Effect of Ligand Structures of Copper Redox Shuttles on Photovoltaic Performance of Dye-Sensitized Solar Cells

In recent years, copper­(I/II) complexes have emerged as alternative redox shuttles in dye-sensitized solar cells (DSSCs), exhibiting more positive redox potential than iodine- and cobalt-based redox shuttles. In particular, copper­(I/II) complexes with 1,10-phenanthroline- or 2,2′-bipyridyl-based ligands attained moderate to high power conversion efficiencies (6–11%) with a high open-circuit voltage (V OC) over 1.0 V due to the positive potentials. Although copper­(I/II) complexes with 1,10-phenanthroline-based ligands with 2,9-substituents have been developed, the effect of their ligand structures on the photovoltaic performance of DSSCs have not been fully addressed due to limited synthetic access to 1,10-phenanthroline derivatives. In this study, we designed and synthesized a series of copper­(I/II) complexes with 1,10-phenanthroline ligands with different substituents at the 2,9-positions: bis­(2-n-butyl-1,10-phenanthroline)­copper­(I/II) ([Cu­(bp)2]1+/2+), bis­(2-ethyl-9-methyl-1,10-phenanthroline)­copper­(I/II) ([Cu­(emp)2]1+/2+), bis­(2,9-diethyl-1,10-phenanthroline)­copper­(I/II) ([Cu­(dep)2]1+/2+), and bis­(2,9-diphenyl-1,10-phenanthroline)­copper­(I/II) ([Cu­(dpp)2]1+/2+). The more positive redox potentials of [Cu­(emp)2]1+/2+ and [Cu­(dep)2]1+/2+, compared to that of bis­(2,9-dimethyl-1,10-phenanthroline)­copper­(I/II) ([Cu­(dmp)2]1+/2+), originate from the larger steric hindrance of the ethyl group instead of the methyl group, whereas the redox potential of [Cu­(bp)2]1+/2+ is significantly shifted to the negative direction because of the lower steric hindrance of the 2-monosubstituted 1,10-phenanthroline ligands. The efficiency of the DSSC with [Cu­(bp)2]1+/2+ (5.90%) is almost comparable to the DSSC with [Cu­(dmp)2]1+/2+ (6.29%). In contrast, the DSSCs with [Cu­(emp)2]1+/2+ (3.25%), [Cu­(dep)2]1+/2+ (2.56%), and [Cu­(dpp)2]1+/2+ (2.21%) exhibited lower efficiencies than those with [Cu­(dmp)2]1+/2+ and [Cu­(bp)2]1+/2+. The difference can be rationalized by the electron collection efficiencies. Considering the similar photovoltaic properties of the DSSCs with [Cu­(bp)2]1+/2+ and [Cu­(dmp)2]1+/2+, the use of copper­(I/II) complexes with 2-monosubstituted 1,10-phenanthroline ligands as the redox shuttle may be useful to improve the short-circuit current density while retaining the rather high V OC value when dyes with a smaller bandgap (i.e., better light harvesting) are developed.