In this work, using the DFT and TDDFT, we have theoretically studied the electronic and optical properties of the two recently synthesized coadsorbents Y1 and Y2, which were aimed to enhance the efficiency of the black dye-sensitized solar cells. To determine the solvatochromic shifts, both the implicit and mixed implicit–explicit models have been used. The connection between the solvatochromic shifts and the changes in dipole moments in the excitation process is discussed. The difference in excitation charge transfer is utilized to explain the experimentally observed difference in J_sc for Y1 and Y2. Investigating the interactions of I₂ molecules in the electrolyte solution with the coadsorbents showed that with Y1 the recombination loss was weakened through decreasing the I₂ concentration near the TiO₂ surface, whereas with Y2 it was increased. As a result, the higher values of both J_sc and V_oc with the Y1 coadsorbent explain its experimentally observed higher efficiency. The present study sheds light on how to design and engineer newer coadsorbents or organic dyes for higher efficiencies.