Stable Dye-Sensitized Solar Cells Based on Copper(II/I) Redox Mediators Bearing a Pentadentate Ligand

In recent years, copper redox mediators have attracted growing interest in dye-sensitized solar cells (DSCs). However, experiments revealed that ubiquitously used Lewis-base additives in the electrolytes coordinate to the Cu^II species, which restricts further enhancement of device performance and stability. We report the application of copper complexes endowed with diamine-tripyridine pentadentate ligands, [Cu(tpe)]^2+/+ (tpe=N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine) and [Cu(tme)]^2+/+ (tme=N-benzyl-N,N',N'-tris(6-methylpyridin-2-ylmethyl)ethylenediamine), as redox mediators in DSCs. Experimental measurements demonstrate that the coordination environment of Cu(II) complexes with pentadentate ligands remains unchanged in the presence of TBP, which is in stark contrast to the state-of-the-art bipyridyl counterpart. DSCs based on [Cu(tme)]^2+/+ complexes exhibit an excellent long-term stability and maintain more than 90 % of the initial efficiency after 400 h under continuous illumination, which outperform the reference devices incorporating the bipyridyl counterpart (less than 80 %) under identical conditions.