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-sensit ized nanocrystalline TiO2 solar cells are investigated by intensity mo dulated photovoltage spectroscopy (IMVS). A theoretical model of IMVS for dye-sensitized nanocrystalline semiconductor electrodes is develop ed, and analytical expressions for the frequency dependence of the pho tovoltage response at open circuit are derived. The model considers ch arge trapping/detrapping and electron transfer from the conduction ban d and surface states of the semiconductor to redox species at the soli d/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 modifi cations 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 lead s to a significant band edge shift concomitant with a more negative V, . Surface-modified dye-covered TiO2 electrodes exhibit a much higher p hotovoltage, for a given concentration of accumulated photogenerated e lectrons, than the unmodified dye-covered electrode. The accumulated c harge in the TiO2 electrode is not sufficient to induce a major potent ial drop across the Helmholtz layer and cannot thus explain the observ ed 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 elect rode, arising from the accumulation of photogenerated electrons in the conduction band, accounts for the observed Vac. The second-order natu re 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 predominant ly via trapped electrons in surface states.