It is well known that the Voc of solar cell devices is directly proportional to the energy gap between the HOMO and the LUMO [11]. The electrochemical oxidation and reduction onset potentials (Eox and Ered) determined from CV results are used to calculate HOMO and LUMO levels. The onset potentials are determined from the intersection of tangents between rising current and baseline charging current of the CV curves. As shown in Table 2, the ELUMO level of the N719 dye was lower than that of the PL dye, which should cause a lower Voc for the N719 dye-based DSSCs. However, Fig. 4 indicates that the higher EHOMO level of the PL dyes might inhibit the charge carrier regeneration and transfer from the electrolyte into the dyes. Consequently, the voltage produced by the N719 dye-based solar cells was greater than that of the PL dye-based ones. The addition of benzoic acid to increase the acidity of the PL dyes significantly changed the position of the EHOMO and ELUMO levels of the dyes. The energy gap between the HOMO–LUMO levels of the dyes and the electrolytes was the main cause of the high Voc generated from the solar cells with PL dye and pH 5. Therefore, the EHOMO and ELUMO levels of the dyes, the TiO2 semiconductors, and the electrolytes should be tuned carefully to minimize the energy loss and to produce high Voc [11].