The molecular structures, UV-Vis absorption spectra, and energy level structures of the dyes D-SS and D-ST were simulated using density functional theory, time-dependent density functional theory (TDDFT), and natural bond orbital analysis, which provided the physical mechanisms of dye-sensitized solar cells (DSSCs) containing D-ST and D-SS. The UV-Vis absorption spectrum of D-SS showed a significant red shift compared with that of D-ST and the molar absorption coefficient of D-SS was higher than that of D-ST. D-SS molecules should have a higher solar radiation photon-harvesting ability than D-ST molecules, but the energy level of the highest occupied molecular orbital (HOMO) of D-SS was higher than the redox energy level of the electrolyte (I-/I-3). As a result, an optically excited D-SS molecule cannot be successfully recovered by accepting an electron from the electrolyte after being oxidized by injecting an electron towards the TiO2 electrode. This limits the photon harvesting ability of D-SS molecules, and thereby significantly decreases the photovoltaic properties and energy conversion efficiency of DSSCs containing D-SS. This allows the photovoltaic properties of DSSCs containing D-SS to be understood, especially why its photovoltaic energy conversion efficiency is lower than that of DSSCs containing D-ST. The position of the HOMO energy level of dye-sensitized molecules is very important for the operation of DSSCs, and that of the organic sensitizer molecules used in DSSCs must be lower than the redox energy