The improvement in photovoltaic performance of dye-sensitized solar cells was found after aging in the dark and was analyzed by linear sweep voltammetry and electrochemical impedance spectroscopy. The promotion was found to arise from the formation of blocking layers on the surface of nanocrystalline TiO2, resulting most likely from the intermolecular electrostatic action between the 4-tert-butylpyridine and the 1,2-dimethyl-3-propylimidazolium ions. These blocking layers can retard the interfacial reaction of the electron with I-3(-) ions without deteriorating the rate of regeneration of the oxidized dye molecules. By virtue of the blocking layers, the retarding recombination of electrons with the I ions significantly increased the electron lifetime and enlarged the electron diffusion length, resulting in a higher open-circuit voltage and an improvement in charge collection efficiency, and eventually an enhancement of the energy conversion efficiency. (C) 2011 Elsevier Ltd. All rights reserved.