The absorption spectra of dye solutions and dyes adsorbed on TiO2 surface were recorded using a UV–VIS spectrophotometer (Shimadzu, model UV-3101). Solar energy conversion efficiency (the photocurrent–voltage (I –V ) curve) was measured by using two computerized digital Keithley multimeters under simulated sunlight (AM
1.5, 100 mW cm 2 Þ. Based on I –V curve, the fill factor (FF)
is defined as