ZnO Nanostructures for Dye-Sensitized Solar Cells Using the TEMPO⁺/TEMPO Redox Mediator and Ruthenium(II) Photosensitizers with 1,2,3-Triazole-Derived Ligands

A series of thiocyanate-free bis(tridentate) ruthenium(II) complexes incorporating 1,2,3-triazole-derived NNN-, NCN-, and CNC-coordinating ligands has been employed for sensitizing ZnO photoanodes for dye-sensitized solar cells (DSSCs). Additionally, the first use of the TEMPO⁺/TEMPO (2,2,6,6-tetramethyl-piperidine-1-oxyl) redox mediator as a surrogate for the I₃ ⁻/I⁻ redox couple in ZnO nanostructured DSSCs is presented. Compared with I₃ ⁻/I⁻-based electrolytes, shorter charge lifetimes and diffusion lengths were determined for the TEMPO⁺/TEMPO-based electrolyte. Nonetheless, similar power conversion efficiencies (PCEs) were achieved with both electrolytes for the RuNCN and RuCNC complexes, whereas higher PCEs are enabled by the iodine-free electrolyte in case of RuNNN. The combination of the molecular sensitizers and the TEMPO-based electrolyte exhibits relatively high external quantum efficiency (EQE) and promising PCEs, ranging from 4.48 to 1.47 %, which are—in part—comparable to that of ZnO-DSSCs with the benchmark N749 black dye. The TEMPO-based electrolyte also exhibits less absorption compared with its I₃ ⁻/I⁻ counterpart, a favorable feature for enhancing the light harvesting ability of the photoanode. Furthermore, the results show the effect of the dye-sensitization procedure on the PCE values: The use of ethanol as the solvent compared with methanol increases the DSSC's efficiency, which is attributed to improved chemisorption of the sensitizer onto the ZnO surface.