Band edge movement and recombination kinetics in dye-sensitized nanocrystalline TiO2 solar cells: A study by intensity modulated photovoltage spectroscopy

The charge-recombination kinetics and band edge movement in dye-sensitized nanocrystalline TiO2 solar cells are investigated by intensity modulated photovoltage spectroscopy (IMVS). A theoretical model of IMVS for dye-sensitized nanocrystalline semiconductor electrodes is developed, and analytical expressions for the frequency dependence of the photovoltage response at open circuit are derived. The model considers charge trapping/detrapping and electron transfer from the conduction band and surface states of the semiconductor to redox species at the solid/solution interface. IMVS is shown to be valuable in elucidating the contributions of band edge shift and recombination kinetics to changes of the open-circuit photovoltage (V-oc) resulting from surface modifications of the semiconductor. IMVS measurements indicate that surface treatment of [RuL2(NCS)(2)] (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid)-sensitized TiO2 electrodes with 4-tert-butylpyridine or ammonia leads to a significant band edge shift concomitant with a more negative V,. Surface-modified dye-covered TiO2 electrodes exhibit a much higher photovoltage, for a given concentration of accumulated photogenerated electrons, than the unmodified dye-covered electrode. The accumulated charge in the TiO2 electrode is not sufficient to induce a major potential drop across the Helmholtz layer and cannot thus explain the observed photovoltage. The surface charge density is also not sufficient to support an accumulation layer strong enough to have a major influence on the photovoltage. The movement of the Fermi level of the TiO2 electrode, arising from the accumulation of photogenerated electrons in the conduction band, accounts for the observed Vac. The second-order nature of the recombination reaction with respect to I-3(-) concentration is confirmed. Furthermore, the IMVS study indicates that recombination at the nanocrystallite/redox electrolyte interface occurs predominantly via trapped electrons in surface states.