Managing wetting behavior and collection efficiency in photoelectrochemical devices based on water electrolytes; improvement in efficiency of water/iodide dye sensitised cells to 4%

Abstract

We have fabricated Dye Sensitised Solar Cells (DSSCs) using only water as the solvent and guanidinium iodide-iodine as the redox couple that operate at 4% energy efficiency under 1-sun illumination. This result is $5 times higher than the best previously reported values. We show that it is critical to facilitate the wetting of water electrolytes into the mesoporous TiO 2 dye films, especially when using hydrophobic dyes. We show chenodeoxycholic acid to be a good surfactant for this purpose. By separate variation of iodide and iodine concentrations in series of cells we show that the optimum concentrations for water based DSSCs are quite different from those used in organic electrolytes. We argue that this is due to the much lower stability constant of tri-iodide in water, relative to organic solvent. Finally we also vary the TiO 2 thickness and pore structure to achieve the above stated efficiency.

Aqueous dye-sensitized solar cells

“…In this respect, O'Regan and coworkers observed a positive effect on both wetting and collection efficiency by means of guanidinium and iodide ions, respectively. 206 In particular, by using a high concentration of guanidinium iodide (GuI 8.0 M), a remarkable increase in the J sc values (from 7.89 to 10.02 mA cm À2 ) was obtained. On the other hand, V oc remained unchanged, due to the balance of the following two opposite phenomena: the reduction of the recombination rate due to the decrease of free iodine amount and the lowering of the redox potential of the solution due to the increased iodide concentration.…”

The electrochemical characteristics of the mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium iodide

“…The iodide-iodine redox couple is widely used in dye sensitized solar cell systems to increase the energy efficiency of the devices [16,28]. Strong influence of the I À ions addition on the electrochemical characteristics of Bi(1 1 1)|EMImBF 4 + EMImI interface has been shown in our recent study [2].…”

“…RTILs have nearly ideal properties for EDLC and dye-sensitized solar cell applications: high ionic concentration and good thermal stability. Drawbacks of using ionic liquids in EDLC are their fairly high viscosity and relatively large size (molar volume) of ions, obtaining the high value of surface area occupied by one adsorbed ion [15,16].…”

Solvent‐Dependent Structure of the I3− Ion Derived from Photoelectron Spectroscopy and Ab Initio Molecular Dynamics Simulations

“…[2] Although, the maximum reported efficiency for such liquid electrolyte-based DSSCs has been for organic solvents (acetonitrile), [3] which has surpassed 13 % (with a cobalt redox mediator), according to some recent reports, [4] the role of water electrolyte has also regained attention in some very recent developments. [5] Hence, examining the solution structures of these ions in several solvents is of practical relevance to understand their electrochemistry at complex interfaces and in electrolytic media. On a parallel note with more fundamental importance, studies concerning I 3 À in solution provide fertile ground for the investigation of solvent-induced symmetry breaking of the ion.…”