The absorption spectra and excited states of four triphenylamine-based organic dye molecules (TC1, TC2, TC3 and TC4) for dye sensitized solar cells (DSSC) have been theoretically investigated with density function theory (DFT) and time-dependent density function theory (TDDFT). In order to simulate the optical properties of the real DSSCs, sensitizer/titanium dioxide cluster interface systems TC4-(TiO₂)_n (n = 1–8) are also modeled. It is found that different adsorption sites and sizes of the clusters will exert critical effects on the molecular orbital energy levels of the complexes, and ultimately on their light-harvesting capabilities. In all the tested sensitizer/titanium dioxide cluster interface systems, the binding of the organic dye molecule to the 3-fold-coordinated titanium of the cluster (TiO₂)₆ could most benefit the widening of the spectrum response region and the interactions between the sensitizer and the cluster. From the intuitive physical pictures given by the three-dimensional (3D) real-space analysis methods of transition and charge difference densities, one reveals the photo-induced charge transfer microcosmic process of the sampled organic sensitizer/titanium dioxide cluster system TC4-(TiO₂)₆.