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), exhibiting high photovoltage of over 1.0 V and outstanding performance under ambient lighting. However, emerging experimental evidence reveals that the ubiquitously used Lewis base additives in the electrolytes, such as 4- tert -butylpyridine (TBP), coordinate to the Cu(II) species, which restricts further enhancement of device performance and stability for these electrolytes. Here, we report for the first time the application of copper complexes endowed with diamine-tripyridine pentadentate ligands, namely [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. Cyclic voltammetry, absorption, and electron paramagnetic resonance spectroscopies demonstrate that the coordination environment of copper(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. Strikingly, 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. The present work thus provides a guidance for future design of stable copper redox mediators in DSCs.