Design of Os^II-based Sensitizers for Dye-Sensitized Solar Cells: Influence of Heterocyclic Ancillaries

A series of Os^II sensitizers (TFOS-x, in which x=1, 2, or 3) with a single 4,4′-dicarboxy-2,2′-dipyridine (H₂dcbpy) anchor and two chelating 2-pyridyl (or 2-pyrimidyl) triazolate ancillaries was successfully prepared. Single-crystal X-ray structural analysis showed that the core geometry of the Os^II-based sensitizers consisted of one H₂dcbpy unit and two eclipsed cis-triazolate fragments; this was notably different from the Ru^II-based counterparts, in which the azolate (both pyrazolate and triazolate) fragments are located at the mutual trans-positions. The basic properties were extensively probed by using spectroscopic and electrochemical methods as well as time-dependent density functional theory (TD-DFT) calculations. Fabrication of dye-sensitized solar cells (DSCs) was then attempted by using the I^-/I₃^--based electrolyte solution. One such DSC device, which utilized TFOS-2 as the sensitizer, showed promising performance characteristics with a short-circuit current density (J_SC) of 15.7 mA cm^-2, an open-circuit voltage of 610 mV, a fill factor of 0.63, and a power conversion efficiency of 6.08 % under AM 1.5G simulated one-sun irradiation. Importantly, adequate incident photon-to-current conversion efficiency performances were observed for all TFOS derivatives over the wide spectral region of 450 to 950 nm, showing a panchromatic light harvesting capability that extended into the near-infrared regime. Our results underlined a feasible strategy for maximizing J_SC and increasing the efficiency of DSCs.