Dye-Sensitized Solar Cell Based on Novel Star-Shaped Ruthenium Polypyridyl Sensitizer: New Insight into the Relationship between Molecular Designing and Its Outstanding Charge Carrier Dynamics

In this work, we have introduced a star-shaped sensitizer for dye sensitized solar cells (DSSCs) consist of three anchoring groups of benzene sulfonic acid with a significant improvement of interfacial electron transfer rate of dye/TiO(2)(-)anatase nanostructures compared to corresponding sensitizers containing only one anchoring group. Physical chemistry aspect of the robust star-shaped sensitizer has been investigated, employing the Current over Voltage analysis (I-V) and Transient Absorption Spectroscopy (TAS) methods. The computational studies based on density functional theory (DFT) and quantum calculations were clearly indicated that the rate of the Interfacial electron transfer (IET) significantly depends on the binding mode of a sensitizer to semiconductor surface and the nature of the electronic population in lowest unoccupied molecular orbitals (LUMOs) that initially localized on the adsorbate molecule, which is in a good agreement with experimental results. Our main result is that, among different possible binding modes of anchoring group to TiO2 surface, the bridging mode has the shortest life time of 28 fs in the presence of visible light as the excitation source. To investigate negative effect of the sensitizer molecules aggregation on TiO2 surface, on the performance of the cell, was examined the dye loading time on TiO2 surface for 10, 16 and 20 hours, which 16 hours of dye loading on surface exhibited the best DSSC performance. Moreover, transient absorption studies consistently show that the star-shaped dye (consist of three anchoring groups) has better performance compared to the other dyes (consist of one anchoring groups) (1 and 3), in all aspects of critical parameters for DSSCs, including electron lifetime in TiO2, electron injection, dye regeneration and recombination resistance. The present work explains the fundamental physical chemistry aspects of the excited-state in star-shaped ruthenium polypyridyl complexes, along with opening new insight into the design of new sensitizers for enhancing the interfacial electron transfer rate and lowering the charge recombination process, which results in high power conversion efficiency of DSSC.