injection efficiency eta(inj). 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 J(sc) and FF, which is essentially related to the cooperative effect of adsorbed cations. (C) 2013 Elsevier Ltd. All rights reserved.