Rapid Static Sensitizer Regeneration Enabled by Ion Pairing

An anionic CoII complex, [Co(TTT) (NCS)3]− (TTT = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine and NCS = isothiocyanate), was synthesized for use in dye-sensitized solar cells (DSSCs). The CoII complex was found to ion-pair with the hexacationic sensitizer [Ru(tmam)2(dcb)]6+ (tmam = 4,4′-bis(trimethylaminomethyl)-2,2′-bipyridine and dcb = 4,4′-(CO2H)2-2,2′-bipyridine) anchored to TiO2 thin films immersed in acetonitrile solution. Visible light excitation of the ion pairs resulted in excited-state injection followed by rapid static regeneration of the oxidized sensitizer (<10 ns). The static component to regeneration gave an ion-pair equilibrium constant of 6000 M–1. This value is an order of magnitude smaller than the equilibrium constant determined for [Ru(tmam)2(deeb)]6+ (deeb = 4,4′-(CO2Et)2-2,2′-bipyridine) dissolved in acetonitrile. DSSC studies employing [Co(TTT) (NCS)3]− or the cationic [Co(DTB)3]2+ (DTB = 4,4′-di-tert-butyl-2,2′-bipyridine) as redox mediators revealed a 3 fold photocurrent increase in the presence of the anionic cobalt complex. As the regeneration step was greatly enhanced through the formation of Coulombic ion pairs, both electron injection and regeneration were complete within 10 ns which is unprecedented for dye-sensitization. The results obtained reveal that ground-state ion-pairing can be a powerful strategy for DSSC optimization.