A density functional theory (DFT) method was used to study the monomer and intermolecular charge-transfer complexes of 22 different alkylpyridines with diiodine. DFT calculations revealed that the sigma* orbital of iodine interacts with the nitrogen lone pair in pyridines. The open-circuit photovoltage (V-oc) values of a bis(tetrabutylammonium)cisbis(thiocyanato)bis(2,2'-bipyridine-4-carboxylic acid, 4'-carboxylate)ruthenium(II) (N719) dye-sensitized nanocrystalline TiO2 solar cell with an I-/I-3(-) redox electrolyte in acetonitrile using alkylpyridines additive were compared to computational calculations on the interaction between pyridines and I-2 by a DFT method. The optimized geometries, frequency analyses, Mulliken population analyses, and interaction energies suggest that the V-oc value of the solar cell is higher, the more alkylpyridine complexes with I-2. (c) 2005 Elsevier B.V. All rights reserved.