Characterisation of DSSC-electrolytes based on 1-ethyl-3-methylimidazolium dicyanamide: Measurement of triiodide diffusion coefficient, viscosity, and photovoltaic performance

A comprehensive characterisation of an ionic liquid based electrolyte for dye-sensitised solar cells (DSSC) was performed by determination of triiodide diffusion coefficients, viscosities and photovoltaic performances. The electrolyte, consisting of 1-ethyl-3-methylimidazolium dicyanamide, 1-methyl-3-propylimidazolium iodide (MPII), and iodine, was examined at varying ionic liquid molar ratio and fixed iodine concentration, as well as at fixed ionic liquid molar ratio and varying iodine concentration. All measurements were conducted at controlled temperature over a broad temperature range to analyse the electrolyte properties in view of thermal stress of the DSSC during later practical application. The triiodide diffusion coefficient increases with decreasing MPII concentration or increasing temperature, caused by decreasing electrolyte viscosity. Additionally, strong non-Stokesian behaviour over the whole ionic liquid mixing range was found, decreasing at higher temperatures. Photovoltaic measurements showed that the DSSC efficiency is limited by triiodide diffusion at low temperatures and low iodine concentrations whereas at high temperatures it is limited by recombination processes, leading to a maximum for DSSC efficiency at a certain temperature. The exchange between diffusion limitation of DSSC efficiency and limitation by recombination and therefore the maximum for DSSC efficiency is shifted to lower temperatures by increasing iodine concentration. (c) 2007 Elsevier B.V. All rights reserved.