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

摘要

Lithium ion (Li~+) and imidazolium cations (Im~+s) had been reported to have competitive effects on the photoinduced electrons in TiO_2-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 TiO_2, 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 η_(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.