Ru(II) sensitizers bearing dianionic biazolate ancillaries: ligand synergy for high performance dye sensitized solar cells

We report on a new series of Ru(II) sensitizers, TFRS-61-66, suited for high performance dye sensitized solar cells (DSCs). The molecular design incorporates novel dianionic bipyrazolate or bitriazolate ancillaries to replace the dual thiocyanates in N3 (or N719) with the aim of improving device stability. These newly designed dianionic biazolate ligands make feasible the fine-tuning of the ground state oxidation potential. This, together with the synergistic incorporation of electron-donating thiophene or thiophene-containing appendages to one bipyridine for improved light-harvesting, leads to the achievement of DSCs with a prominent J_SC of 17.4 mA cm^-2 and hence a high photon conversion efficiency of 9.6% using TFRS-63, which is comparable with the N3 control device (9.5% in this study) but with notably superior device stability. Comprehensive electrochemical impedance, charge extraction, transient photovoltage and transient absorption spectroscopy measurements were carried out to gain insights into the fundamentals of these new dyes and the associated device properties. These biazolate substituted Ru(II) sensitizers add another dimension in the search to replace thiocyanate ligands in Ru(II) sensitizers showing high performance in DSC devices.