Investigation of the effect of alkyl chain length on charge transfer at TiO2/dye/electrolyte interface

Dye-adsorbed nanoporous TiO2 films were prepared with Ru(H(2)dcbpy)(bpy)(NCS)(2), Ru(H(2)dcbpy)(dmbpy)-(NCS)(2), and Ru(H(2)dcbpy)(dnbpy)(NCS)(2), which have the same structure with different alkyl chain lengths of 0, 1, and 9 attached to a bipyridine ligand, and with conventional dyes, Ru(BU4NHdcbPY)2(NCS)2, known as N719, and Ru(dcb)(2)(NCS)(2), known as N3. The films were immersed in electrolytes containing I-/I-3(-) redox couples, and lifetime of conduction band electrons, which were injected from the excited states of the dyes, was measured. When Li+ was employed as a counter charge of the redox couple, no noticeable difference was observed regardless of the difference of alkyl chain length of the dyes. Transient absorption measurements of dye cation also showed comparable dye cation lifetime, suggesting that the alkyl chain length of the ruthenium complexes has little influence on the charge transfers from TiO2 to I-3(-) and from I- to dye cation. Electron lifetime was also measured for dye-sensitized solar cells (DSCs) using electrolytes containing tetrabuthylammonium or tetrahexylammonium cations instead of Li+. Except for N719, comparable electron lifetime was observed regardless of the species of the cations, indicating that the alkyl chain length of quaternary ammonium cations also did not influence the interfacial charge transfer. These results show that alkyl chain itself does not impede the approach of I-/I-3(-) anions to the TiO2 surface, but the simultaneous control and concerted effect of the density of adsorbed dye and the size of cations are important to retard undesired interfacial charge transfers.