Tuning Open Circuit Photovoltages with Tripodal Sensitizers

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Abstract
The sensitizers [Ru(bpy)2(deeb)](PF6)2 (1), [Ru(bpy)2(bpy)-(E-Ph)-Ad](PF6)2 (2), and [Ru(bpy)2(bpy)-(E-Ph)2-Ad](PF6)2 (3), where deeb is 4,4'-(COOCH2CH3)2-2,2'-bipyridine, E-Ph is phenylethynyl, and Ad are tripod shaped bpy ligands based on 1,3,5,7-tetraphenyladamantane, were anchored to mesoporous nanocrystalline (anatase) TiO2 thin films and studied in regenerative solar cells with 0.1 M LiI/0.005 M I2 dichloromethane electrolyte. Over three decades of 488 nm irradiance, the open circuit photovoltage increased markedly with the distance between the Ru center and the surface binding groups, 1 (7 A) < 2 (18 A) < 3 (24 A). The diode equation accurately models the irradiance dependent data and indicates that the TiO2(e-) --> I3- (and/or I2) charge recombination rate constants were decreased by a factor of 20 for 2/TiO2 and 280 for 3/TiO2 relative to 1/TiO2. The results suggest that control of the sensitizer-TiO2 orientation is important for efficient power optimization.