Nickel-Based Two-Electron Redox Shuttle for Dye-Sensitized Solar Cells in Low Light Applications

Dye-sensitized solar cells (DSCs) are important to indoor solar powered devices and energy sustainable buildings because of their remarkable performance under indoor/ambient light conditions. Triiodide/iodide (I₃^–/I^–) has been used as the most common redox mediator in DSCs because of its desirable kinetic properties and multielectron redox cycle. However, the low redox potential, corrosiveness, competitive visible light absorption, and lack of tunability of this redox mediator limit its performance in many DSC devices. Here we report a class of transition metal complex redox shuttles which operate on a similar multielectron redox cycle as I₃^–/I^– while maintaining desirable kinetics and improving on its limitations. These complexes, nickel dithiocarbamates, were evaluated as redox shuttles in DSCs, which exhibited excellent performance under low light conditions. The recombination behavior of the redox shuttles with electrons in TiO₂, dye regeneration behavior, and counter electrode electron transfer resistance were studied via chronoamperometry and electrochemical impedance spectroscopy (EIS). Further, DSC devices were studied with the Ni-based redox shuttles via incident photon-to-current conversion efficiencies (IPCEs) and current–voltage (J–V) curves under varied light intensities. The Ni-based redox shuttles showed up to 20.4% power conversion efficiency under fluorescent illumination, which was higher than I₃^–/I^–-based devices (13%) at similar electrolyte concentrations. Taken together, these results show that nickel dithiocarbamate redox shuttles have faster rates of dye regeneration than the I₃^–/I^– shuttle but suffer from faster recombination of photoinjected electrons with oxidized Ni­(IV) species, which decrease photovoltages.