Dye-sensitized solar cells are a potential low-cost alternative to silicon solar cells. A current limitation to DSSCs is that most dyes used in these cells have bandgaps that are too large for ideal sunlight absorption. In this thesis, cis-Os(dcbpy)_{2}I_2 dye, a sensitizer with an absorption onset near 1.2 eV is introduced that has significantly better light-absorbing capability in the near-infrared spectral regions compared to benchmark RuN3, and related, dyes. To understand the effect of overpotential required to drive iodide/triiodide redox chemistry on the performance of DSSCs containing RuN3 or OsI_2 dyes bound to TiO_2, their performance was examined with varied ratios of [I^-] to [I_3^-]. Measurements showed that the maximum light-to-electrical power conversion efficiency for RuN3 DSSCs occurred at an electrolyte potential of 0.35 V corresponded to 6.3 times more I^- than I_3^-. Whereas for OsI_2 DSSCs, an insignificant amount of power was generated due to slow electron-transfer kinetics between the redox shuttle and OsI_2. It was proposed that the addition of dmFc to the electrolyte could improve OsI_2 DSSCs performance by directly catalyzing I^- oxidation. To validate this hypothesis, foot-of-the-wave analysis was applied to the cyclic voltammetry responses of TiO_2 photoelectrodes dyed with OsI_2. It demonstrated that dmFc catalyzed I_3^- reduction faster than I^- oxidation. Photoelectrochemical measurements unexpectedly showed that at the presence of dmFc in the electrolyte of OsI_2 DSSCs, electrons transferred from excited OsI_2 to the electrolyte solution, instead of injecting into the TiO_2 film, making these films photocathodic even though TiO_2 is n-type.