Cyclometalated sensitizers for DSSCs employing cobalt redox shuttles

Abstract

Two novel thiocyanate-free heteroleptic ruthenium (II) cyclometalated complexes with long chain hydrophobic groups, [Ru(ppy)(dnbpy)(dcbpy)](PF6) (ss-14) and [Ru(dFppy)(dnbpy)(dcbpy)](PF6) (ss-22) (dnbpy=4,4′-dinonyl-2,2′-bipyridine; dcbpy=2,2′-bipyridine-4,4′-dicarboxylic acid; ppy=2-phenylpyridine; dFppy=2-(2,4-difluorophenyl)pyridine) were synthesized, characterized and applied in dye sensitized solar cells (DSSC) using the [Co(dmbpy)3]2+/3+ (dmbpy=4,4′-dimethyl-2,2′-bipyridine) redox shuttle. The optical and electrochemical properties were compared to that of benchmark DSSC dye Z907 and a previously reported parent cyclometalated complex [Ru(ppy)(bpy)(dcbpy)](PF6) (1d). Functionalization of the bpy ligand with dinonyl groups combined with the addition of electron withdrawing fluorine substituents on the 2-phenylpyridine ligand resulted in enhanced light absorption and longer electron lifetimes compared to 1d, and a sufficiently positive HOMO energy level for regeneration. As a consequence of such sensitizer engineering, an overall power conversion efficiency in excess of 3% with [Co(dmbpy)3]2+/3+ was achieved.