Bilayer Dye Protected Aqueous Photocathodes for Tandem Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are of broad interest for both solar energy conversion and storage. Furthermore, utilizing water as the electrolyte solvent makes this already cost-effective system even more practical and environmentally friendly, although using water as the electrolyte solvent introduces new issues such as dye anchor instability and semiconductor degradation. Herein, a bilayer dye design was used which simultaneously improves the aqueous stability by creating a hydrophobic layer over the semiconductor and anchoring group which repels water and ionic/polar species of the electrolyte. We propose the larger recombination in the aqueous based iodide/triiodide redox is due to the low equilibrium constant of triiodide formation in aqueous solvents as opposed to nonaqueous solvents. Consequently, higher concentrations of free iodine exist, which has been suggested recently in the literature to be the predominant pathway for recombination in both n-type and p-type nonaqueous dye-sensitized systems. Additionally, iodide/iodine has a complex pH-dependent equilibrium in water where formation of oxoiodides can now contribute to recombination. When our bilayer dye was used on NiO, aqueous p-type DSSCs were fabricated using the iodide/triiodide redox and displayed photocurrents up to 3.0 ± 0.12 mA/cm 2 measured under 1 sun conditions (AM1.5, 100 mW/cm 2 ). The aqueous p-type DSSC photocurrent rivals that of nonaqueous p-type DSSCs while also being stable for months with no sign of photocathode degradation but rather electrolyte decomposition. Furthermore, an aqueous tandem DSSC was fabricated with a completely aqueous electrolyte with all earth-abundant materials with no use of any precious metals.