Relative diffusion rates (R-rel) of I-3(-) species in titania nanopores modified with various dye molecules were measured. Introduction of polar atoms such as O or F decreased R-rel and open circuit voltage (V-oc) of dye-sensitized solar cells (DSC). The results were explained by the fact that slow diffusion of I-3(-) species in nanopores increases opportunities of charge recombination between electrons in titania layer and I-3(-) existing in the titania nanopores. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3583638] All rights reserved.