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
FF ¼ ðI max V max Þ=ðI sc V oc Þ, (1)
where I max and V max are the photocurrent and photo- voltage for maximum power output (Pmax Þ, I sc and V oc are the short-circuit photocurrent and open-circuit photovol- tage, respectively. The overall energy conversion efficiency (ZÞ is defined as
Z ¼ ðI sc V oc FFÞ=Pin , (2)
where Pin is the power of incident light.