Dye-Sensitized Solar Cells Based on Cu(I) Complexes Containing Catechol Anchor Groups That Operate with Aqueous Electrolytes

We report the design and synthesis of two novel phenanthroline-based ligands, a catechol-functionalized derivative (L1: 4,7-(4-catechol)-2,9-dimethyl-1,10-phenanthroline) ligand and its methoxy analogue (L1′). These ligands were used to prepare four Cu(I) complexes: two homoleptic bis-diimine Cu(I) complexes (C1 and C1′) and two xantphos-based heteroleptic diimine-diphosphine derivatives (C2 and C2′). Their photophysical and electrochemical properties were characterized by steady-state and time-resolved spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. Implementation of L1, C1, and C2 in n-type DSSCs gave a photoconversion efficiency (PCE) of 1.88% for the heteroleptic Cu(I) complex C2, representing a 35-fold increase compared to previously reported diimine-diphosphine Cu(I)-based DSSCs. Incident photon-to-current efficiency (IPCE) measurements and electrochemical impedance spectroscopy confirmed efficient photoinduced charge injection and interfacial electron transfer. Moreover, the strong binding affinity of the catechol anchors enabled the fabrication of DSSCs with an aqueous electrolyte, which showed stable performance for at least 10 days. The working principle of these cells is described as a dual chromophore system, where the catechol-TiO₂ interaction, operating through a type-II sensitization mechanism, acts as the primary chromophore, while the Cu(I) complex serves as an antenna chromophore.