Plant carotenoids (Cars) with 8-10 conjugated double bonds, having higher singlet energies than those of bacterial Cars with 9-13 conjugated double bonds, were added (by 20%) as redox spacers to a titania-based Gratzel-type solar cell using a chlorophyll derivative (PPB a) as the sensitizer. No clear indication of singlet-energy transfer from Car to PPB a was seen, but clear enhancement of photocurrent with the decreasing one electron-oxidation potential of Car was observed. An empirical equation correlating the increase in photocurrent to difference in one electron-oxidation potentials (PPB a minus Car) and the oscillator strength of Car is proposed. (c) 2006 Elsevier B.V. All rights reserved.