A New Direction in Dye-Sensitized Solar Cells Redox Mediator Development: In Situ Fine-Tuning of the Cobalt(II)/(III) Redox Potential through Lewis Base Interactions

Abstract

Dye-sensitized solar cells (DSCs) are an attractive renewable energy technology currently under intense investigation. In recent years, one area of major interest has been the exploration of alternatives to the classical iodide/triiodide redox shuttle, with particular attention focused on cobalt complexes with the general formula [Co(L)(n)](2+/3+). We introduce a new approach to designing redox mediators that involves the application of [Co(PY5Me(2))(MeCN)](2+/3+) complexes, where PY5Me(2) is the pentadentate ligand, 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine. It is shown, by X-ray crystallography, that the axial acetonitrile (MeCN) ligand can be replaced by more strongly coordinating Lewis bases (B) to give complexes with the general formula [Co(PY5Me(2))(B)](2+/3+), where B = 4-tert-butylpyridine (tBP) or N-methylbenzimidazole (NMBI). These commonly applied DSC electrolyte components are used for the first time to fine-tune the potential of the redox couple to the requirements of the dye through coordinative interactions with the Co(II/III) centers. Application of electrolytes based on the [Co(PY5Me(2))(NMBI)](2+/3+) complex in combination with a commercially available organic sensitizer has enabled us to attain DSC efficiencies of 8.4% and 9.2% at a simulated light intensity of 100% sun (1000 W m(-2) AM1.5 G) and at 10% sun, respectively, higher than analogous devices applying the [Co(bpy)(3)](2+/3+) redox couple, and an open circuit voltage (V(oc)) of almost 1.0 V at 100% sun for devices constructed with the tBP complex.

Are High-Potential Cobalt Tris(bipyridyl) Complexes Sufficiently Stable to Be Efficient Mediators in Dye-Sensitized Solar Cells? Synthesis, Characterization, and Stability Tests

“…Some examples of cobalt complexes with highly coordinating ligands are already beginning to appear in the literature. 53,54 The present studies support that direction for future DSSC research. …”

Theoretical study on a high-efficient porphyrin-sensitizer in a local electric field: How does the local electric field affects the performance of dye-sensitized solar cells?

“…The typical DSSC are constructed with four major parts: an anchored molecular sensitizer [7][8][9][10][11][12][13], a wide band gap semiconductor (typically the wide band gap semiconductor is TiO 2 ) [14,15], and a redox electrolyte (usually the redox couple) [16,17] as well as counter electrode [18]. The photo-to-current conversion mechanism of DSSC can be described as follow [19][20][21]: the photo absorptions of sensitizers result in electron injection from the excited dyes to the conduction band of the oxide semiconductor film.…”

Towards Compatibility between Ruthenium Sensitizers and Cobalt Electrolytes in Dye‐Sensitized Solar Cells

“…[10] In contrast to the iodide/triiodide couple (I À /I 3 À ), the electrochemical potential of these species can be adjusted through substitution and/or the modification of the ligand skeleton. [11][12][13][14][15] This tuning minimizes energy losses in the device by optimizing the driving force for regeneration, which allows for considerable improvement of the open-circuit voltage relative to iodine electrolytes.…”