The identification of an efficient and stable redox mediator is of paramount importance for commercialization of dye-sensitized solar cells (DSCs). Herein, we report a new class of copper complexes containing diamine-dipyridine tetradentate ligands (L1 = N,N'-dibenzyl-N,N'-bis-(pyridin-2-ylmethyl)ethylenediamine; L2 = N,N'-dibenzyl-N,N'-bis (6-methyl-pyridin-2-ylmethyl)ethylenediamine) as redox mediators in DSCs. Devices constructed with Cu(L2) redox couple afford an impressive power conversion efficiency (PCE) of 9.2% measured under simulated one sun irradiation (100 mW cm(-2), AM 1.5G), which is among the top efficiencies reported thus far for DSCs with copper complex-based redox mediators. Remarkably, the excellent air, photo, and electrochemical stability of the Cu(L2) complexes renders an outstanding long-term stability of the whole DSC device, maintaining similar to 90% of the initial efficiency over 500 h under continuous full sun irradiation. This work unfolds a new platform for developing highly efficient and stable redox mediators for large-scale application of DSCs.