Nickel(II) Bis(diethyldithiocarbamate) As a Redox Mediator in Dye Sensitized Solar Cells

Abstract

Since the first report of a dye sensitized solar cell (DSSC) in 1991, the 2e- redox cycle of triiodide/iodide served as the archetype for redox mediators. However, major shortcomings of I3 -/I- include corrosion with metal surfaces and its redox potential of ERedox = -0.34 V vs Fc+/0, which limits the maximum photovoltage to ~0.8 V. Increasing ERedox using alternative mediators has been a primary research focus. Although [CoIII/II(bpy)3]3+/2+ and its derivatives have been shown to possess more positive ERedox values, these mediators suffer from slow electron transfer kinetics. Here we present the combination of a multielectron redox cycle with a tunable transition metal complex which could provide the best solution for increasing ERedox while maintaining a 2e- redox cycle. This research focuses on using [NiIV(dtc)3]+/NiII(dtc)2 as a redox mediator in DSSCs where dtc- is a bidentate dithiocarbamate ligand, which undergoes 2e- oxidation from Ni(II) to Ni(IV) during dye regeneration. Steady state photoluminescence quenching of [Ru(bpy)3]2+ shows comparable behavior between Ni(dtc)2 and iodide with a Ksv = 1.72×10-9 M-1 for Ni(dtc)2 compared with 3.67×10-9 M-1 for I-. Photoelectrochemical measurements were made using FTO/TiO2 electrodes sensitized with the N3 dye in the presence of I- or Ni(dtc)2. Cyclic voltammetry experiments were carried out using a 0.001 M Ni(dtc)2 with external light source (white light LED) with different light intensities ranging from 0 to 125 mW/cm2. Similar currents were obtained in the case of Ni(dtc)2 compared to iodide for all measured light intensities. The measured open-circuit potential was also found to be larger in case of Ni(dtc)2, consistent with the ERedox = 0.01 V for the Ni(IV/II) couple. This value is 0.32 V larger than I3 -/I- and could produce a theoretical photovoltage value of 1.15 V which is 0.14 V more than the maximum value reported so far with a cobalt based mediator. These studies show that [NiIV(dtc)3]+/Ni(dtc)2 has the potential to serve as an alternative redox mediator to I3 -/I-.