(Digital Presentation) A Two-Electron Transition Metal Based Redox Mediator for Dye-Sensitized Solar Cells

Abstract

Since the first report of a Dye Sensitized Solar Cells (DSSC), I3 -/I- has been as staple redox mediator because of its efficient 2e- redox cycle. However, tuning this mediator is challenging given its atomic nature. Here we present the combination of a multielectron redox cycle with a tunable transition metal complex as an alternative redox mediator in DSSCs. This research focuses on using the [NiIV(R2dtc)3]+/NiII(R2dtc)2 redox couple, where dtc- is a bidentate dithiocarbamate ligand with R = ethyl and butyl groups. DSSC device measurements under low light intensity and 1 sun illumination showed the highest efficiency of 8% at 10,000 lux for [NiIV/II(Et2dtc)3/2]+/0, whereas [NiIV/II(Bu2dtc)3/2]+/0 displayed a high of 20% at 4660 lux. To investigate the individual roles of reduced and oxidized mediators, comparative photoelectrochemical studies between reduced mediators, Ni(dtc)2 and I- were performed in a photoelectrochemical cell with an external white light LED along with a photoanode consisting of FTO/TiO2 sensitized with the N3 dye. Ni(Et2dtc)2 and Ni(Bu2dtc)2 showed 75 μA and 100 μA more photocurrent respectively than iodide in the case of the highest irradiance (100 mW/cm2). These studies show the NiII(R2dtc)2 is a superior mediator for dye regeneration. Likewise, the photovoltage was much greater in the case of nickel mediators, NiII(Et2dtc)2 (800 mV) and NiII(Bu2dtc)2 (860 mV) than iodide (560 mV), consistent with ERedox = 0.02 V for the Ni(IV/II) couple, which is 0.32 V greater than ERedox for I3 -/I-. Surprisingly, as [NiIV(R2dtc)3]+ was introduced into the electrolyte the observed photocurrent became dynamic with time and continued to show photoanodic current even at 100% NiIV. These results are consistent with [NiIV(R2dtc)3]+ photodissociation to yield NiII(dtc)2. Preliminary studies on this photodissociation have been done by a UV-Vis kinetic analysis in various solvents. [NiIV(R2dtc)3]+ showed a prolonged dissociation in dichloromethane and chloroform while dissociation was much quicker in acetonitrile. An unknown intermediate was observed in the case of alcohol solvents which suggests a two-step mechanism for dissociation. Taken together, NiIV/II represents alternative redox mediator with outstanding dye regeneration capabilities, but complications due to photodissociation of [NiIV(R2dtc)3]+. Further studies are underway to investigate the impact of photodissociation on the redox cycle and electron-hole recombination in DSSCs.