Use of lithium iodide and tetrapropylammonium iodide in gel electrolytes for improved performance of quasi-solid-state dye-sensitized solar cells: Recording an efficiency of 6.40%

Use of gel electrolytes in place of liquid electrolytes in DSCs is one of the proposed alternatives to circumvent some of the problems suffered by the liquid electrolyte-based DSCs such as electrolyte leakage, solvent evaporation and reaction of I ions with atmospheric oxygen due to sealing imperfections. Even though the use of gel electrolytes sacrifices the cell efficiency up to some extent due to their low ion mobility, there are promising properties of gel electrolytes such as thermal stability, lower flammability and environmental friendliness. Use of Li+ and tetrapropylammonium (Pr4N+)-based iodide salts in gel electrolytes is quite common. However, the effects of Li+ and Pr4N+ on vital processes such as recombination and diffusion of I-3 through the device have not been fully understood. In this study we have attempted to examine the influence of Li+ and Pr4N+ cations present in the electrolyte on the cell performance. The general structure of the devices used was FTO/TiO2 working electrode/dye (N719)/gel electrolyte/lightly platinized FTO counter electrode. As a first step, devices based on two types of electrolytes were prepared using only LiI salt in one set and only Pr4N+I salt in the other. The salt concentrations were optimized independently for best cell performance in each case. Then a composite salt was prepared using a mixture of LiI and Pr4N+I and the relative proportions of the two salts were varied until the efficiency reached a maximum. The reason for this choice was the fact that the bulk cations such as Pr4N+ have high dissociation constant resulting in easy release of I to the electrolyte whereas Li+ ions tend to increase the amorphous nature of the electrolyte supporting the hopping mechanism of transport of I-3 through the electrolyte. A best efficiency of 6.40% was obtained for the composite salt having a molar ratio of 1:3 of Pr4N+I to LiI, while the individual salts gave efficiencies 5.36% and 6.10%, respectively under identical conditions. We have employed XRD, FTIR, LSV and AC impedance measurements along with the literature support to explain the observed performance of the DSC devices fabricated with different salt compositions. (C) 2016 Elsevier Ltd. All rights reserved.