An Extremely High Molar Extinction Coefficient Ruthenium Sensitizer in Dye-Sensitized Solar Cells: The Effects of π-Conjugation Extension

We report a heteroleptic ruthenium complex (C107) featuring the electron-rich 5-octyl-2,2′-bis(3,4-ethylenedioxythiophene) moiety conjugated with 2,2′-bipyridine and exhibiting 10.7% power conversion efficiency measured at the AM1.5G conditions, thanks to the enhanced light-harvesting that is closely related to photocurrent. This C107sensitizer has an extremely high molar extinction coefficient of 27.4 × 103M−1cm−1at 559 nm in comparison to its analogue C103(20.5 × 103M−1cm−1at 550 nm) or Z907(12.2 × 103M−1cm−1at 521 nm) with the corresponding 5-hexyl-3,4-ethylenedioxythiophene- or nonyl-substituted bipyridyl unit. The augmentation of molar extinction coefficients and the bathochromic shift of low-energy absorption peaks along with the π-conjugation extension are detailed by TD-DFT calculations. The absorptivity of mesoporous titania films grafted with Z907, C103, or C107sublinearly increases with the molar extinction coefficient of sensitizers, which is consistent with the finding derived from the surface coverage measurements that the packing density of those sensitizers decreases with the geometric enlargement of ancillary ligands. When the dye-coated titania film is immersed in a high-efficiency redox electrolyte, a lower density molecule grafting on titania nanocrystals leads to more deep electronic states and a faster charge recombination at the titania/dye/electrolyte interface at a given electron Fermi level, explaining the observation of a larger dark current and a lower open-circuit photovoltage. Electrical impedance analysis further reveals that the electron diffusion length in nanocrystalline titania films is shortened along with the π-conjugation extension of ancillary ligands.