Recombination reactions in dye-sensitized solar cells (DSSCs) may substantially decrease the open-circuit voltage (V-oc) with cobalt complex redox electrolyte. Managing steric hindrance in the dye structure is necessary to inhibit recombination reactions and thereby increase the V-oc and achieve high power-conversion efficiency (PCE). New dyes with large-sized donors based on triphenylamine and modified with 4-(hexyloxy)phenyl groups were developed to identify an effective inhibitor for the recombination reaction in DSSCs with a cobalt complex redox electrolyte. The 4-(hexyloxy)phenyl tetra-adducts dye MK-123 effectively inhibited the recombination reaction, and the DSSC fabricated using this dye exhibited the highest V-oc (greater than 900 mV) among the cells with the investigated dyes. However, the short-circuit current (J(sc)) of the MK-123 cell was lower than that of the cell with the simple triphenylamine donor dye, MK-89. In contrast, the cell with bis-adducts dye MK-136 also exhibited an increase in its V-oc without a decrease in its J(sc). Among the investigated dyes, MK-136 exhibited the highest PCE of 8.9%. The effects of the steric hindrance of the 4-(hexyloxy)phenyl substituent are discussed.