Transport and Interfacial Transfer of Electrons in Dye-Sensitized Solar Cells Utilizing a Co(dbbip)2 Redox Shuttle

The transport and interfacial transfer of electrons in dye-sensitized solar cells utilizing the Co(dbbip)(2) (dbbip = 2,6-bis(l'-butylbenzimidazol-2'-yl)pyridine) redox couple as an alternative to the conventional I-3(-)/l(-) couple have been investigated using intensity modulated photocurrent and photovoltage spectroscopy (IMPS/IMVS) combined with in situ near IR absorption spectroscopy. Attempts to use impedance spectroscopy to determine the electron diffusion length were unsuccessful due to overlap of the cathode and electron transport impedances. Values of the electron diffusion length in the range 5-8 pm were derived by IMPS/IMVS as well as by analysis of the ratio of the normalized photocurrent action spectra measured for illumination through the counter electrode and through the TiO2, electrode. These values indicate that loss of electrons by electron transfer to the Co(III) species will be important for TiO2 films thicker than about 5 pm, unless steps are taken to passivate the surface to retard back electron transfer.