Novel phenanthroline-based ruthenium complexes for dye-sensitized solar cells: Enhancement in performance through fluoro-substitution

Three new ruthenium sensitizers [Ru(H₂dcbpy)(L _X)(NCS)₂] [H₂dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine, and L_X = 2-(4-fluorophenyl)-1H-imidazo[4, 5-f][1,10]phenanthroline (RuL_F), 4-(1H-imidazo[4,5-f][1,10] phenanthrolin-2-yl)-N,N-dimethylaniline (RuL_N), or 2-(4-methylphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (RuL_C)], have been designed and synthesized. Under standard global AM 1.5 solar conditions, the device using the RuL_F sensitizer gave a conversion efficiency of 7.28%, compared to 7.50% optimized for the N719 dye. The power-conversion efficiency of RuL_F is 24% and 66% higher than those of RuL_N and RuL_C, respectively, due to a remarkable fluoro-substitution effect. The origins of this effect have been thoroughly investigated by spectroscopic, electrochemical, and density functional theory (DFT) studies. Our results show the fluoro-substitution serves as the key factor for improving the device performance, the role of which is to increase the dye density on TiO₂, alter the molecular packing, and tune the electron donating property. These findings may provide an alternative strategy for the design of a new type of ruthenium sensitizer by the introduction of C-F bonds to the auxochromic ligand to improve the device performance.