A Bis(tridentate) cobalt Polypyridine Complex as Mediator in Dye-Sensitized Solar Cells

Abstract

Dye-sensitized solar cells equipped with cationic and neutral Ru-II-based sensitizers [Ru(ddpd){tpy(COOH)(3)}](2+) [1(2+); ddpd = N,N'-dimethyl-N,N'-di(pyridin-2-yl)pyridin-2,6-diamine, tpy(COOH)(3) = 2,2 '' 6',2 ''-terpyridine-4,4',4 ''-tricarboxylic acid] and [Ru(ddpd){tpy(COOH)(COO)(2)}] (2) with and without the coadsorbent chenodeoxycholic acid were constructed with I-3(-)/I- or the Co-III/II-based redox mediators [Co-(bpy)(3)](3+/2+) (3(3+/2+); bpy = 2,2'-bipyridine) and [Co-(ddpd)(2)](3+/2+) (4(3+/2+)) in the presence of LiClO4 and 4-tert-butylpyridine. The best photovoltaic performance was achieved by using the 4(3+/2+) shuttle and the neutral sensitizer 2 without coadsorbent. The higher short-circuit photocurrent density and higher electron recombination lifetimes obtained with this combination suggest slow electron recombination kinetics at the TiO2 surface with the Co-III complex 4(3+). The slow electron transfer to 4(3+) is tentatively ascribed to the high-lying pi* orbitals of the electron-rich ddpd ligands, which result in a weak electronic coupling. This contrasts with the faster recombination with 3(3+), which features the low-energy pi* orbitals of the bpy ligands.