Thiocyanate-Free Ru(II) Sensitizers with a 4,4 ′ -Dicarboxyvinyl-2,2 ′ -bipyridine Anchor for Dye-Sensitized Solar Cells

A new class of thiocyanate-free Ru(II) sensitizers with 4,4′-dicarboxyvinyl-2,2′-bipyridine anchor and two trans-oriented pyrid-2-yl pyrazolate (or triazolate) functional chromophores is synthesized, characterized, and evaluated in dye-sensitized solar cells (DSCs). Despite their enhanced red response and absorptivity when compared to the parent sensitizer TFRS-2 that possesses standard 4,4′-dicarboxyl-2,2′-bipyridine anchor and shows the best conversion efficiency of η = 9.82%, the newly synthesized carboxyvinyl-pyrazolate sensitizers, TFRS-11-TFRS-13, exhibit inferior performance characteristics in terms of short-circuit current density (J_SC), open-circuit voltage (V_OC), and power conversion efficiency (η), the latter being recorded to be in the range 5.60-7.62%. The reduction in device efficiencies is attributed to a combination of poor packing of these sensitizers on the TiO₂ surface and less positive ground-state oxidation potentials, which, respectively, increase charge recombination with I₃^- in electrolytes and impede the regeneration of sensitizers by I^- anions. The latter obstacle can be circumvented in part by the replacement of the pyrazolates with triazolates, forming the TFRS-14 sensitizer, which exhibits an improved J_SC, V_OC, and η of 16.4 mAcm^-2, 0.77 V, and 9.02%, respectively.