An Alkyloxyphenyl Group as a Sterically Hindered Substituent on a Triphenylamine Donor Dye for Effective Recombination Inhibition in Dye-Sensitized Solar Cells

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.